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🔌 Adapter
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📚 Array
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allallEqualanyarrayToCSVbifurcatebifurcateBychunkcompactcountBycountOccurrencesdeepFlattendifferencedifferenceBydifferenceWithdropdropRightdropRightWhiledropWhileeveryNthfilterFalsyfilterNonUniquefilterNonUniqueByfindLastfindLastIndexflattenforEachRightgroupByheadindexOfAllinitialinitialize2DArrayinitializeArrayWithRangeinitializeArrayWithRangeRightinitializeArrayWithValuesinitializeNDArrayintersectionintersectionByintersectionWithisSortedjoinJSONtoCSVlastlongestItemmapObjectmaxNminNnonenthElementoffsetpartitionpermutationspullpullAtIndexpullAtValuepullByreducedFilterreduceSuccessivereduceWhichrejectremovesamplesampleSizeshankshufflesimilaritysortedIndexsortedIndexBysortedLastIndexsortedLastIndexBystableSortsymmetricDifferencesymmetricDifferenceBysymmetricDifferenceWithtailtaketakeRighttakeRightWhiletakeWhiletoHashunionunionByunionWithuniqueElementsuniqueElementsByuniqueElementsByRightuniqueSymmetricDifferenceunzipunzipWithwithoutxProdzipzipObjectzipWith
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arrayToHtmlListbottomVisiblecopyToClipboardcountercreateElementcreateEventHubcurrentURLdetectDeviceTypeelementContainselementIsVisibleInViewportgetImagesgetScrollPositiongetStylehasClasshashBrowserhidehttpsRedirectinsertAfterinsertBeforeisBrowserTabFocusednodeListToArrayobserveMutationsoffononUserInputChangeprefixrecordAnimationFramesredirectrunAsyncscrollToTopsetStyleshowsmoothScrolltoggleClasstriggerEventUUIDGeneratorBrowser
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approximatelyEqualaverageaverageBybinomialCoefficientclampNumberdegreesToRadsdigitizedistanceelofactorialfibonaccigcdgeometricProgressionhammingDistanceinRangeisDivisibleisEvenisNegativeZeroisPrimelcmluhnCheckmaxBymedianmidpointminBypercentilepowersetprimesradsToDegreesrandomIntArrayInRangerandomIntegerInRangerandomNumberInRangeroundsdbmstandardDeviationsumsumBysumPowertoSafeInteger
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bindAlldeepClonedeepFreezedeepMapKeysdefaultsdigequalsfindKeyfindLastKeyflattenObjectforOwnforOwnRightfunctionsgetinvertKeyValueslowercaseKeysmapKeysmapValuesmatchesmatchesWithmergenestobjectFromPairsobjectToPairsomitomitByorderBypickpickByrenameKeysshallowClonesizetransformtruthCheckCollectionunflattenObject
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byteSizecapitalizecapitalizeEveryWordcompactWhitespaceCSVToArrayCSVToJSONdecapitalizeescapeHTMLescapeRegExpfromCamelCaseindentStringisAbsoluteURLisAnagramisLowerCaseisUpperCasemapStringmaskpadpalindromepluralizeremoveNonASCIIreverseStringsortCharactersInStringsplitLinesstringPermutationsstripHTMLTagstoCamelCasetoKebabCasetoSnakeCasetoTitleCasetruncateStringunescapeHTMLURLJoinwords
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创建一个可以接收n个参数的函数, 忽略其他额外的参数。
调用提供的函数fn,参数最多为n个, 使用 Array.prototype.slice(0,n) 和展开操作符 (...)。
const ary = (fn, n) => (...args) => fn(...args.slice(0, n));示例
const firstTwoMax = ary(Math.max, 2);
[[2, 6, 'a'], [8, 4, 6], [10]].map(x => firstTwoMax(...x)); // [6, 8, 10]给定一个key和一组参数，给定一个上下文时调用它们。主要用于合并。
使用闭包调用上下文中key对应的值，�即带有存储参数的函数。
const call = (key, ...args) => context => context[key](...args);示例
Promise.resolve([1, 2, 3])
.then(call('map', x => 2 * x))
.then(console.log); // [ 2, 4, 6 ]
const map = call.bind(null, 'map');
Promise.resolve([1, 2, 3])
.then(map(x => 2 * x))
.then(console.log); // [ 2, 4, 6 ]将一个接收数组参数的函数改变为可变参数的函数。
给定一个函数，返回一个闭包，该闭包将所有输入收集到一个数组接受函数中。
const collectInto = fn => (...args) => fn(args);示例
const Pall = collectInto(Promise.all.bind(Promise));
let p1 = Promise.resolve(1);
let p2 = Promise.resolve(2);
let p3 = new Promise(resolve => setTimeout(resolve, 2000, 3));
Pall(p1, p2, p3).then(console.log); // [1, 2, 3] (after about 2 seconds)�Flip以一个函数作为参数，然后把第一个参数作为最后一个参数。
返回一个可变参数的闭包，在应用其他参数前，先把第一个以外的其他参数作为第一个参数。
const flip = fn => (first, ...rest) => fn(...rest, first);示例
let a = { name: 'John Smith' };
let b = {};
const mergeFrom = flip(Object.assign);
let mergePerson = mergeFrom.bind(null, a);
mergePerson(b); // == b
b = {};
Object.assign(b, a); // == b创建一个函数，这个函数可以调用每一个被传入的并且才有参数的函数，然后返回结果。
使用 Array.prototype.map() 和 Function.prototype.apply()将每个函数应用给定的参数。
const over = (...fns) => (...args) => fns.map(fn => fn.apply(null, args));示例
const minMax = over(Math.min, Math.max);
minMax(1, 2, 3, 4, 5); // [1,5]创建一个函数，它可以调用提供的被转换参数的函数。
使用Array.prototype.map()将transforms应用于args，并结合扩展运算符(…)将转换后的参数传递给fn。
const overArgs = (fn, transforms) => (...args) => fn(...args.map((val, i) => transforms[i](val)));示例
const square = n => n * n;
const double = n => n * 2;
const fn = overArgs((x, y) => [x, y], [square, double]);
fn(9, 3); // [81, 6]为异步函数执行从左到右的函数组合。
在扩展操作符(…)中使用Array.prototype.reduce() 来使用Promise.then()执行从左到右的函数组合。
这些函数可以返回简单值、Promise的组合，也可以定义为通过await返回的async值。
所有函数必须是一元的。
const pipeAsyncFunctions = (...fns) => arg => fns.reduce((p, f) => p.then(f), Promise.resolve(arg));示例
const sum = pipeAsyncFunctions(
x => x + 1,
x => new Promise(resolve => setTimeout(() => resolve(x + 2), 1000)),
x => x + 3,
async x => (await x) + 4
);
(async() => {
console.log(await sum(5)); // 15 (after one second)
})();执行从左到右的函数组合。
在展开操作符(…)中使用Array.prototype.reduce()来执行从左到右的函数组合。
第一个(最左边的)函数可以接受一个或多个参数; 其余的函数必须是一元的。
const pipeFunctions = (...fns) => fns.reduce((f, g) => (...args) => g(f(...args)));示例
const add5 = x => x + 5;
const multiply = (x, y) => x * y;
const multiplyAndAdd5 = pipeFunctions(multiply, add5);
multiplyAndAdd5(5, 2); // 15把一个异步函数转换成返回promise的。
使用局部套用返回一个函数，该函数返回一个调用原始函数的Promise。
使用的...操作符来传入所有参数。
const promisify = func => (...args) =>
new Promise((resolve, reject) =>
func(...args, (err, result) => (err ? reject(err) : resolve(result)))
);示例
const delay = promisify((d, cb) => setTimeout(cb, d));
delay(2000).then(() => console.log('Hi!')); // // Promise resolves after 2s创建一个调用提供的函数的函数，该函数的参数按照指定的索引排列。
利用 Array.prototype.map() 根据 indexes 和展开操作符 (...) 对参数进行重新排序，将转换后的参数传递给 fn。
const rearg = (fn, indexes) => (...args) => fn(...indexes.map(i => args[i]));示例
var rearged = rearg(
function(a, b, c) {
return [a, b, c];
},
[2, 0, 1]
);
rearged('b', 'c', 'a'); // ['a', 'b', 'c']接受一个可变参数函数并返回一个闭包，该闭包接受一个参数数组以映射到函数的输入。
使用闭包和扩展操作符(…)将参数数组映射到函数的输入。
const spreadOver = fn => argsArr => fn(...argsArr);示例
const arrayMax = spreadOver(Math.max);
arrayMax([1, 2, 3]); // 3创建一个最多接受一个参数的函数，忽略任何其他参数。
只把第一个参数传递给要调用的函数fn。
const unary = fn => val => fn(val);示例
['6', '8', '10'].map(unary(parseInt)); // [6, 8, 10]如果被提供的断言函数接收数组中每个元素作为参数都返回true，则返回true，否则返回false。
使用 Array.prototype.every()来测试是否第二个参数fn以集合中每个元素作为参数都返回true，使用Boolean作为默认值。
const all = (arr, fn = Boolean) => arr.every(fn);示例
all([4, 2, 3], x => x > 1); // true
all([1, 2, 3]); // true检查是否数组中所有的元素都是相等的。
使用 Array.prototype.every() 来检测是否数组中的所有元素都和第一个元素相等。
const allEqual = arr => arr.every(val => val === arr[0]);示例
allEqual([1, 2, 3, 4, 5, 6]); // false
allEqual([1, 1, 1, 1]); // true如果被提供的断言函数接收数组中任意一个元素作为参数都返回true，则返回true，否则返回false。
使用 Array.prototype.every()来测试是否第二个参数fn以集合中任意一个元素作为参数都返回true，使用Boolean作为默认值。
const any = (arr, fn = Boolean) => arr.some(fn);示例
any([0, 1, 2, 0], x => x >= 2); // true
any([0, 0, 1, 0]); // true将2D数组转换为逗号分隔值(CSV)字符串。
使用 Array.prototype.map() 和 Array.prototype.join(delimiter) 将一个一维数组转换为字符串。
使用 Array.prototype.join('\n') 将所有行合并成CSV字符串, 用换行符分割每一行。
如果没有第二哥参数, delimiter会使用一个默认分隔符 ,.
const arrayToCSV = (arr, delimiter = ',') =>
arr.map(v => v.map(x => `"${x}"`).join(delimiter)).join('\n');示例
arrayToCSV([['a', 'b'], ['c', 'd']]); // '"a","b"\n"c","d"'
arrayToCSV([['a', 'b'], ['c', 'd']], ';'); // '"a";"b"\n"c";"d"'将数据分为两组，如果元素在 filter数组中对应的是true，集合中相应的元素应加入第一个数组，否则加入第二个数组。
基于filter使用Array.prototype.reduce() 和 Array.prototype.push()将元素分组。
const bifurcate = (arr, filter) =>
arr.reduce((acc, val, i) => (acc[filter[i] ? 0 : 1].push(val), acc), [[], []]);示例
bifurcate(['beep', 'boop', 'foo', 'bar'], [true, true, false, true]); // [ ['beep', 'boop', 'bar'], ['foo'] ]根据断言函数将数据分成两组，断言函数将指定集合中的元素属于哪个组。如果断言函数返回true，元素加入第一个数组，否则加入第二个数组。
给予fn接收元素的返回值，使用Array.prototype.reduce() 和 Array.prototype.push()将元素分组。
const bifurcateBy = (arr, fn) =>
arr.reduce((acc, val, i) => (acc[fn(val, i) ? 0 : 1].push(val), acc), [[], []]);示例
bifurcateBy(['beep', 'boop', 'foo', 'bar'], x => x[0] === 'b'); // [ ['beep', 'boop', 'bar'], ['foo'] ]将数组分成指定大小的较小数组。
使用 Array.from()创建一个新的数组，该数组与将要生成的块的数量相匹配。
使用 Array.prototype.slice() 将新数组的每个元素映射到长度为size的块。
如果原始的数组不能被均匀的分割，最后的一块将包含剩余的元素。
const chunk = (arr, size) =>
Array.from({ length: Math.ceil(arr.length / size) }, (v, i) =>
arr.slice(i * size, i * size + size)
);示例
chunk([1, 2, 3, 4, 5], 2); // [[1,2],[3,4],[5]]删除数组中错误的元素
使用 Array.prototype.filter() 过滤掉错误的元素 (false, null, 0, "", undefined, NaN).
const compact = arr => arr.filter(Boolean);示例
compact([0, 1, false, 2, '', 3, 'a', 'e' * 23, NaN, 's', 34]); // [ 1, 2, 3, 'a', 's', 34 ]基于给定的函数将数组中的元素进行分组，并返回每个组中的元素数。
使用Array.prototype.map()来将数组中的每个元素映射到函数或属性名。
使用 Array.prototype.reduce() 创建一个对象，其中的键是从映射结果生成的。
const countBy = (arr, fn) =>
arr.map(typeof fn === 'function' ? fn : val => val[fn]).reduce((acc, val) => {
acc[val] = (acc[val] || 0) + 1;
return acc;
}, {});示例
countBy([6.1, 4.2, 6.3], Math.floor); // {4: 1, 6: 2}
countBy(['one', 'two', 'three'], 'length'); // {3: 2, 5: 1}计算数组中某个元素出现的次数。
Use Array.prototype.reduce()在每次遇到数组中的特定值时递增计数器。
const countOccurrences = (arr, val) => arr.reduce((a, v) => (v === val ? a + 1 : a), 0);示例
countOccurrences([1, 1, 2, 1, 2, 3], 1); // 3将一个多层嵌套的数组转转换成一个一元数组。
使用递归.
Use Array.prototype.concat() with an empty array ([]) and the spread operator (...) to flatten an array.
使用 Array.prototype.concat() 和一个空数组([])以及展开运算符(...)来平铺一个数组。
当每个元素还是一个数字时，递归铺平他。
const deepFlatten = arr => [].concat(...arr.map(v => (Array.isArray(v) ? deepFlatten(v) : v)));示例
deepFlatten([1, [2], [[3], 4], 5]); // [1,2,3,4,5]返回两个数组间的差异值。
从数组b中创建一个 Set ，然后用使用另一个数组a的Array.prototype.filter() 方法过滤掉b中的元素。
const difference = (a, b) => {
const s = new Set(b);
return a.filter(x => !s.has(x));
};示例
difference([1, 2, 3], [1, 2, 4]); // [3]将提供的函数应用于两个数组的每个数组元素后，返回两个数组中不同的元素。
通过b中的每个元素调用 fn后创建一个 Set ，然后将Array.prototype.filter() 与fn调用后的a结合使用，只保留先前创建的集合中不包含的值。
const differenceBy = (a, b, fn) => {
const s = new Set(b.map(fn));
return a.filter(x => !s.has(fn(x)));
};示例
differenceBy([2.1, 1.2], [2.3, 3.4], Math.floor); // [1.2]
differenceBy([{ x: 2 }, { x: 1 }], [{ x: 1 }], v => v.x); // [ { x: 2 } ]筛选出比较器函数不返回true的数组中的所有值。
使用 Array.prototype.filter() 和 Array.prototype.findIndex() 查找合适的值。
const differenceWith = (arr, val, comp) => arr.filter(a => val.findIndex(b => comp(a, b)) === -1);示例
differenceWith([1, 1.2, 1.5, 3, 0], [1.9, 3, 0], (a, b) => Math.round(a) === Math.round(b)); // [1, 1.2]返回一个新数组，从原数组左边删除n个元素。
使用 Array.prototype.slice() 从左侧删除指定数量的元素。
const drop = (arr, n = 1) => arr.slice(n);slice(n) 表示取数组下标n以后的元素（含n）
示例
drop([1, 2, 3]); // [2,3]
drop([1, 2, 3], 2); // [3]
drop([1, 2, 3], 42); // []返回一个新数组，从原数组右边删除n个元素。
使用 Array.prototype.slice()从右边删除指定数目的元素。
const dropRight = (arr, n = 1) => arr.slice(0, -n);slice(0, -n) 表示取数组第一个到倒数第n个元素（不含-n）
示例
dropRight([1, 2, 3]); // [1,2]
dropRight([1, 2, 3], 2); // [1]
dropRight([1, 2, 3], 42); // []从数组尾部移除数组中的元素，直到传递的函数返回true。返回数组中剩余的元素。
遍历数组，使用Array.prototype.slice()删除数组的最后一个元素，直到函数的返回值为true。返回剩余的元素。
const dropRightWhile = (arr, func) => {
while (arr.length > 0 && !func(arr[arr.length - 1])) arr = arr.slice(0, -1);
return arr;
};示例
dropRightWhile([1, 2, 3, 4], n => n < 3); // [1, 2]移除数组中的元素，直到传递的函数返回true。返回数组中剩余的元素。
遍历数组，使用Array.prototype.slice()删除数组的第一个元素，直到函数的返回值为true。返回剩余的元素。
const dropWhile = (arr, func) => {
while (arr.length > 0 && !func(arr[0])) arr = arr.slice(1);
return arr;
};示例
dropWhile([1, 2, 3, 4], n => n >= 3); // [3,4]返回数组中所有下标是n的倍数的元素。
使用 Array.prototype.filter() 创建包含给定数组中所有下标是n的倍数的元素的新数组。
const everyNth = (arr, nth) => arr.filter((e, i) => i % nth === nth - 1);示例
everyNth([1, 2, 3, 4, 5, 6], 2); // [ 2, 4, 6 ]把数组中的虚值过滤掉。
使用 Array.prototype.filter()创建一个只包含真值的新数组。
const filterFalsy = arr => arr.filter(Boolean);
falsy(虚值)是在Boolean上下文中已认定可转换为‘假‘的值。例如：false，0，""，null，undefined 和 NaN 。
Truthy(真值)指的是在 布尔值 上下文中转换后的值为真的值。所有值都是真值，除非它们被定义为falsy。
示例
filterFalsy(['', true, {}, false, 'sample', 1, 0]); // [true, {}, 'sample', 1]过滤掉数组中重复的值。
使用 Array.prototype.filter()创建一个只包含谓唯一值的数组。
const filterNonUnique = arr => arr.filter(i => arr.indexOf(i) === arr.lastIndexOf(i));示例
filterNonUnique([1, 2, 2, 3, 4, 4, 5]); // [1, 3, 5]基于给定的比较器函数，过滤掉数组中重复的元素。
使用Array.prototype.filter() 和 Array.prototype.every() 创建一个新数组，该数组只包含唯一值，基于给定的比较器函数 fn。
比较器函数接收四个参数：正在被比较的两个元素和他们的索引。
const filterNonUniqueBy = (arr, fn) =>
arr.filter((v, i) => arr.every((x, j) => (i === j) === fn(v, x, i, j)));示例
filterNonUniqueBy(
[
{ id: 0, value: 'a' },
{ id: 1, value: 'b' },
{ id: 2, value: 'c' },
{ id: 1, value: 'd' },
{ id: 0, value: 'e' }
],
(a, b) => a.id == b.id
); // [ { id: 2, value: 'c' } ]返回所提供函数返回真值的最后一个元素。
使用 Array.prototype.filter() 将调用fn后返回虚值的元素过滤掉, 然后调用Array.prototype.pop() 来获取最后一个元素。
const findLast = (arr, fn) => arr.filter(fn).pop();示例
findLast([1, 2, 3, 4], n => n % 2 === 1); // 3返回所提供函数返回真值的最后一个元素的索引。
使用 Array.prototype.map() 将每个元素映射到具有其索引和值的数组。
使用 Array.prototype.filter() 将调用fn后返回虚值的元素过滤掉, 然后调用Array.prototype.pop() 来获取最后一个元素的索引。
const findLastIndex = (arr, fn) =>
arr
.map((val, i) => [i, val])
.filter(([i, val]) => fn(val, i, arr))
.pop()[0];示例
findLastIndex([1, 2, 3, 4], n => n % 2 === 1); // 2 (index of the value 3)根据指定的深度展平一个数组。
使用递归，每层递归 depth 递减1。
使用 Array.prototype.reduce() 和 Array.prototype.concat() 来合并数组或者元素。
基本情况下，当depth 等于1时停止递归。
忽略第二个参数的情况下， depth 默认为1（单层展开）。
const flatten = (arr, depth = 1) =>
arr.reduce((a, v) => a.concat(depth > 1 && Array.isArray(v) ? flatten(v, depth - 1) : v), []);示例
flatten([1, [2], 3, 4]); // [1, 2, 3, 4]
flatten([1, [2, [3, [4, 5], 6], 7], 8], 2); // [1, 2, 3, [4, 5], 6, 7, 8]对数组中的每个元素执行一次所提供的函数，从数组的最后一个元素开始。
使用 Array.prototype.slice(0) 克隆给定的数组，并使用Array.prototype.reverse() 将它反转，然后使用Array.prototype.forEach()遍历反转后的数组。
const forEachRight = (arr, callback) =>
arr
.slice(0)
.reverse()
.forEach(callback);示例
forEachRight([1, 2, 3, 4], val => console.log(val)); // '4', '3', '2', '1'基于给定的函数将数组分组。
使用 Array.prototype.map() 将组数中的值映射到一个函数或者属性名。
使用 Array.prototype.reduce() 创建一个对象，其中的键由映射的结果生成。
const groupBy = (arr, fn) =>
arr.map(typeof fn === 'function' ? fn : val => val[fn]).reduce((acc, val, i) => {
acc[val] = (acc[val] || []).concat(arr[i]);
return acc;
}, {});示例
groupBy([6.1, 4.2, 6.3], Math.floor); // {4: [4.2], 6: [6.1, 6.3]}
groupBy(['one', 'two', 'three'], 'length'); // {3: ['one', 'two'], 5: ['three']}返回列表的头部
使用 arr[0] 返回传递数组的第一个元素。
const head = arr => arr[0];示例
head([1, 2, 3]); // 1返回一个数组中所有 val 的索引。
如果 val 不存在，返回 [] 。
使用 Array.prototype.reduce() 遍历元素，将匹配的元素索引存储下来，返回索引数组。
const indexOfAll = (arr, val) => arr.reduce((acc, el, i) => (el === val ? [...acc, i] : acc), []);示例
indexOfAll([1, 2, 3, 1, 2, 3], 1); // [0,3]
indexOfAll([1, 2, 3], 4); // []返回数组中除最后一个元素外的所有元素。
使用 arr.slice(0,-1) 返回数组中除最后一个元素外的所有元素。
const initial = arr => arr.slice(0, -1);示例
initial([1, 2, 3]); // [1,2]根据给定的宽、高和值初始化一个二维数组。
使用 Array.prototype.map() 生成h行，其中每一行都是大小为w的新数组，并用值初始化。如果没有提供该值，则默认为null。
const initialize2DArray = (w, h, val = null) =>
Array.from({ length: h }).map(() => Array.from({ length: w }).fill(val));示例
initialize2DArray(2, 2, 0); // [[0,0], [0,0]]初始化一个数组，该数组包括从 start 到 end 指定范围的数字，并且包括共同的公差 step 。
使用 Array.from() 创建一个所需长度 (end - start + 1)/step 的数组，然后指定一个匹配函数将指定范围内的所需值填充到数组中。
你可以省略 start 使用默认值0;
你可以省略 step 使用默认值1;
const initializeArrayWithRange = (end, start = 0, step = 1) =>
Array.from({ length: Math.ceil((end - start + 1) / step) }, (v, i) => i * step + start);示例
initializeArrayWithRange(5); // [0,1,2,3,4,5]
initializeArrayWithRange(7, 3); // [3,4,5,6,7]
initializeArrayWithRange(9, 0, 2); // [0,2,4,6,8]初始化一个数组，该数组包括从 start 到 end 指定范围的数字（反向的），并且包括共同的公差 step 。
使用 Array.from(Math.ceil((end+1-start)/step)) 创建一个期望长度的数组（为了兼容结束，元素的数量等同于(end-start)/step 或 (end+1-start)/step），使用Array.prototype.map()来填充期望范围内的值。
你可以省略 start 使用默认值0。
你可以省略 step 使用默认值1。
const initializeArrayWithRangeRight = (end, start = 0, step = 1) =>
Array.from({ length: Math.ceil((end + 1 - start) / step) }).map(
(v, i, arr) => (arr.length - i - 1) * step + start
);示例
initializeArrayWithRangeRight(5); // [5,4,3,2,1,0]
initializeArrayWithRangeRight(7, 3); // [7,6,5,4,3]
initializeArrayWithRangeRight(9, 0, 2); // [8,6,4,2,0]初始化一个数组，并且使用指定的值填充它。
使用 Array(n) 创建一个期望长度的数组，使用 fill(v) 用期望的值填充数组。
你可以省略参数 val 使用默认值0。
const initializeArrayWithValues = (n, val = 0) => Array(n).fill(val);示例
initializeArrayWithValues(5, 2); // [2, 2, 2, 2, 2]使用给定的值创建一个n维数组。
使用递归。使用 Array.prototype.map() 来生成行，这些行每一个都是使用initializeNDArray初始化的新数组。
const initializeNDArray = (val, ...args) =>
args.length === 0
? val
: Array.from({ length: args[0] }).map(() => initializeNDArray(val, ...args.slice(1)));示例
initializeNDArray(1, 3); // [1,1,1]
initializeNDArray(5, 2, 2, 2); // [[[5,5],[5,5]],[[5,5],[5,5]]]返回两个数组中都存在的元素列表。
从 b创建一个 Set ，然后在a上使用Array.prototype.filter()来只保留 b中包含的元素。
const intersection = (a, b) => {
const s = new Set(b);
return a.filter(x => s.has(x));
};示例
intersection([1, 2, 3], [4, 3, 2]); // [2, 3]将提供的函数应应用到两个数组的每个元素上，然后返回两个数组中都存在的元素列表
通过将 fn 应用到 b的所有元素来创建一个 Set，然后在 a 上调用 Array.prototype.filter() 来只保留调用 fn 后 b 中包含的元素。
const intersectionBy = (a, b, fn) => {
const s = new Set(b.map(fn));
return a.filter(x => s.has(fn(x)));
};示例
intersectionBy([2.1, 1.2], [2.3, 3.4], Math.floor); // [2.1]返回两个数组中都存在的元素列表，使用给定的比较函数。
结合使用Array.prototype.filter()和 Array.prototype.findIndex() 来确定交叉值。
const intersectionWith = (a, b, comp) => a.filter(x => b.findIndex(y => comp(x, y)) !== -1);示例
intersectionWith([1, 1.2, 1.5, 3, 0], [1.9, 3, 0, 3.9], (a, b) => Math.round(a) === Math.round(b)); // [1.5, 3, 0]如果数组是升序排序的，返回 1 ，如果数组是降序排序的返回 -1，如果数组没有排序返回0 。
计算前两个元素的顺序 direction。使用 Object.entries() 来遍历数组，病成对比较。如果 direction 改变，返回 0 ，如果到达最后一个元素，返回 direction 。
const isSorted = arr => {
let direction = -(arr[0] - arr[1]);
for (let [i, val] of arr.entries()) {
direction = !direction ? -(arr[i - 1] - arr[i]) : direction;
if (i === arr.length - 1) return !direction ? 0 : direction;
else if ((val - arr[i + 1]) * direction > 0) return 0;
}
};示例
isSorted([0, 1, 2, 2]); // 1
isSorted([4, 3, 2]); // -1
isSorted([4, 3, 5]); // 0将数组中所有的元素连接成一个字符串，并返回这个字符串。使用一个分隔符和结束分隔符。
使用 Array.prototype.reduce() 将元素合并成字符串。
忽略第二个参数，separator，默认情况下使用一个默认分隔符','。
忽略第三个参数， end，使用和separator相同的值作为默认值。
const join = (arr, separator = ',', end = separator) =>
arr.reduce(
(acc, val, i) =>
i === arr.length - 2
? acc + val + end
: i === arr.length - 1
? acc + val
: acc + val + separator,
''
);示例
join(['pen', 'pineapple', 'apple', 'pen'], ',', '&'); // "pen,pineapple,apple&pen"
join(['pen', 'pineapple', 'apple', 'pen'], ','); // "pen,pineapple,apple,pen"
join(['pen', 'pineapple', 'apple', 'pen']); // "pen,pineapple,apple,pen"将对象数组转换为仅包含指定的columns的逗号分隔值(CSV)字符串。
使用 Array.prototype.join(delimiter) 合并columns中的所有名称以创建第一行。、
使用 Array.prototype.map() 和 Array.prototype.reduce() 为每个对象创建一行，用空字符串替换不存在的值，只映射“列”中的值。
使用Array.prototype.join('\n')将所有行组合成一个字符串。
忽略第三个参数 delimiter,使用默认分隔符 ,。
const JSONtoCSV = (arr, columns, delimiter = ',') =>
[
columns.join(delimiter),
...arr.map(obj =>
columns.reduce(
(acc, key) => `${acc}${!acc.length ? '' : delimiter}"${!obj[key] ? '' : obj[key]}"`,
''
)
)
].join('\n');示例
JSONtoCSV([{ a: 1, b: 2 }, { a: 3, b: 4, c: 5 }, { a: 6 }, { b: 7 }], ['a', 'b']); // 'a,b\n"1","2"\n"3","4"\n"6",""\n"","7"'
JSONtoCSV([{ a: 1, b: 2 }, { a: 3, b: 4, c: 5 }, { a: 6 }, { b: 7 }], ['a', 'b'], ';'); // 'a;b\n"1";"2"\n"3";"4"\n"6";""\n"";"7"'返回数组中最后一个元素。
使用 arr.length - 1 来计算给定数组最后一个元素的索引，然后返回它。
const last = arr => arr[arr.length - 1];示例
last([1, 2, 3]); // 3获取任意数量的可迭代对象或具有 length 属性的对象，并返回其中最长的一个。
如果多个对象具有相同的长度，则返回第一个对象。 如果没有提供参数，则返回“undefined”。
使用Array.prototype.reduce()，比较对象的length以找到最长的对象。
const longestItem = (...vals) => vals.reduce((a, x) => (x.length > a.length ? x : a));示例
longestItem('this', 'is', 'a', 'testcase'); // 'testcase'
longestItem(...['a', 'ab', 'abc']); // 'abc'
longestItem(...['a', 'ab', 'abc'], 'abcd'); // 'abcd'
longestItem([1, 2, 3], [1, 2], [1, 2, 3, 4, 5]); // [1, 2, 3, 4, 5]
longestItem([1, 2, 3], 'foobar'); // 'foobar'使用函数将数组的值映射到对象，其中键值对由作为键的原始值和映射的值组成。
使用匿名内部函数作用域声明未定义的内存空间，使用闭包存储返回值。使用一个新的 Array 来存储数组，其中包含函数在其数据集上的映射，并使用逗号操作符返回第二个步骤，而不需要从一个上下文移动到另一个上下文(由于闭包和操作顺序)。
const mapObject = (arr, fn) =>
(a => (
(a = [arr, arr.map(fn)]), a[0].reduce((acc, val, ind) => ((acc[val] = a[1][ind]), acc), {})
))();示例
const squareIt = arr => mapObject(arr, a => a * a);
squareIt([1, 2, 3]); // { 1: 1, 2: 4, 3: 9 }返回给定数组中前 n 大的元素。
如果 n 比给定的数组长度还要大，返回原始数组（按降序排列）。
使用 Array.prototype.sort() 结合展开操作符 (...) 创建一个数组的浅克隆，并按照降序排序。
使用 Array.prototype.slice() 获取指定数量的元素。
忽略第二个参数, n, 返回一个单元素数组。
const maxN = (arr, n = 1) => [...arr].sort((a, b) => b - a).slice(0, n);示例
maxN([1, 2, 3]); // [3]
maxN([1, 2, 3], 2); // [3,2]返回给定数组中前 n 小的元素。
如果 n 比给定的数组长度还要大，返回原始数组（按升序排列）。
使用 Array.prototype.sort() 结合展开操作符 (...) 创建一个数组的浅克隆，并按照升序排序。
使用 Array.prototype.slice() 获取指定数量的元素。
忽略第二个参数, n, 返回一个单元素数组。
const minN = (arr, n = 1) => [...arr].sort((a, b) => a - b).slice(0, n);示例
minN([1, 2, 3]); // [1]
minN([1, 2, 3], 2); // [1,2]Returns true if the provided predicate function returns false for all elements in a collection, false otherwise.
Use Array.prototype.some() to test if any elements in the collection return true based on fn.
Omit the second argument, fn, to use Boolean as a default.
const none = (arr, fn = Boolean) => !arr.some(fn);示例
none([0, 1, 3, 0], x => x == 2); // true
none([0, 0, 0]); // trueReturns the nth element of an array.
Use Array.prototype.slice() to get an array containing the nth element at the first place.
If the index is out of bounds, return undefined.
Omit the second argument, n, to get the first element of the array.
const nthElement = (arr, n = 0) => (n === -1 ? arr.slice(n) : arr.slice(n, n + 1))[0];示例
nthElement(['a', 'b', 'c'], 1); // 'b'
nthElement(['a', 'b', 'b'], -3); // 'a'Moves the specified amount of elements to the end of the array.
Use Array.prototype.slice() twice to get the elements after the specified index and the elements before that.
Use the spread operator(...) to combine the two into one array.
If offset is negative, the elements will be moved from end to start.
const offset = (arr, offset) => [...arr.slice(offset), ...arr.slice(0, offset)];示例
offset([1, 2, 3, 4, 5], 2); // [3, 4, 5, 1, 2]
offset([1, 2, 3, 4, 5], -2); // [4, 5, 1, 2, 3]Groups the elements into two arrays, depending on the provided function's truthiness for each element.
Use Array.prototype.reduce() to create an array of two arrays.
Use Array.prototype.push() to add elements for which fn returns true to the first array and elements for which fn returns false to the second one.
const partition = (arr, fn) =>
arr.reduce(
(acc, val, i, arr) => {
acc[fn(val, i, arr) ? 0 : 1].push(val);
return acc;
},
[[], []]
);示例
const users = [{ user: 'barney', age: 36, active: false }, { user: 'fred', age: 40, active: true }];
partition(users, o => o.active); // [[{ 'user': 'fred', 'age': 40, 'active': true }],[{ 'user': 'barney', 'age': 36, 'active': false }]]
Generates all permutations of an array's elements (contains duplicates).
Use recursion.
For each element in the given array, create all the partial permutations for the rest of its elements.
Use Array.prototype.map() to combine the element with each partial permutation, then Array.prototype.reduce() to combine all permutations in one array.
Base cases are for array length equal to 2 or 1.
const permutations = arr => {
if (arr.length <= 2) return arr.length === 2 ? [arr, [arr[1], arr[0]]] : arr;
return arr.reduce(
(acc, item, i) =>
acc.concat(
permutations([...arr.slice(0, i), ...arr.slice(i + 1)]).map(val => [item, ...val])
),
[]
);
};示例
permutations([1, 33, 5]); // [ [ 1, 33, 5 ], [ 1, 5, 33 ], [ 33, 1, 5 ], [ 33, 5, 1 ], [ 5, 1, 33 ], [ 5, 33, 1 ] ]Mutates the original array to filter out the values specified.
Use Array.prototype.filter() and Array.prototype.includes() to pull out the values that are not needed.
Use Array.prototype.length = 0 to mutate the passed in an array by resetting it's length to zero and Array.prototype.push() to re-populate it with only the pulled values.
(For a snippet that does not mutate the original array see without)
const pull = (arr, ...args) => {
let argState = Array.isArray(args[0]) ? args[0] : args;
let pulled = arr.filter((v, i) => !argState.includes(v));
arr.length = 0;
pulled.forEach(v => arr.push(v));
};示例
let myArray = ['a', 'b', 'c', 'a', 'b', 'c'];
pull(myArray, 'a', 'c'); // myArray = [ 'b', 'b' ]Mutates the original array to filter out the values at the specified indexes.
Use Array.prototype.filter() and Array.prototype.includes() to pull out the values that are not needed.
Use Array.prototype.length = 0 to mutate the passed in an array by resetting it's length to zero and Array.prototype.push() to re-populate it with only the pulled values.
Use Array.prototype.push() to keep track of pulled values
const pullAtIndex = (arr, pullArr) => {
let removed = [];
let pulled = arr
.map((v, i) => (pullArr.includes(i) ? removed.push(v) : v))
.filter((v, i) => !pullArr.includes(i));
arr.length = 0;
pulled.forEach(v => arr.push(v));
return removed;
};示例
let myArray = ['a', 'b', 'c', 'd'];
let pulled = pullAtIndex(myArray, [1, 3]); // myArray = [ 'a', 'c' ] , pulled = [ 'b', 'd' ]Mutates the original array to filter out the values specified. Returns the removed elements.
Use Array.prototype.filter() and Array.prototype.includes() to pull out the values that are not needed.
Use Array.prototype.length = 0 to mutate the passed in an array by resetting it's length to zero and Array.prototype.push() to re-populate it with only the pulled values.
Use Array.prototype.push() to keep track of pulled values
const pullAtValue = (arr, pullArr) => {
let removed = [],
pushToRemove = arr.forEach((v, i) => (pullArr.includes(v) ? removed.push(v) : v)),
mutateTo = arr.filter((v, i) => !pullArr.includes(v));
arr.length = 0;
mutateTo.forEach(v => arr.push(v));
return removed;
};示例
let myArray = ['a', 'b', 'c', 'd'];
let pulled = pullAtValue(myArray, ['b', 'd']); // myArray = [ 'a', 'c' ] , pulled = [ 'b', 'd' ]Mutates the original array to filter out the values specified, based on a given iterator function.
Check if the last argument provided in a function.
Use Array.prototype.map() to apply the iterator function fn to all array elements.
Use Array.prototype.filter() and Array.prototype.includes() to pull out the values that are not needed.
Use Array.prototype.length = 0 to mutate the passed in an array by resetting it's length to zero and Array.prototype.push() to re-populate it with only the pulled values.
const pullBy = (arr, ...args) => {
const length = args.length;
let fn = length > 1 ? args[length - 1] : undefined;
fn = typeof fn == 'function' ? (args.pop(), fn) : undefined;
let argState = (Array.isArray(args[0]) ? args[0] : args).map(val => fn(val));
let pulled = arr.filter((v, i) => !argState.includes(fn(v)));
arr.length = 0;
pulled.forEach(v => arr.push(v));
};示例
var myArray = [{ x: 1 }, { x: 2 }, { x: 3 }, { x: 1 }];
pullBy(myArray, [{ x: 1 }, { x: 3 }], o => o.x); // myArray = [{ x: 2 }]Filter an array of objects based on a condition while also filtering out unspecified keys.
Use Array.prototype.filter() to filter the array based on the predicate fn so that it returns the objects for which the condition returned a truthy value.
On the filtered array, use Array.prototype.map() to return the new object using Array.prototype.reduce() to filter out the keys which were not supplied as the keys argument.
const reducedFilter = (data, keys, fn) =>
data.filter(fn).map(el =>
keys.reduce((acc, key) => {
acc[key] = el[key];
return acc;
}, {})
);示例
const data = [
{
id: 1,
name: 'john',
age: 24
},
{
id: 2,
name: 'mike',
age: 50
}
];
reducedFilter(data, ['id', 'name'], item => item.age > 24); // [{ id: 2, name: 'mike'}]Applies a function against an accumulator and each element in the array (from left to right), returning an array of successively reduced values.
Use Array.prototype.reduce() to apply the given function to the given array, storing each new result.
const reduceSuccessive = (arr, fn, acc) =>
arr.reduce((res, val, i, arr) => (res.push(fn(res.slice(-1)[0], val, i, arr)), res), [acc]);示例
reduceSuccessive([1, 2, 3, 4, 5, 6], (acc, val) => acc + val, 0); // [0, 1, 3, 6, 10, 15, 21]Returns the minimum/maximum value of an array, after applying the provided function to set comparing rule.
Use Array.prototype.reduce() in combination with the comparator function to get the appropriate element in the array.
You can omit the second parameter, comparator, to use the default one that returns the minimum element in the array.
const reduceWhich = (arr, comparator = (a, b) => a - b) =>
arr.reduce((a, b) => (comparator(a, b) >= 0 ? b : a));示例
reduceWhich([1, 3, 2]); // 1
reduceWhich([1, 3, 2], (a, b) => b - a); // 3
reduceWhich(
[{ name: 'Tom', age: 12 }, { name: 'Jack', age: 18 }, { name: 'Lucy', age: 9 }],
(a, b) => a.age - b.age
); // {name: "Lucy", age: 9}Takes a predicate and array, like Array.prototype.filter(), but only keeps x if pred(x) === false.
const reject = (pred, array) => array.filter((...args) => !pred(...args));示例
reject(x => x % 2 === 0, [1, 2, 3, 4, 5]); // [1, 3, 5]
reject(word => word.length > 4, ['Apple', 'Pear', 'Kiwi', 'Banana']); // ['Pear', 'Kiwi']Removes elements from an array for which the given function returns false.
Use Array.prototype.filter() to find array elements that return truthy values and Array.prototype.reduce() to remove elements using Array.prototype.splice().
The func is invoked with three arguments (value, index, array).
const remove = (arr, func) =>
Array.isArray(arr)
? arr.filter(func).reduce((acc, val) => {
arr.splice(arr.indexOf(val), 1);
return acc.concat(val);
}, [])
: [];示例
remove([1, 2, 3, 4], n => n % 2 === 0); // [2, 4]Returns a random element from an array.
Use Math.random() to generate a random number, multiply it by length and round it off to the nearest whole number using Math.floor().
This method also works with strings.
const sample = arr => arr[Math.floor(Math.random() * arr.length)];示例
sample([3, 7, 9, 11]); // 9Gets n random elements at unique keys from array up to the size of array.
Shuffle the array using the Fisher-Yates algorithm.
Use Array.prototype.slice() to get the first n elements.
Omit the second argument, n to get only one element at random from the array.
const sampleSize = ([...arr], n = 1) => {
let m = arr.length;
while (m) {
const i = Math.floor(Math.random() * m--);
[arr[m], arr[i]] = [arr[i], arr[m]];
}
return arr.slice(0, n);
};示例
sampleSize([1, 2, 3], 2); // [3,1]
sampleSize([1, 2, 3], 4); // [2,3,1]Has the same functionality as Array.prototype.splice(), but returning a new array instead of mutating the original array.
Use Array.prototype.slice() and Array.prototype.concat() to get a new array with the new contents after removing existing elements and/or adding new elements.
Omit the second argument, index, to start at 0.
Omit the third argument, delCount, to remove 0 elements.
Omit the fourth argument, elements, in order to not add any new elements.
const shank = (arr, index = 0, delCount = 0, ...elements) =>
arr
.slice(0, index)
.concat(elements)
.concat(arr.slice(index + delCount));示例
const names = ['alpha', 'bravo', 'charlie'];
const namesAndDelta = shank(names, 1, 0, 'delta'); // [ 'alpha', 'delta', 'bravo', 'charlie' ]
const namesNoBravo = shank(names, 1, 1); // [ 'alpha', 'charlie' ]
console.log(names); // ['alpha', 'bravo', 'charlie']Randomizes the order of the values of an array, returning a new array.
Uses the Fisher-Yates algorithm to reorder the elements of the array.
const shuffle = ([...arr]) => {
let m = arr.length;
while (m) {
const i = Math.floor(Math.random() * m--);
[arr[m], arr[i]] = [arr[i], arr[m]];
}
return arr;
};示例
const foo = [1, 2, 3];
shuffle(foo); // [2, 3, 1], foo = [1, 2, 3]Returns an array of elements that appear in both arrays.
Use Array.prototype.filter() to remove values that are not part of values, determined using Array.prototype.includes().
const similarity = (arr, values) => arr.filter(v => values.includes(v));示例
similarity([1, 2, 3], [1, 2, 4]); // [1, 2]Returns the lowest index at which value should be inserted into array in order to maintain its sort order.
Check if the array is sorted in descending order (loosely).
Use Array.prototype.findIndex() to find the appropriate index where the element should be inserted.
const sortedIndex = (arr, n) => {
const isDescending = arr[0] > arr[arr.length - 1];
const index = arr.findIndex(el => (isDescending ? n >= el : n <= el));
return index === -1 ? arr.length : index;
};示例
sortedIndex([5, 3, 2, 1], 4); // 1
sortedIndex([30, 50], 40); // 1Returns the lowest index at which value should be inserted into array in order to maintain its sort order, based on a provided iterator function.
Check if the array is sorted in descending order (loosely).
Use Array.prototype.findIndex() to find the appropriate index where the element should be inserted, based on the iterator function fn.
const sortedIndexBy = (arr, n, fn) => {
const isDescending = fn(arr[0]) > fn(arr[arr.length - 1]);
const val = fn(n);
const index = arr.findIndex(el => (isDescending ? val >= fn(el) : val <= fn(el)));
return index === -1 ? arr.length : index;
};示例
sortedIndexBy([{ x: 4 }, { x: 5 }], { x: 4 }, o => o.x); // 0Returns the highest index at which value should be inserted into array in order to maintain its sort order.
Check if the array is sorted in descending order (loosely).
Use Array.prototype.reverse() and Array.prototype.findIndex() to find the appropriate last index where the element should be inserted.
const sortedLastIndex = (arr, n) => {
const isDescending = arr[0] > arr[arr.length - 1];
const index = arr.reverse().findIndex(el => (isDescending ? n <= el : n >= el));
return index === -1 ? 0 : arr.length - index;
};示例
sortedLastIndex([10, 20, 30, 30, 40], 30); // 4Returns the highest index at which value should be inserted into array in order to maintain its sort order, based on a provided iterator function.
Check if the array is sorted in descending order (loosely).
Use Array.prototype.map() to apply the iterator function to all elements of the array.
Use Array.prototype.reverse() and Array.prototype.findIndex() to find the appropriate last index where the element should be inserted, based on the provided iterator function.
const sortedLastIndexBy = (arr, n, fn) => {
const isDescending = fn(arr[0]) > fn(arr[arr.length - 1]);
const val = fn(n);
const index = arr
.map(fn)
.reverse()
.findIndex(el => (isDescending ? val <= el : val >= el));
return index === -1 ? 0 : arr.length - index;
};示例
sortedLastIndexBy([{ x: 4 }, { x: 5 }], { x: 4 }, o => o.x); // 1Performs stable sorting of an array, preserving the initial indexes of items when their values are the same. Does not mutate the original array, but returns a new array instead.
Use Array.prototype.map() to pair each element of the input array with its corresponding index.
Use Array.prototype.sort() and a compare function to sort the list, preserving their initial order if the items compared are equal.
Use Array.prototype.map() to convert back to the initial array items.
const stableSort = (arr, compare) =>
arr
.map((item, index) => ({ item, index }))
.sort((a, b) => compare(a.item, b.item) || a.index - b.index)
.map(({ item }) => item);示例
const arr = [0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10];
const stable = stableSort(arr, () => 0); // [0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10]Returns the symmetric difference between two arrays, without filtering out duplicate values.
Create a Set from each array, then use Array.prototype.filter() on each of them to only keep values not contained in the other.
const symmetricDifference = (a, b) => {
const sA = new Set(a),
sB = new Set(b);
return [...a.filter(x => !sB.has(x)), ...b.filter(x => !sA.has(x))];
};示例
symmetricDifference([1, 2, 3], [1, 2, 4]); // [3, 4]
symmetricDifference([1, 2, 2], [1, 3, 1]); // [2, 2, 3]Returns the symmetric difference between two arrays, after applying the provided function to each array element of both.
Create a Set by applying fn to each array's elements, then use Array.prototype.filter() on each of them to only keep values not contained in the other.
const symmetricDifferenceBy = (a, b, fn) => {
const sA = new Set(a.map(v => fn(v))),
sB = new Set(b.map(v => fn(v)));
return [...a.filter(x => !sB.has(fn(x))), ...b.filter(x => !sA.has(fn(x)))];
};示例
symmetricDifferenceBy([2.1, 1.2], [2.3, 3.4], Math.floor); // [ 1.2, 3.4 ]Returns the symmetric difference between two arrays, using a provided function as a comparator.
Use Array.prototype.filter() and Array.prototype.findIndex() to find the appropriate values.
const symmetricDifferenceWith = (arr, val, comp) => [
...arr.filter(a => val.findIndex(b => comp(a, b)) === -1),
...val.filter(a => arr.findIndex(b => comp(a, b)) === -1)
];示例
symmetricDifferenceWith(
[1, 1.2, 1.5, 3, 0],
[1.9, 3, 0, 3.9],
(a, b) => Math.round(a) === Math.round(b)
); // [1, 1.2, 3.9]Returns all elements in an array except for the first one.
Return Array.prototype.slice(1) if the array's length is more than 1, otherwise, return the whole array.
const tail = arr => (arr.length > 1 ? arr.slice(1) : arr);示例
tail([1, 2, 3]); // [2,3]
tail([1]); // [1]Returns an array with n elements removed from the beginning.
Use Array.prototype.slice() to create a slice of the array with n elements taken from the beginning.
const take = (arr, n = 1) => arr.slice(0, n);示例
take([1, 2, 3], 5); // [1, 2, 3]
take([1, 2, 3], 0); // []Returns an array with n elements removed from the end.
Use Array.prototype.slice() to create a slice of the array with n elements taken from the end.
const takeRight = (arr, n = 1) => arr.slice(arr.length - n, arr.length);示例
takeRight([1, 2, 3], 2); // [ 2, 3 ]
takeRight([1, 2, 3]); // [3]Removes elements from the end of an array until the passed function returns true. Returns the removed elements.
Loop through the array, using a Array.prototype.reduceRight() and accumulating elements while the function returns falsy value.
const takeRightWhile = (arr, func) =>
arr.reduceRight((acc, el) => (func(el) ? acc : [el, ...acc]), []);示例
takeRightWhile([1, 2, 3, 4], n => n < 3); // [3, 4]Removes elements in an array until the passed function returns true. Returns the removed elements.
Loop through the array, using a for...of loop over Array.prototype.entries() until the returned value from the function is true.
Return the removed elements, using Array.prototype.slice().
const takeWhile = (arr, func) => {
for (const [i, val] of arr.entries()) if (func(val)) return arr.slice(0, i);
return arr;
};示例
takeWhile([1, 2, 3, 4], n => n >= 3); // [1, 2]Reduces a given Array-like into a value hash (keyed data store).
Given an Iterable or Array-like structure, call Array.prototype.reduce.call() on the provided object to step over it and return an Object, keyed by the reference value.
const toHash = (object, key) =>
Array.prototype.reduce.call(
object,
(acc, data, index) => ((acc[!key ? index : data[key]] = data), acc),
{}
);示例
toHash([4, 3, 2, 1]); // { 0: 4, 1: 3, 2: 2, 3: 1 }
toHash([{ a: 'label' }], 'a'); // { label: { a: 'label' } }
// A more in depth example:
let users = [{ id: 1, first: 'Jon' }, { id: 2, first: 'Joe' }, { id: 3, first: 'Moe' }];
let managers = [{ manager: 1, employees: [2, 3] }];
// We use function here because we want a bindable reference, but a closure referencing the hash would work, too.
managers.forEach(
manager =>
(manager.employees = manager.employees.map(function(id) {
return this[id];
}, toHash(users, 'id')))
);
managers; // [ { manager:1, employees: [ { id: 2, first: "Joe" }, { id: 3, first: "Moe" } ] } ]Returns every element that exists in any of the two arrays once.
Create a Set with all values of a and b and convert to an array.
const union = (a, b) => Array.from(new Set([...a, ...b]));示例
union([1, 2, 3], [4, 3, 2]); // [1,2,3,4]Returns every element that exists in any of the two arrays once, after applying the provided function to each array element of both.
Create a Set by applying all fn to all values of a.
Create a Set from a and all elements in b whose value, after applying fn does not match a value in the previously created set.
Return the last set converted to an array.
const unionBy = (a, b, fn) => {
const s = new Set(a.map(fn));
return Array.from(new Set([...a, ...b.filter(x => !s.has(fn(x)))]));
};示例
unionBy([2.1], [1.2, 2.3], Math.floor); // [2.1, 1.2]Returns every element that exists in any of the two arrays once, using a provided comparator function.
Create a Set with all values of a and values in b for which the comparator finds no matches in a, using Array.prototype.findIndex().
const unionWith = (a, b, comp) =>
Array.from(new Set([...a, ...b.filter(x => a.findIndex(y => comp(x, y)) === -1)]));示例
unionWith([1, 1.2, 1.5, 3, 0], [1.9, 3, 0, 3.9], (a, b) => Math.round(a) === Math.round(b)); // [1, 1.2, 1.5, 3, 0, 3.9]Returns all unique values of an array.
Use ES6 Set and the ...rest operator to discard all duplicated values.
const uniqueElements = arr => [...new Set(arr)];示例
uniqueElements([1, 2, 2, 3, 4, 4, 5]); // [1, 2, 3, 4, 5]Returns all unique values of an array, based on a provided comparator function.
Use Array.prototype.reduce() and Array.prototype.some() for an array containing only the first unique occurence of each value, based on the comparator function, fn.
The comparator function takes two arguments: the values of the two elements being compared.
const uniqueElementsBy = (arr, fn) =>
arr.reduce((acc, v) => {
if (!acc.some(x => fn(v, x))) acc.push(v);
return acc;
}, []);示例
uniqueElementsBy(
[
{ id: 0, value: 'a' },
{ id: 1, value: 'b' },
{ id: 2, value: 'c' },
{ id: 1, value: 'd' },
{ id: 0, value: 'e' }
],
(a, b) => a.id == b.id
); // [ { id: 0, value: 'a' }, { id: 1, value: 'b' }, { id: 2, value: 'c' } ]Returns all unique values of an array, based on a provided comparator function.
Use Array.prototype.reduce() and Array.prototype.some() for an array containing only the last unique occurence of each value, based on the comparator function, fn.
The comparator function takes two arguments: the values of the two elements being compared.
const uniqueElementsByRight = (arr, fn) =>
arr.reduceRight((acc, v) => {
if (!acc.some(x => fn(v, x))) acc.push(v);
return acc;
}, []);示例
uniqueElementsByRight(
[
{ id: 0, value: 'a' },
{ id: 1, value: 'b' },
{ id: 2, value: 'c' },
{ id: 1, value: 'd' },
{ id: 0, value: 'e' }
],
(a, b) => a.id == b.id
); // [ { id: 0, value: 'e' }, { id: 1, value: 'd' }, { id: 2, value: 'c' } ]Returns the unique symmetric difference between two arrays, not containing duplicate values from either array.
Use Array.prototype.filter() and Array.prototype.includes() on each array to remove values contained in the other, then create a Set from the results, removing duplicate values.
const uniqueSymmetricDifference = (a, b) => [
...new Set([...a.filter(v => !b.includes(v)), ...b.filter(v => !a.includes(v))])
];示例
uniqueSymmetricDifference([1, 2, 3], [1, 2, 4]); // [3, 4]
uniqueSymmetricDifference([1, 2, 2], [1, 3, 1]); // [2, 3]Creates an array of arrays, ungrouping the elements in an array produced by zip.
Use Math.max.apply() to get the longest subarray in the array, Array.prototype.map() to make each element an array.
Use Array.prototype.reduce() and Array.prototype.forEach() to map grouped values to individual arrays.
const unzip = arr =>
arr.reduce(
(acc, val) => (val.forEach((v, i) => acc[i].push(v)), acc),
Array.from({
length: Math.max(...arr.map(x => x.length))
}).map(x => [])
);示例
unzip([['a', 1, true], ['b', 2, false]]); // [['a', 'b'], [1, 2], [true, false]]
unzip([['a', 1, true], ['b', 2]]); // [['a', 'b'], [1, 2], [true]]Creates an array of elements, ungrouping the elements in an array produced by zip and applying the provided function.
Use Math.max.apply() to get the longest subarray in the array, Array.prototype.map() to make each element an array.
Use Array.prototype.reduce() and Array.prototype.forEach() to map grouped values to individual arrays.
Use Array.prototype.map() and the spread operator (...) to apply fn to each individual group of elements.
const unzipWith = (arr, fn) =>
arr
.reduce(
(acc, val) => (val.forEach((v, i) => acc[i].push(v)), acc),
Array.from({
length: Math.max(...arr.map(x => x.length))
}).map(x => [])
)
.map(val => fn(...val));示例
unzipWith([[1, 10, 100], [2, 20, 200]], (...args) => args.reduce((acc, v) => acc + v, 0)); // [3, 30, 300]Filters out the elements of an array, that have one of the specified values.
Use Array.prototype.filter() to create an array excluding(using !Array.includes()) all given values.
(For a snippet that mutates the original array see pull)
const without = (arr, ...args) => arr.filter(v => !args.includes(v));示例
without([2, 1, 2, 3], 1, 2); // [3]Creates a new array out of the two supplied by creating each possible pair from the arrays.
Use Array.prototype.reduce(), Array.prototype.map() and Array.prototype.concat() to produce every possible pair from the elements of the two arrays and save them in an array.
const xProd = (a, b) => a.reduce((acc, x) => acc.concat(b.map(y => [x, y])), []);示例
xProd([1, 2], ['a', 'b']); // [[1, 'a'], [1, 'b'], [2, 'a'], [2, 'b']]Creates an array of elements, grouped based on the position in the original arrays.
Use Math.max.apply() to get the longest array in the arguments.
Creates an array with that length as return value and use Array.from() with a map-function to create an array of grouped elements.
If lengths of the argument-arrays vary, undefined is used where no value could be found.
const zip = (...arrays) => {
const maxLength = Math.max(...arrays.map(x => x.length));
return Array.from({ length: maxLength }).map((_, i) => {
return Array.from({ length: arrays.length }, (_, k) => arrays[k][i]);
});
};示例
zip(['a', 'b'], [1, 2], [true, false]); // [['a', 1, true], ['b', 2, false]]
zip(['a'], [1, 2], [true, false]); // [['a', 1, true], [undefined, 2, false]]Given an array of valid property identifiers and an array of values, return an object associating the properties to the values.
Since an object can have undefined values but not undefined property pointers, the array of properties is used to decide the structure of the resulting object using Array.prototype.reduce().
const zipObject = (props, values) =>
props.reduce((obj, prop, index) => ((obj[prop] = values[index]), obj), {});示例
zipObject(['a', 'b', 'c'], [1, 2]); // {a: 1, b: 2, c: undefined}
zipObject(['a', 'b'], [1, 2, 3]); // {a: 1, b: 2}Creates an array of elements, grouped based on the position in the original arrays and using function as the last value to specify how grouped values should be combined.
Check if the last argument provided is a function.
Use Math.max() to get the longest array in the arguments.
Creates an array with that length as return value and use Array.from() with a map-function to create an array of grouped elements.
If lengths of the argument-arrays vary, undefined is used where no value could be found.
The function is invoked with the elements of each group (...group).
const zipWith = (...array) => {
const fn = typeof array[array.length - 1] === 'function' ? array.pop() : undefined;
return Array.from(
{ length: Math.max(...array.map(a => a.length)) },
(_, i) => (fn ? fn(...array.map(a => a[i])) : array.map(a => a[i]))
);
};示例
zipWith([1, 2], [10, 20], [100, 200], (a, b, c) => a + b + c); // [111,222]
zipWith(
[1, 2, 3],
[10, 20],
[100, 200],
(a, b, c) => (a != null ? a : 'a') + (b != null ? b : 'b') + (c != null ? c : 'c')
); // [111, 222, '3bc']Converts the given array elements into <li> tags and appends them to the list of the given id.
Use Array.prototype.map(), document.querySelector(), and an anonymous inner closure to create a list of html tags.
const arrayToHtmlList = (arr, listID) =>
(el => (
(el = document.querySelector('#' + listID)),
(el.innerHTML += arr.map(item => `<li>${item}</li>`).join(''))
))();示例
arrayToHtmlList(['item 1', 'item 2'], 'myListID');Returns true if the bottom of the page is visible, false otherwise.
Use scrollY, scrollHeight and clientHeight to determine if the bottom of the page is visible.
const bottomVisible = () =>
document.documentElement.clientHeight + window.scrollY >=
(document.documentElement.scrollHeight || document.documentElement.clientHeight);示例
bottomVisible(); // trueCopy a string to the clipboard.
Only works as a result of user action (i.e. inside a click event listener).
Create a new <textarea> element, fill it with the supplied data and add it to the HTML document.
Use Selection.getRangeAt()to store the selected range (if any).
Use document.execCommand('copy') to copy to the clipboard.
Remove the <textarea> element from the HTML document.
Finally, use Selection().addRange() to recover the original selected range (if any).
const copyToClipboard = str => {
const el = document.createElement('textarea');
el.value = str;
el.setAttribute('readonly', '');
el.style.position = 'absolute';
el.style.left = '-9999px';
document.body.appendChild(el);
const selected =
document.getSelection().rangeCount > 0 ? document.getSelection().getRangeAt(0) : false;
el.select();
document.execCommand('copy');
document.body.removeChild(el);
if (selected) {
document.getSelection().removeAllRanges();
document.getSelection().addRange(selected);
}
};示例
copyToClipboard('Lorem ipsum'); // 'Lorem ipsum' copied to clipboard.Creates a counter with the specified range, step and duration for the specified selector.
Check if step has the proper sign and change it accordingly.
Use setInterval() in combination with Math.abs() and Math.floor() to calculate the time between each new text draw.
Use document.querySelector().innerHTML to update the value of the selected element.
Omit the fourth parameter, step, to use a default step of 1.
Omit the fifth parameter, duration, to use a default duration of 2000ms.
const counter = (selector, start, end, step = 1, duration = 2000) => {
let current = start,
_step = (end - start) * step < 0 ? -step : step,
timer = setInterval(() => {
current += _step;
document.querySelector(selector).innerHTML = current;
if (current >= end) document.querySelector(selector).innerHTML = end;
if (current >= end) clearInterval(timer);
}, Math.abs(Math.floor(duration / (end - start))));
return timer;
};示例
counter('#my-id', 1, 1000, 5, 2000); // Creates a 2-second timer for the element with id="my-id"Creates an element from a string (without appending it to the document). If the given string contains multiple elements, only the first one will be returned.
Use document.createElement() to create a new element.
Set its innerHTML to the string supplied as the argument.
Use ParentNode.firstElementChild to return the element version of the string.
const createElement = str => {
const el = document.createElement('div');
el.innerHTML = str;
return el.firstElementChild;
};示例
const el = createElement(
`<div class="container">
<p>Hello!</p>
</div>`
);
console.log(el.className); // 'container'Creates a pub/sub (publish–subscribe) event hub with emit, on, and off methods.
Use Object.create(null) to create an empty hub object that does not inherit properties from Object.prototype.
For emit, resolve the array of handlers based on the event argument and then run each one with Array.prototype.forEach() by passing in the data as an argument.
For on, create an array for the event if it does not yet exist, then use Array.prototype.push() to add the handler
to the array.
For off, use Array.prototype.findIndex() to find the index of the handler in the event array and remove it using Array.prototype.splice().
const createEventHub = () => ({
hub: Object.create(null),
emit(event, data) {
(this.hub[event] || []).forEach(handler => handler(data));
},
on(event, handler) {
if (!this.hub[event]) this.hub[event] = [];
this.hub[event].push(handler);
},
off(event, handler) {
const i = (this.hub[event] || []).findIndex(h => h === handler);
if (i > -1) this.hub[event].splice(i, 1);
}
});示例
const handler = data => console.log(data);
const hub = createEventHub();
let increment = 0;
// Subscribe: listen for different types of events
hub.on('message', handler);
hub.on('message', () => console.log('Message event fired'));
hub.on('increment', () => increment++);
// Publish: emit events to invoke all handlers subscribed to them, passing the data to them as an argument
hub.emit('message', 'hello world'); // logs 'hello world' and 'Message event fired'
hub.emit('message', { hello: 'world' }); // logs the object and 'Message event fired'
hub.emit('increment'); // `increment` variable is now 1
// Unsubscribe: stop a specific handler from listening to the 'message' event
hub.off('message', handler);Returns the current URL.
Use window.location.href to get current URL.
const currentURL = () => window.location.href;示例
currentURL(); // 'https://google.com'Detects wether the website is being opened in a mobile device or a desktop/laptop.
Use a regular expression to test the navigator.userAgent property to figure out if the device is a mobile device or a desktop/laptop.
const detectDeviceType = () =>
/Android|webOS|iPhone|iPad|iPod|BlackBerry|IEMobile|Opera Mini/i.test(navigator.userAgent)
? 'Mobile'
: 'Desktop';示例
detectDeviceType(); // "Mobile" or "Desktop"Returns true if the parent element contains the child element, false otherwise.
Check that parent is not the same element as child, use parent.contains(child) to check if the parent element contains the child element.
const elementContains = (parent, child) => parent !== child && parent.contains(child);示例
elementContains(document.querySelector('head'), document.querySelector('title')); // true
elementContains(document.querySelector('body'), document.querySelector('body')); // falseReturns true if the element specified is visible in the viewport, false otherwise.
Use Element.getBoundingClientRect() and the window.inner(Width|Height) values
to determine if a given element is visible in the viewport.
Omit the second argument to determine if the element is entirely visible, or specify true to determine if
it is partially visible.
const elementIsVisibleInViewport = (el, partiallyVisible = false) => {
const { top, left, bottom, right } = el.getBoundingClientRect();
const { innerHeight, innerWidth } = window;
return partiallyVisible
? ((top > 0 && top < innerHeight) || (bottom > 0 && bottom < innerHeight)) &&
((left > 0 && left < innerWidth) || (right > 0 && right < innerWidth))
: top >= 0 && left >= 0 && bottom <= innerHeight && right <= innerWidth;
};示例
// e.g. 100x100 viewport and a 10x10px element at position {top: -1, left: 0, bottom: 9, right: 10}
elementIsVisibleInViewport(el); // false - (not fully visible)
elementIsVisibleInViewport(el, true); // true - (partially visible)Fetches all images from within an element and puts them into an array
Use Element.prototype.getElementsByTagName() to fetch all <img> elements inside the provided element, Array.prototype.map() to map every src attribute of their respective <img> element, then create a Set to eliminate duplicates and return the array.
const getImages = (el, includeDuplicates = false) => {
const images = [...el.getElementsByTagName('img')].map(img => img.getAttribute('src'));
return includeDuplicates ? images : [...new Set(images)];
};示例
getImages(document, true); // ['image1.jpg', 'image2.png', 'image1.png', '...']
getImages(document, false); // ['image1.jpg', 'image2.png', '...']Returns the scroll position of the current page.
Use pageXOffset and pageYOffset if they are defined, otherwise scrollLeft and scrollTop.
You can omit el to use a default value of window.
const getScrollPosition = (el = window) => ({
x: el.pageXOffset !== undefined ? el.pageXOffset : el.scrollLeft,
y: el.pageYOffset !== undefined ? el.pageYOffset : el.scrollTop
});示例
getScrollPosition(); // {x: 0, y: 200}Returns the value of a CSS rule for the specified element.
Use Window.getComputedStyle() to get the value of the CSS rule for the specified element.
const getStyle = (el, ruleName) => getComputedStyle(el)[ruleName];示例
getStyle(document.querySelector('p'), 'font-size'); // '16px'Returns true if the element has the specified class, false otherwise.
Use element.classList.contains() to check if the element has the specified class.
const hasClass = (el, className) => el.classList.contains(className);示例
hasClass(document.querySelector('p.special'), 'special'); // trueCreates a hash for a value using the SHA-256 algorithm. Returns a promise.
Use the SubtleCrypto API to create a hash for the given value.
const hashBrowser = val =>
crypto.subtle.digest('SHA-256', new TextEncoder('utf-8').encode(val)).then(h => {
let hexes = [],
view = new DataView(h);
for (let i = 0; i < view.byteLength; i += 4)
hexes.push(('00000000' + view.getUint32(i).toString(16)).slice(-8));
return hexes.join('');
});示例
hashBrowser(JSON.stringify({ a: 'a', b: [1, 2, 3, 4], foo: { c: 'bar' } })).then(console.log); // '04aa106279f5977f59f9067fa9712afc4aedc6f5862a8defc34552d8c7206393'Hides all the elements specified.
Use NodeList.prototype.forEach() to apply display: none to each element specified.
const hide = (...el) => [...el].forEach(e => (e.style.display = 'none'));示例
hide(document.querySelectorAll('img')); // Hides all <img> elements on the pageRedirects the page to HTTPS if its currently in HTTP. Also, pressing the back button doesn't take it back to the HTTP page as its replaced in the history.
Use location.protocol to get the protocol currently being used. If it's not HTTPS, use location.replace() to replace the existing page with the HTTPS version of the page. Use location.href to get the full address, split it with String.prototype.split() and remove the protocol part of the URL.
const httpsRedirect = () => {
if (location.protocol !== 'https:') location.replace('https://' + location.href.split('//')[1]);
};示例
httpsRedirect(); // If you are on http://mydomain.com, you are redirected to https://mydomain.comInserts an HTML string after the end of the specified element.
Use el.insertAdjacentHTML() with a position of 'afterend' to parse htmlString and insert it after the end of el.
const insertAfter = (el, htmlString) => el.insertAdjacentHTML('afterend', htmlString);示例
insertAfter(document.getElementById('myId'), '<p>after</p>'); // <div id="myId">...</div> <p>after</p>Inserts an HTML string before the start of the specified element.
Use el.insertAdjacentHTML() with a position of 'beforebegin' to parse htmlString and insert it before the start of el.
const insertBefore = (el, htmlString) => el.insertAdjacentHTML('beforebegin', htmlString);示例
insertBefore(document.getElementById('myId'), '<p>before</p>'); // <p>before</p> <div id="myId">...</div>Returns true if the browser tab of the page is focused, false otherwise.
Use the Document.hidden property, introduced by the Page Visibility API to check if the browser tab of the page is visible or hidden.
const isBrowserTabFocused = () => !document.hidden;示例
isBrowserTabFocused(); // trueConverts a NodeList to an array.
Use spread operator inside new array to convert a NodeList to an array.
const nodeListToArray = nodeList => [...nodeList];示例
nodeListToArray(document.childNodes); // [ <!DOCTYPE html>, html ]Returns a new MutationObserver and runs the provided callback for each mutation on the specified element.
Use a MutationObserver to observe mutations on the given element.
Use Array.prototype.forEach() to run the callback for each mutation that is observed.
Omit the third argument, options, to use the default options (all true).
const observeMutations = (element, callback, options) => {
const observer = new MutationObserver(mutations => mutations.forEach(m => callback(m)));
observer.observe(
element,
Object.assign(
{
childList: true,
attributes: true,
attributeOldValue: true,
characterData: true,
characterDataOldValue: true,
subtree: true
},
options
)
);
return observer;
};示例
const obs = observeMutations(document, console.log); // Logs all mutations that happen on the page
obs.disconnect(); // Disconnects the observer and stops logging mutations on the pageRemoves an event listener from an element.
Use EventTarget.removeEventListener() to remove an event listener from an element.
Omit the fourth argument opts to use false or specify it based on the options used when the event listener was added.
const off = (el, evt, fn, opts = false) => el.removeEventListener(evt, fn, opts);示例
const fn = () => console.log('!');
document.body.addEventListener('click', fn);
off(document.body, 'click', fn); // no longer logs '!' upon clicking on the pageAdds an event listener to an element with the ability to use event delegation.
Use EventTarget.addEventListener() to add an event listener to an element. If there is a target property supplied to the options object, ensure the event target matches the target specified and then invoke the callback by supplying the correct this context.
Returns a reference to the custom delegator function, in order to be possible to use with off.
Omit opts to default to non-delegation behavior and event bubbling.
const on = (el, evt, fn, opts = {}) => {
const delegatorFn = e => e.target.matches(opts.target) && fn.call(e.target, e);
el.addEventListener(evt, opts.target ? delegatorFn : fn, opts.options || false);
if (opts.target) return delegatorFn;
};示例
const fn = () => console.log('!');
on(document.body, 'click', fn); // logs '!' upon clicking the body
on(document.body, 'click', fn, { target: 'p' }); // logs '!' upon clicking a `p` element child of the body
on(document.body, 'click', fn, { options: true }); // use capturing instead of bubblingRun the callback whenever the user input type changes (mouse or touch). Useful for enabling/disabling code depending on the input device. This process is dynamic and works with hybrid devices (e.g. touchscreen laptops).
Use two event listeners. Assume mouse input initially and bind a touchstart event listener to the document.
On touchstart, add a mousemove event listener to listen for two consecutive mousemove events firing within 20ms, using performance.now().
Run the callback with the input type as an argument in either of these situations.
const onUserInputChange = callback => {
let type = 'mouse',
lastTime = 0;
const mousemoveHandler = () => {
const now = performance.now();
if (now - lastTime < 20)
(type = 'mouse'), callback(type), document.removeEventListener('mousemove', mousemoveHandler);
lastTime = now;
};
document.addEventListener('touchstart', () => {
if (type === 'touch') return;
(type = 'touch'), callback(type), document.addEventListener('mousemove', mousemoveHandler);
});
};示例
onUserInputChange(type => {
console.log('The user is now using', type, 'as an input method.');
});Returns the prefixed version (if necessary) of a CSS property that the browser supports.
Use Array.prototype.findIndex() on an array of vendor prefix strings to test if document.body has one of them defined in its CSSStyleDeclaration object, otherwise return null.
Use String.prototype.charAt() and String.prototype.toUpperCase() to capitalize the property, which will be appended to the vendor prefix string.
const prefix = prop => {
const capitalizedProp = prop.charAt(0).toUpperCase() + prop.slice(1);
const prefixes = ['', 'webkit', 'moz', 'ms', 'o'];
const i = prefixes.findIndex(
prefix => typeof document.body.style[prefix ? prefix + capitalizedProp : prop] !== 'undefined'
);
return i !== -1 ? (i === 0 ? prop : prefixes[i] + capitalizedProp) : null;
};示例
prefix('appearance'); // 'appearance' on a supported browser, otherwise 'webkitAppearance', 'mozAppearance', 'msAppearance' or 'oAppearance'Invokes the provided callback on each animation frame.
Use recursion.
Provided that running is true, continue invoking window.requestAnimationFrame() which invokes the provided callback.
Return an object with two methods start and stop to allow manual control of the recording.
Omit the second argument, autoStart, to implicitly call start when the function is invoked.
const recordAnimationFrames = (callback, autoStart = true) => {
let running = true,
raf;
const stop = () => {
running = false;
cancelAnimationFrame(raf);
};
const start = () => {
running = true;
run();
};
const run = () => {
raf = requestAnimationFrame(() => {
callback();
if (running) run();
});
};
if (autoStart) start();
return { start, stop };
};示例
const cb = () => console.log('Animation frame fired');
const recorder = recordAnimationFrames(cb); // logs 'Animation frame fired' on each animation frame
recorder.stop(); // stops logging
recorder.start(); // starts again
const recorder2 = recordAnimationFrames(cb, false); // `start` needs to be explicitly called to begin recording framesRedirects to a specified URL.
Use window.location.href or window.location.replace() to redirect to url.
Pass a second argument to simulate a link click (true - default) or an HTTP redirect (false).
const redirect = (url, asLink = true) =>
asLink ? (window.location.href = url) : window.location.replace(url);示例
redirect('https://google.com');Runs a function in a separate thread by using a Web Worker, allowing long running functions to not block the UI.
Create a new Worker using a Blob object URL, the contents of which should be the stringified version of the supplied function.
Immediately post the return value of calling the function back.
Return a promise, listening for onmessage and onerror events and resolving the data posted back from the worker, or throwing an error.
const runAsync = fn => {
const worker = new Worker(
URL.createObjectURL(new Blob([`postMessage((${fn})());`]), {
type: 'application/javascript; charset=utf-8'
})
);
return new Promise((res, rej) => {
worker.onmessage = ({ data }) => {
res(data), worker.terminate();
};
worker.onerror = err => {
rej(err), worker.terminate();
};
});
};示例
const longRunningFunction = () => {
let result = 0;
for (let i = 0; i < 1000; i++)
for (let j = 0; j < 700; j++) for (let k = 0; k < 300; k++) result = result + i + j + k;
return result;
};
/*
NOTE: Since the function is running in a different context, closures are not supported.
The function supplied to `runAsync` gets stringified, so everything becomes literal.
All variables and functions must be defined inside.
*/
runAsync(longRunningFunction).then(console.log); // 209685000000
runAsync(() => 10 ** 3).then(console.log); // 1000
let outsideVariable = 50;
runAsync(() => typeof outsideVariable).then(console.log); // 'undefined'Smooth-scrolls to the top of the page.
Get distance from top using document.documentElement.scrollTop or document.body.scrollTop.
Scroll by a fraction of the distance from the top. Use window.requestAnimationFrame() to animate the scrolling.
const scrollToTop = () => {
const c = document.documentElement.scrollTop || document.body.scrollTop;
if (c > 0) {
window.requestAnimationFrame(scrollToTop);
window.scrollTo(0, c - c / 8);
}
};示例
scrollToTop();Sets the value of a CSS rule for the specified element.
Use element.style to set the value of the CSS rule for the specified element to val.
const setStyle = (el, ruleName, val) => (el.style[ruleName] = val);示例
setStyle(document.querySelector('p'), 'font-size', '20px'); // The first <p> element on the page will have a font-size of 20pxShows all the elements specified.
Use the spread operator (...) and Array.prototype.forEach() to clear the display property for each element specified.
const show = (...el) => [...el].forEach(e => (e.style.display = ''));示例
show(...document.querySelectorAll('img')); // Shows all <img> elements on the pageSmoothly scrolls the element on which it's called into the visible area of the browser window.
Use .scrollIntoView method to scroll the element.
Pass { behavior: 'smooth' } to .scrollIntoView so it scrolls smoothly.
const smoothScroll = element =>
document.querySelector(element).scrollIntoView({
behavior: 'smooth'
});示例
smoothScroll('#fooBar'); // scrolls smoothly to the element with the id fooBar
smoothScroll('.fooBar'); // scrolls smoothly to the first element with a class of fooBarToggle a class for an element.
Use element.classList.toggle() to toggle the specified class for the element.
const toggleClass = (el, className) => el.classList.toggle(className);示例
toggleClass(document.querySelector('p.special'), 'special'); // The paragraph will not have the 'special' class anymoreTriggers a specific event on a given element, optionally passing custom data.
Use new CustomEvent() to create an event from the specified eventType and details.
Use el.dispatchEvent() to trigger the newly created event on the given element.
Omit the third argument, detail, if you do not want to pass custom data to the triggered event.
const triggerEvent = (el, eventType, detail) =>
el.dispatchEvent(new CustomEvent(eventType, { detail }));示例
triggerEvent(document.getElementById('myId'), 'click');
triggerEvent(document.getElementById('myId'), 'click', { username: 'bob' });Generates a UUID in a browser.
Use crypto API to generate a UUID, compliant with RFC4122 version 4.
const UUIDGeneratorBrowser = () =>
([1e7] + -1e3 + -4e3 + -8e3 + -1e11).replace(/[018]/g, c =>
(c ^ (crypto.getRandomValues(new Uint8Array(1))[0] & (15 >> (c / 4)))).toString(16)
);示例
UUIDGeneratorBrowser(); // '7982fcfe-5721-4632-bede-6000885be57d'Gets the day of the year from a Date object.
Use new Date() and Date.prototype.getFullYear() to get the first day of the year as a Date object, subtract it from the provided date and divide with the milliseconds in each day to get the result.
Use Math.floor() to appropriately round the resulting day count to an integer.
const dayOfYear = date =>
Math.floor((date - new Date(date.getFullYear(), 0, 0)) / 1000 / 60 / 60 / 24);示例
dayOfYear(new Date()); // 272Returns the human readable format of the given number of milliseconds.
Divide ms with the appropriate values to obtain the appropriate values for day, hour, minute, second and millisecond.
Use Object.entries() with Array.prototype.filter() to keep only non-zero values.
Use Array.prototype.map() to create the string for each value, pluralizing appropriately.
Use String.prototype.join(', ') to combine the values into a string.
const formatDuration = ms => {
if (ms < 0) ms = -ms;
const time = {
day: Math.floor(ms / 86400000),
hour: Math.floor(ms / 3600000) % 24,
minute: Math.floor(ms / 60000) % 60,
second: Math.floor(ms / 1000) % 60,
millisecond: Math.floor(ms) % 1000
};
return Object.entries(time)
.filter(val => val[1] !== 0)
.map(([key, val]) => `${val} ${key}${val !== 1 ? 's' : ''}`)
.join(', ');
};示例
formatDuration(1001); // '1 second, 1 millisecond'
formatDuration(34325055574); // '397 days, 6 hours, 44 minutes, 15 seconds, 574 milliseconds'Returns a string of the form HH:MM:SS from a Date object.
Use Date.prototype.toTimeString() and String.prototype.slice() to get the HH:MM:SS part of a given Date object.
const getColonTimeFromDate = date => date.toTimeString().slice(0, 8);示例
getColonTimeFromDate(new Date()); // "08:38:00"Returns the difference (in days) between two dates.
Calculate the difference (in days) between two Date objects.
const getDaysDiffBetweenDates = (dateInitial, dateFinal) =>
(dateFinal - dateInitial) / (1000 * 3600 * 24);示例
getDaysDiffBetweenDates(new Date('2017-12-13'), new Date('2017-12-22')); // 9Converts an integer to a suffixed string, adding am or pm based on its value.
Use the modulo operator (%) and conditional checks to transform an integer to a stringified 12-hour format with meridiem suffix.
const getMeridiemSuffixOfInteger = num =>
num === 0 || num === 24
? 12 + 'am'
: num === 12
? 12 + 'pm'
: num < 12
? (num % 12) + 'am'
: (num % 12) + 'pm';示例
getMeridiemSuffixOfInteger(0); // "12am"
getMeridiemSuffixOfInteger(11); // "11am"
getMeridiemSuffixOfInteger(13); // "1pm"
getMeridiemSuffixOfInteger(25); // "1pm"Check if a date is after another date.
Use the greater than operator (>) to check if the first date comes after the second one.
const isAfterDate = (dateA, dateB) => dateA > dateB;示例
isAfterDate(new Date(2010, 10, 21), new Date(2010, 10, 20)); // trueCheck if a date is before another date.
Use the less than operator (<) to check if the first date comes before the second one.
const isBeforeDate = (dateA, dateB) => dateA < dateB;示例
isBeforeDate(new Date(2010, 10, 20), new Date(2010, 10, 21)); // trueCheck if a date is the same as another date.
Use Date.prototype.toISOString() and strict equality checking (===) to check if the first date is the same as the second one.
const isSameDate = (dateA, dateB) => dateA.toISOString() === dateB.toISOString();示例
isSameDate(new Date(2010, 10, 20), new Date(2010, 10, 20)); // trueReturns the maximum of the given dates.
Use Math.max.apply() to find the maximum date value, new Date() to convert it to a Date object.
const maxDate = (...dates) => new Date(Math.max.apply(null, ...dates));示例
const array = [
new Date(2017, 4, 13),
new Date(2018, 2, 12),
new Date(2016, 0, 10),
new Date(2016, 0, 9)
];
maxDate(array); // 2018-03-11T22:00:00.000ZReturns the minimum of the given dates.
Use Math.min.apply() to find the minimum date value, new Date() to convert it to a Date object.
const minDate = (...dates) => new Date(Math.min.apply(null, ...dates));示例
const array = [
new Date(2017, 4, 13),
new Date(2018, 2, 12),
new Date(2016, 0, 10),
new Date(2016, 0, 9)
];
minDate(array); // 2016-01-08T22:00:00.000ZResults in a string representation of tomorrow's date.
Use new Date() to get the current date, increment by one using Date.getDate() and set the value to the result using Date.setDate().
Use Date.prototype.toISOString() to return a string in yyyy-mm-dd format.
const tomorrow = () => {
let t = new Date();
t.setDate(t.getDate() + 1);
return t.toISOString().split('T')[0];
};示例
tomorrow(); // 2018-10-18 (if current date is 2018-10-18)Attempts to invoke a function with the provided arguments, returning either the result or the caught error object.
Use a try... catch block to return either the result of the function or an appropriate error.
const attempt = (fn, ...args) => {
try {
return fn(...args);
} catch (e) {
return e instanceof Error ? e : new Error(e);
}
};示例
var elements = attempt(function(selector) {
return document.querySelectorAll(selector);
}, '>_>');
if (elements instanceof Error) elements = []; // elements = []Creates a function that invokes fn with a given context, optionally adding any additional supplied parameters to the beginning of the arguments.
Return a function that uses Function.prototype.apply() to apply the given context to fn.
Use Array.prototype.concat() to prepend any additional supplied parameters to the arguments.
const bind = (fn, context, ...boundArgs) => (...args) => fn.apply(context, [...boundArgs, ...args]);示例
function greet(greeting, punctuation) {
return greeting + ' ' + this.user + punctuation;
}
const freddy = { user: 'fred' };
const freddyBound = bind(greet, freddy);
console.log(freddyBound('hi', '!')); // 'hi fred!'Creates a function that invokes the method at a given key of an object, optionally adding any additional supplied parameters to the beginning of the arguments.
Return a function that uses Function.prototype.apply() to bind context[fn] to context.
Use the spread operator (...) to prepend any additional supplied parameters to the arguments.
const bindKey = (context, fn, ...boundArgs) => (...args) =>
context[fn].apply(context, [...boundArgs, ...args]);示例
const freddy = {
user: 'fred',
greet: function(greeting, punctuation) {
return greeting + ' ' + this.user + punctuation;
}
};
const freddyBound = bindKey(freddy, 'greet');
console.log(freddyBound('hi', '!')); // 'hi fred!'Chains asynchronous functions.
Loop through an array of functions containing asynchronous events, calling next when each asynchronous event has completed.
const chainAsync = fns => {
let curr = 0;
const last = fns[fns.length - 1];
const next = () => {
const fn = fns[curr++];
fn === last ? fn() : fn(next);
};
next();
};示例
chainAsync([
next => {
console.log('0 seconds');
setTimeout(next, 1000);
},
next => {
console.log('1 second');
setTimeout(next, 1000);
},
() => {
console.log('2 second');
}
]);Performs right-to-left function composition.
Use Array.prototype.reduce() to perform right-to-left function composition.
The last (rightmost) function can accept one or more arguments; the remaining functions must be unary.
const compose = (...fns) => fns.reduce((f, g) => (...args) => f(g(...args)));示例
const add5 = x => x + 5;
const multiply = (x, y) => x * y;
const multiplyAndAdd5 = compose(
add5,
multiply
);
multiplyAndAdd5(5, 2); // 15Performs left-to-right function composition.
Use Array.prototype.reduce() to perform left-to-right function composition.
The first (leftmost) function can accept one or more arguments; the remaining functions must be unary.
const composeRight = (...fns) => fns.reduce((f, g) => (...args) => g(f(...args)));示例
const add = (x, y) => x + y;
const square = x => x * x;
const addAndSquare = composeRight(add, square);
addAndSquare(1, 2); // 9Accepts a converging function and a list of branching functions and returns a function that applies each branching function to the arguments and the results of the branching functions are passed as arguments to the converging function.
Use Array.prototype.map() and Function.prototype.apply() to apply each function to the given arguments.
Use the spread operator (...) to call coverger with the results of all other functions.
const converge = (converger, fns) => (...args) => converger(...fns.map(fn => fn.apply(null, args)));示例
const average = converge((a, b) => a / b, [
arr => arr.reduce((a, v) => a + v, 0),
arr => arr.length
]);
average([1, 2, 3, 4, 5, 6, 7]); // 4Curries a function.
Use recursion.
If the number of provided arguments (args) is sufficient, call the passed function fn.
Otherwise, return a curried function fn that expects the rest of the arguments.
If you want to curry a function that accepts a variable number of arguments (a variadic function, e.g. Math.min()), you can optionally pass the number of arguments to the second parameter arity.
const curry = (fn, arity = fn.length, ...args) =>
arity <= args.length ? fn(...args) : curry.bind(null, fn, arity, ...args);示例
curry(Math.pow)(2)(10); // 1024
curry(Math.min, 3)(10)(50)(2); // 2Creates a debounced function that delays invoking the provided function until at least ms milliseconds have elapsed since the last time it was invoked.
Each time the debounced function is invoked, clear the current pending timeout with clearTimeout() and use setTimeout() to create a new timeout that delays invoking the function until at least ms milliseconds has elapsed. Use Function.prototype.apply() to apply the this context to the function and provide the necessary arguments.
Omit the second argument, ms, to set the timeout at a default of 0 ms.
const debounce = (fn, ms = 0) => {
let timeoutId;
return function(...args) {
clearTimeout(timeoutId);
timeoutId = setTimeout(() => fn.apply(this, args), ms);
};
};示例
window.addEventListener(
'resize',
debounce(() => {
console.log(window.innerWidth);
console.log(window.innerHeight);
}, 250)
); // Will log the window dimensions at most every 250msDefers invoking a function until the current call stack has cleared.
Use setTimeout() with a timeout of 1ms to add a new event to the browser event queue and allow the rendering engine to complete its work. Use the spread (...) operator to supply the function with an arbitrary number of arguments.
const defer = (fn, ...args) => setTimeout(fn, 1, ...args);示例
// Example A:
defer(console.log, 'a'), console.log('b'); // logs 'b' then 'a'
// Example B:
document.querySelector('#someElement').innerHTML = 'Hello';
longRunningFunction(); // Browser will not update the HTML until this has finished
defer(longRunningFunction); // Browser will update the HTML then run the functionInvokes the provided function after wait milliseconds.
Use setTimeout() to delay execution of fn.
Use the spread (...) operator to supply the function with an arbitrary number of arguments.
const delay = (fn, wait, ...args) => setTimeout(fn, wait, ...args);示例
delay(
function(text) {
console.log(text);
},
1000,
'later'
); // Logs 'later' after one second.Logs the name of a function.
Use console.debug() and the name property of the passed method to log the method's name to the debug channel of the console.
const functionName = fn => (console.debug(fn.name), fn);示例
functionName(Math.max); // max (logged in debug channel of console)Returns the number of times a function executed per second.
hz is the unit for hertz, the unit of frequency defined as one cycle per second.
Use performance.now() to get the difference in milliseconds before and after the iteration loop to calculate the time elapsed executing the function iterations times.
Return the number of cycles per second by converting milliseconds to seconds and dividing it by the time elapsed.
Omit the second argument, iterations, to use the default of 100 iterations.
const hz = (fn, iterations = 100) => {
const before = performance.now();
for (let i = 0; i < iterations; i++) fn();
return (1000 * iterations) / (performance.now() - before);
};示例
// 10,000 element array
const numbers = Array(10000)
.fill()
.map((_, i) => i);
// Test functions with the same goal: sum up the elements in the array
const sumReduce = () => numbers.reduce((acc, n) => acc + n, 0);
const sumForLoop = () => {
let sum = 0;
for (let i = 0; i < numbers.length; i++) sum += numbers[i];
return sum;
};
// `sumForLoop` is nearly 10 times faster
Math.round(hz(sumReduce)); // 572
Math.round(hz(sumForLoop)); // 4784Returns the memoized (cached) function.
Create an empty cache by instantiating a new Map object.
Return a function which takes a single argument to be supplied to the memoized function by first checking if the function's output for that specific input value is already cached, or store and return it if not. The function keyword must be used in order to allow the memoized function to have its this context changed if necessary.
Allow access to the cache by setting it as a property on the returned function.
const memoize = fn => {
const cache = new Map();
const cached = function(val) {
return cache.has(val) ? cache.get(val) : cache.set(val, fn.call(this, val)) && cache.get(val);
};
cached.cache = cache;
return cached;
};示例
// See the `anagrams` snippet.
const anagramsCached = memoize(anagrams);
anagramsCached('javascript'); // takes a long time
anagramsCached('javascript'); // returns virtually instantly since it's now cached
console.log(anagramsCached.cache); // The cached anagrams mapNegates a predicate function.
Take a predicate function and apply the not operator (!) to it with its arguments.
const negate = func => (...args) => !func(...args);示例
[1, 2, 3, 4, 5, 6].filter(negate(n => n % 2 === 0)); // [ 1, 3, 5 ]Ensures a function is called only once.
Utilizing a closure, use a flag, called, and set it to true once the function is called for the first time, preventing it from being called again. In order to allow the function to have its this context changed (such as in an event listener), the function keyword must be used, and the supplied function must have the context applied.
Allow the function to be supplied with an arbitrary number of arguments using the rest/spread (...) operator.
const once = fn => {
let called = false;
return function(...args) {
if (called) return;
called = true;
return fn.apply(this, args);
};
};示例
const startApp = function(event) {
console.log(this, event); // document.body, MouseEvent
};
document.body.addEventListener('click', once(startApp)); // only runs `startApp` once upon clickCreates a function that invokes fn with partials prepended to the arguments it receives.
Use the spread operator (...) to prepend partials to the list of arguments of fn.
const partial = (fn, ...partials) => (...args) => fn(...partials, ...args);示例
const greet = (greeting, name) => greeting + ' ' + name + '!';
const greetHello = partial(greet, 'Hello');
greetHello('John'); // 'Hello John!'Creates a function that invokes fn with partials appended to the arguments it receives.
Use the spread operator (...) to append partials to the list of arguments of fn.
const partialRight = (fn, ...partials) => (...args) => fn(...args, ...partials);示例
const greet = (greeting, name) => greeting + ' ' + name + '!';
const greetJohn = partialRight(greet, 'John');
greetJohn('Hello'); // 'Hello John!'Runs an array of promises in series.
Use Array.prototype.reduce() to create a promise chain, where each promise returns the next promise when resolved.
const runPromisesInSeries = ps => ps.reduce((p, next) => p.then(next), Promise.resolve());示例
const delay = d => new Promise(r => setTimeout(r, d));
runPromisesInSeries([() => delay(1000), () => delay(2000)]); // Executes each promise sequentially, taking a total of 3 seconds to completeDelays the execution of an asynchronous function.
Delay executing part of an async function, by putting it to sleep, returning a Promise.
const sleep = ms => new Promise(resolve => setTimeout(resolve, ms));示例
async function sleepyWork() {
console.log("I'm going to sleep for 1 second.");
await sleep(1000);
console.log('I woke up after 1 second.');
}Creates a throttled function that only invokes the provided function at most once per every wait milliseconds
Use setTimeout() and clearTimeout() to throttle the given method, fn.
Use Function.prototype.apply() to apply the this context to the function and provide the necessary arguments.
Use Date.now() to keep track of the last time the throttled function was invoked.
Omit the second argument, wait, to set the timeout at a default of 0 ms.
const throttle = (fn, wait) => {
let inThrottle, lastFn, lastTime;
return function() {
const context = this,
args = arguments;
if (!inThrottle) {
fn.apply(context, args);
lastTime = Date.now();
inThrottle = true;
} else {
clearTimeout(lastFn);
lastFn = setTimeout(function() {
if (Date.now() - lastTime >= wait) {
fn.apply(context, args);
lastTime = Date.now();
}
}, Math.max(wait - (Date.now() - lastTime), 0));
}
};
};示例
window.addEventListener(
'resize',
throttle(function(evt) {
console.log(window.innerWidth);
console.log(window.innerHeight);
}, 250)
); // Will log the window dimensions at most every 250msIterates over a callback n times
Use Function.call() to call fn n times or until it returns false.
Omit the last argument, context, to use an undefined object (or the global object in non-strict mode).
const times = (n, fn, context = undefined) => {
let i = 0;
while (fn.call(context, i) !== false && ++i < n) {}
};示例
var output = '';
times(5, i => (output += i));
console.log(output); // 01234Uncurries a function up to depth n.
Return a variadic function.
Use Array.prototype.reduce() on the provided arguments to call each subsequent curry level of the function.
If the length of the provided arguments is less than n throw an error.
Otherwise, call fn with the proper amount of arguments, using Array.prototype.slice(0, n).
Omit the second argument, n, to uncurry up to depth 1.
const uncurry = (fn, n = 1) => (...args) => {
const next = acc => args => args.reduce((x, y) => x(y), acc);
if (n > args.length) throw new RangeError('Arguments too few!');
return next(fn)(args.slice(0, n));
};示例
const add = x => y => z => x + y + z;
const uncurriedAdd = uncurry(add, 3);
uncurriedAdd(1, 2, 3); // 6Builds an array, using an iterator function and an initial seed value.
Use a while loop and Array.prototype.push() to call the function repeatedly until it returns false.
The iterator function accepts one argument (seed) and must always return an array with two elements ([value, nextSeed]) or false to terminate.
const unfold = (fn, seed) => {
let result = [],
val = [null, seed];
while ((val = fn(val[1]))) result.push(val[0]);
return result;
};示例
var f = n => (n > 50 ? false : [-n, n + 10]);
unfold(f, 10); // [-10, -20, -30, -40, -50]Tests a value, x, against a predicate function. If true, return fn(x). Else, return x.
Return a function expecting a single value, x, that returns the appropriate value based on pred.
const when = (pred, whenTrue) => x => (pred(x) ? whenTrue(x) : x);示例
const doubleEvenNumbers = when(x => x % 2 === 0, x => x * 2);
doubleEvenNumbers(2); // 4
doubleEvenNumbers(1); // 1Checks if two numbers are approximately equal to each other.
Use Math.abs() to compare the absolute difference of the two values to epsilon.
Omit the third parameter, epsilon, to use a default value of 0.001.
const approximatelyEqual = (v1, v2, epsilon = 0.001) => Math.abs(v1 - v2) < epsilon;示例
approximatelyEqual(Math.PI / 2.0, 1.5708); // trueReturns the average of two or more numbers.
Use Array.prototype.reduce() to add each value to an accumulator, initialized with a value of 0, divide by the length of the array.
const average = (...nums) => nums.reduce((acc, val) => acc + val, 0) / nums.length;示例
average(...[1, 2, 3]); // 2
average(1, 2, 3); // 2Returns the average of an array, after mapping each element to a value using the provided function.
Use Array.prototype.map() to map each element to the value returned by fn, Array.prototype.reduce() to add each value to an accumulator, initialized with a value of 0, divide by the length of the array.
const averageBy = (arr, fn) =>
arr.map(typeof fn === 'function' ? fn : val => val[fn]).reduce((acc, val) => acc + val, 0) /
arr.length;示例
averageBy([{ n: 4 }, { n: 2 }, { n: 8 }, { n: 6 }], o => o.n); // 5
averageBy([{ n: 4 }, { n: 2 }, { n: 8 }, { n: 6 }], 'n'); // 5Evaluates the binomial coefficient of two integers n and k.
Use Number.isNaN() to check if any of the two values is NaN.
Check if k is less than 0, greater than or equal to n, equal to 1 or n - 1 and return the appropriate result.
Check if n - k is less than k and switch their values accordingly.
Loop from 2 through k and calculate the binomial coefficient.
Use Math.round() to account for rounding errors in the calculation.
const binomialCoefficient = (n, k) => {
if (Number.isNaN(n) || Number.isNaN(k)) return NaN;
if (k < 0 || k > n) return 0;
if (k === 0 || k === n) return 1;
if (k === 1 || k === n - 1) return n;
if (n - k < k) k = n - k;
let res = n;
for (let j = 2; j <= k; j++) res *= (n - j + 1) / j;
return Math.round(res);
};示例
binomialCoefficient(8, 2); // 28Clamps num within the inclusive range specified by the boundary values a and b.
If num falls within the range, return num.
Otherwise, return the nearest number in the range.
const clampNumber = (num, a, b) => Math.max(Math.min(num, Math.max(a, b)), Math.min(a, b));示例
clampNumber(2, 3, 5); // 3
clampNumber(1, -1, -5); // -1Converts an angle from degrees to radians.
Use Math.PI and the degree to radian formula to convert the angle from degrees to radians.
const degreesToRads = deg => (deg * Math.PI) / 180.0;示例
degreesToRads(90.0); // ~1.5708Converts a number to an array of digits.
Convert the number to a string, using the spread operator (...) to build an array.
Use Array.prototype.map() and parseInt() to transform each value to an integer.
const digitize = n => [...`${n}`].map(i => parseInt(i));示例
digitize(123); // [1, 2, 3]Returns the distance between two points.
Use Math.hypot() to calculate the Euclidean distance between two points.
const distance = (x0, y0, x1, y1) => Math.hypot(x1 - x0, y1 - y0);示例
distance(1, 1, 2, 3); // 2.23606797749979Computes the new ratings between two or more opponents using the Elo rating system. It takes an array of pre-ratings and returns an array containing post-ratings. The array should be ordered from best performer to worst performer (winner -> loser).
Use the exponent ** operator and math operators to compute the expected score (chance of winning).
of each opponent and compute the new rating for each.
Loop through the ratings, using each permutation to compute the post-Elo rating for each player in a pairwise fashion.
Omit the second argument to use the default kFactor of 32.
const elo = ([...ratings], kFactor = 32, selfRating) => {
const [a, b] = ratings;
const expectedScore = (self, opponent) => 1 / (1 + 10 ** ((opponent - self) / 400));
const newRating = (rating, i) =>
(selfRating || rating) + kFactor * (i - expectedScore(i ? a : b, i ? b : a));
if (ratings.length === 2) return [newRating(a, 1), newRating(b, 0)];
for (let i = 0, len = ratings.length; i < len; i++) {
let j = i;
while (j < len - 1) {
j++;
[ratings[i], ratings[j]] = elo([ratings[i], ratings[j]], kFactor);
}
}
return ratings;
};示例
// Standard 1v1s
elo([1200, 1200]); // [1216, 1184]
elo([1200, 1200], 64); // [1232, 1168]
// 4 player FFA, all same rank
elo([1200, 1200, 1200, 1200]).map(Math.round); // [1246, 1215, 1185, 1154]
/*
For teams, each rating can adjusted based on own team's average rating vs.
average rating of opposing team, with the score being added to their
own individual rating by supplying it as the third argument.
*/Calculates the factorial of a number.
Use recursion.
If n is less than or equal to 1, return 1.
Otherwise, return the product of n and the factorial of n - 1.
Throws an exception if n is a negative number.
const factorial = n =>
n < 0
? (() => {
throw new TypeError('Negative numbers are not allowed!');
})()
: n <= 1
? 1
: n * factorial(n - 1);示例
factorial(6); // 720Generates an array, containing the Fibonacci sequence, up until the nth term.
Create an empty array of the specific length, initializing the first two values (0 and 1).
Use Array.prototype.reduce() to add values into the array, using the sum of the last two values, except for the first two.
const fibonacci = n =>
Array.from({ length: n }).reduce(
(acc, val, i) => acc.concat(i > 1 ? acc[i - 1] + acc[i - 2] : i),
[]
);示例
fibonacci(6); // [0, 1, 1, 2, 3, 5]Calculates the greatest common divisor between two or more numbers/arrays.
The inner _gcd function uses recursion.
Base case is when y equals 0. In this case, return x.
Otherwise, return the GCD of y and the remainder of the division x/y.
const gcd = (...arr) => {
const _gcd = (x, y) => (!y ? x : gcd(y, x % y));
return [...arr].reduce((a, b) => _gcd(a, b));
};示例
gcd(8, 36); // 4
gcd(...[12, 8, 32]); // 4Initializes an array containing the numbers in the specified range where start and end are inclusive and the ratio between two terms is step.
Returns an error if step equals 1.
Use Array.from(), Math.log() and Math.floor() to create an array of the desired length, Array.prototype.map() to fill with the desired values in a range.
Omit the second argument, start, to use a default value of 1.
Omit the third argument, step, to use a default value of 2.
const geometricProgression = (end, start = 1, step = 2) =>
Array.from({ length: Math.floor(Math.log(end / start) / Math.log(step)) + 1 }).map(
(v, i) => start * step ** i
);示例
geometricProgression(256); // [1, 2, 4, 8, 16, 32, 64, 128, 256]
geometricProgression(256, 3); // [3, 6, 12, 24, 48, 96, 192]
geometricProgression(256, 1, 4); // [1, 4, 16, 64, 256]Calculates the Hamming distance between two values.
Use XOR operator (^) to find the bit difference between the two numbers, convert to a binary string using toString(2).
Count and return the number of 1s in the string, using match(/1/g).
const hammingDistance = (num1, num2) => ((num1 ^ num2).toString(2).match(/1/g) || '').length;示例
hammingDistance(2, 3); // 1Checks if the given number falls within the given range.
Use arithmetic comparison to check if the given number is in the specified range.
If the second parameter, end, is not specified, the range is considered to be from 0 to start.
const inRange = (n, start, end = null) => {
if (end && start > end) [end, start] = [start, end];
return end == null ? n >= 0 && n < start : n >= start && n < end;
};示例
inRange(3, 2, 5); // true
inRange(3, 4); // true
inRange(2, 3, 5); // false
inRange(3, 2); // falseChecks if the first numeric argument is divisible by the second one.
Use the modulo operator (%) to check if the remainder is equal to 0.
const isDivisible = (dividend, divisor) => dividend % divisor === 0;示例
isDivisible(6, 3); // trueReturns true if the given number is even, false otherwise.
Checks whether a number is odd or even using the modulo (%) operator.
Returns true if the number is even, false if the number is odd.
const isEven = num => num % 2 === 0;示例
isEven(3); // falseChecks if the given value is equal to negative zero (-0).
Checks whether a passed value is equal to 0 and if 1 divided by the value equals -Infinity.
const isNegativeZero = val => val === 0 && 1 / val === -Infinity;示例
isNegativeZero(-0); // true
isNegativeZero(0); // falseChecks if the provided integer is a prime number.
Check numbers from 2 to the square root of the given number.
Return false if any of them divides the given number, else return true, unless the number is less than 2.
const isPrime = num => {
const boundary = Math.floor(Math.sqrt(num));
for (var i = 2; i <= boundary; i++) if (num % i === 0) return false;
return num >= 2;
};示例
isPrime(11); // trueReturns the least common multiple of two or more numbers.
Use the greatest common divisor (GCD) formula and the fact that lcm(x,y) = x * y / gcd(x,y) to determine the least common multiple.
The GCD formula uses recursion.
const lcm = (...arr) => {
const gcd = (x, y) => (!y ? x : gcd(y, x % y));
const _lcm = (x, y) => (x * y) / gcd(x, y);
return [...arr].reduce((a, b) => _lcm(a, b));
};示例
lcm(12, 7); // 84
lcm(...[1, 3, 4, 5]); // 60Implementation of the Luhn Algorithm used to validate a variety of identification numbers, such as credit card numbers, IMEI numbers, National Provider Identifier numbers etc.
Use String.prototype.split(''), Array.prototype.reverse() and Array.prototype.map() in combination with parseInt() to obtain an array of digits.
Use Array.prototype.splice(0,1) to obtain the last digit.
Use Array.prototype.reduce() to implement the Luhn Algorithm.
Return true if sum is divisible by 10, false otherwise.
const luhnCheck = num => {
let arr = (num + '')
.split('')
.reverse()
.map(x => parseInt(x));
let lastDigit = arr.splice(0, 1)[0];
let sum = arr.reduce((acc, val, i) => (i % 2 !== 0 ? acc + val : acc + ((val * 2) % 9) || 9), 0);
sum += lastDigit;
return sum % 10 === 0;
};示例
luhnCheck('4485275742308327'); // true
luhnCheck(6011329933655299); // false
luhnCheck(123456789); // falseReturns the maximum value of an array, after mapping each element to a value using the provided function.
Use Array.prototype.map() to map each element to the value returned by fn, Math.max() to get the maximum value.
const maxBy = (arr, fn) => Math.max(...arr.map(typeof fn === 'function' ? fn : val => val[fn]));示例
maxBy([{ n: 4 }, { n: 2 }, { n: 8 }, { n: 6 }], o => o.n); // 8
maxBy([{ n: 4 }, { n: 2 }, { n: 8 }, { n: 6 }], 'n'); // 8Returns the median of an array of numbers.
Find the middle of the array, use Array.prototype.sort() to sort the values.
Return the number at the midpoint if length is odd, otherwise the average of the two middle numbers.
const median = arr => {
const mid = Math.floor(arr.length / 2),
nums = [...arr].sort((a, b) => a - b);
return arr.length % 2 !== 0 ? nums[mid] : (nums[mid - 1] + nums[mid]) / 2;
};示例
median([5, 6, 50, 1, -5]); // 5Calculates the midpoint between two pairs of (x,y) points.
Destructure the array to get x1, y1, x2 and y2, calculate the midpoint for each dimension by dividing the sum of the two endpoints by 2.
const midpoint = ([x1, y1], [x2, y2]) => [(x1 + x2) / 2, (y1 + y2) / 2];示例
midpoint([2, 2], [4, 4]); // [3, 3]
midpoint([4, 4], [6, 6]); // [5, 5]
midpoint([1, 3], [2, 4]); // [1.5, 3.5]Returns the minimum value of an array, after mapping each element to a value using the provided function.
Use Array.prototype.map() to map each element to the value returned by fn, Math.min() to get the maximum value.
const minBy = (arr, fn) => Math.min(...arr.map(typeof fn === 'function' ? fn : val => val[fn]));示例
minBy([{ n: 4 }, { n: 2 }, { n: 8 }, { n: 6 }], o => o.n); // 2
minBy([{ n: 4 }, { n: 2 }, { n: 8 }, { n: 6 }], 'n'); // 2Uses the percentile formula to calculate how many numbers in the given array are less or equal to the given value.
Use Array.prototype.reduce() to calculate how many numbers are below the value and how many are the same value and apply the percentile formula.
const percentile = (arr, val) =>
(100 * arr.reduce((acc, v) => acc + (v < val ? 1 : 0) + (v === val ? 0.5 : 0), 0)) / arr.length;示例
percentile([1, 2, 3, 4, 5, 6, 7, 8, 9, 10], 6); // 55Returns the powerset of a given array of numbers.
Use Array.prototype.reduce() combined with Array.prototype.map() to iterate over elements and combine into an array containing all combinations.
const powerset = arr => arr.reduce((a, v) => a.concat(a.map(r => [v].concat(r))), [[]]);示例
powerset([1, 2]); // [[], [1], [2], [2, 1]]Generates primes up to a given number, using the Sieve of Eratosthenes.
Generate an array from 2 to the given number. Use Array.prototype.filter() to filter out the values divisible by any number from 2 to the square root of the provided number.
const primes = num => {
let arr = Array.from({ length: num - 1 }).map((x, i) => i + 2),
sqroot = Math.floor(Math.sqrt(num)),
numsTillSqroot = Array.from({ length: sqroot - 1 }).map((x, i) => i + 2);
numsTillSqroot.forEach(x => (arr = arr.filter(y => y % x !== 0 || y === x)));
return arr;
};示例
primes(10); // [2,3,5,7]Converts an angle from radians to degrees.
Use Math.PI and the radian to degree formula to convert the angle from radians to degrees.
const radsToDegrees = rad => (rad * 180.0) / Math.PI;示例
radsToDegrees(Math.PI / 2); // 90Returns an array of n random integers in the specified range.
Use Array.from() to create an empty array of the specific length, Math.random() to generate a random number and map it to the desired range, using Math.floor() to make it an integer.
const randomIntArrayInRange = (min, max, n = 1) =>
Array.from({ length: n }, () => Math.floor(Math.random() * (max - min + 1)) + min);示例
randomIntArrayInRange(12, 35, 10); // [ 34, 14, 27, 17, 30, 27, 20, 26, 21, 14 ]Returns a random integer in the specified range.
Use Math.random() to generate a random number and map it to the desired range, using Math.floor() to make it an integer.
const randomIntegerInRange = (min, max) => Math.floor(Math.random() * (max - min + 1)) + min;示例
randomIntegerInRange(0, 5); // 2Returns a random number in the specified range.
Use Math.random() to generate a random value, map it to the desired range using multiplication.
const randomNumberInRange = (min, max) => Math.random() * (max - min) + min;示例
randomNumberInRange(2, 10); // 6.0211363285087005Rounds a number to a specified amount of digits.
Use Math.round() and template literals to round the number to the specified number of digits.
Omit the second argument, decimals to round to an integer.
const round = (n, decimals = 0) => Number(`${Math.round(`${n}e${decimals}`)}e-${decimals}`);示例
round(1.005, 2); // 1.01Hashes the input string into a whole number.
Use String.prototype.split('') and Array.prototype.reduce() to create a hash of the input string, utilizing bit shifting.
const sdbm = str => {
let arr = str.split('');
return arr.reduce(
(hashCode, currentVal) =>
(hashCode = currentVal.charCodeAt(0) + (hashCode << 6) + (hashCode << 16) - hashCode),
0
);
};示例
sdbm('name'); // -3521204949Returns the standard deviation of an array of numbers.
Use Array.prototype.reduce() to calculate the mean, variance and the sum of the variance of the values, the variance of the values, then
determine the standard deviation.
You can omit the second argument to get the sample standard deviation or set it to true to get the population standard deviation.
const standardDeviation = (arr, usePopulation = false) => {
const mean = arr.reduce((acc, val) => acc + val, 0) / arr.length;
return Math.sqrt(
arr.reduce((acc, val) => acc.concat((val - mean) ** 2), []).reduce((acc, val) => acc + val, 0) /
(arr.length - (usePopulation ? 0 : 1))
);
};示例
standardDeviation([10, 2, 38, 23, 38, 23, 21]); // 13.284434142114991 (sample)
standardDeviation([10, 2, 38, 23, 38, 23, 21], true); // 12.29899614287479 (population)Returns the sum of two or more numbers/arrays.
Use Array.prototype.reduce() to add each value to an accumulator, initialized with a value of 0.
const sum = (...arr) => [...arr].reduce((acc, val) => acc + val, 0);示例
sum(1, 2, 3, 4); // 10
sum(...[1, 2, 3, 4]); // 10Returns the sum of an array, after mapping each element to a value using the provided function.
Use Array.prototype.map() to map each element to the value returned by fn, Array.prototype.reduce() to add each value to an accumulator, initialized with a value of 0.
const sumBy = (arr, fn) =>
arr.map(typeof fn === 'function' ? fn : val => val[fn]).reduce((acc, val) => acc + val, 0);示例
sumBy([{ n: 4 }, { n: 2 }, { n: 8 }, { n: 6 }], o => o.n); // 20
sumBy([{ n: 4 }, { n: 2 }, { n: 8 }, { n: 6 }], 'n'); // 20Returns the sum of the powers of all the numbers from start to end (both inclusive).
Use Array.prototype.fill() to create an array of all the numbers in the target range, Array.prototype.map() and the exponent operator (**) to raise them to power and Array.prototype.reduce() to add them together.
Omit the second argument, power, to use a default power of 2.
Omit the third argument, start, to use a default starting value of 1.
const sumPower = (end, power = 2, start = 1) =>
Array(end + 1 - start)
.fill(0)
.map((x, i) => (i + start) ** power)
.reduce((a, b) => a + b, 0);示例
sumPower(10); // 385
sumPower(10, 3); // 3025
sumPower(10, 3, 5); // 2925Converts a value to a safe integer.
Use Math.max() and Math.min() to find the closest safe value.
Use Math.round() to convert to an integer.
const toSafeInteger = num =>
Math.round(Math.max(Math.min(num, Number.MAX_SAFE_INTEGER), Number.MIN_SAFE_INTEGER));示例
toSafeInteger('3.2'); // 3
toSafeInteger(Infinity); // 9007199254740991Decodes a string of data which has been encoded using base-64 encoding.
Create a Buffer for the given string with base-64 encoding and use Buffer.toString('binary') to return the decoded string.
const atob = str => Buffer.from(str, 'base64').toString('binary');示例
atob('Zm9vYmFy'); // 'foobar'Creates a base-64 encoded ASCII string from a String object in which each character in the string is treated as a byte of binary data.
Create a Buffer for the given string with binary encoding and use Buffer.toString('base64') to return the encoded string.
const btoa = str => Buffer.from(str, 'binary').toString('base64');示例
btoa('foobar'); // 'Zm9vYmFy'Add special characters to text to print in color in the console (combined with console.log()).
Use template literals and special characters to add the appropriate color code to the string output. For background colors, add a special character that resets the background color at the end of the string.
const colorize = (...args) => ({
black: `\x1b[30m${args.join(' ')}`,
red: `\x1b[31m${args.join(' ')}`,
green: `\x1b[32m${args.join(' ')}`,
yellow: `\x1b[33m${args.join(' ')}`,
blue: `\x1b[34m${args.join(' ')}`,
magenta: `\x1b[35m${args.join(' ')}`,
cyan: `\x1b[36m${args.join(' ')}`,
white: `\x1b[37m${args.join(' ')}`,
bgBlack: `\x1b[40m${args.join(' ')}\x1b[0m`,
bgRed: `\x1b[41m${args.join(' ')}\x1b[0m`,
bgGreen: `\x1b[42m${args.join(' ')}\x1b[0m`,
bgYellow: `\x1b[43m${args.join(' ')}\x1b[0m`,
bgBlue: `\x1b[44m${args.join(' ')}\x1b[0m`,
bgMagenta: `\x1b[45m${args.join(' ')}\x1b[0m`,
bgCyan: `\x1b[46m${args.join(' ')}\x1b[0m`,
bgWhite: `\x1b[47m${args.join(' ')}\x1b[0m`
});示例
console.log(colorize('foo').red); // 'foo' (red letters)
console.log(colorize('foo', 'bar').bgBlue); // 'foo bar' (blue background)
console.log(colorize(colorize('foo').yellow, colorize('foo').green).bgWhite); // 'foo bar' (first word in yellow letters, second word in green letters, white background for both)Creates a directory, if it does not exist.
Use fs.existsSync() to check if the directory exists, fs.mkdirSync() to create it.
const fs = require('fs');
const createDirIfNotExists = dir => (!fs.existsSync(dir) ? fs.mkdirSync(dir) : undefined);示例
createDirIfNotExists('test'); // creates the directory 'test', if it doesn't existCheck if the current process's arguments contain the specified flags.
Use Array.prototype.every() and Array.prototype.includes() to check if process.argv contains all the specified flags.
Use a regular expression to test if the specified flags are prefixed with - or -- and prefix them accordingly.
const hasFlags = (...flags) =>
flags.every(flag => process.argv.includes(/^-{1,2}/.test(flag) ? flag : '--' + flag));示例
// node myScript.js -s --test --cool=true
hasFlags('-s'); // true
hasFlags('--test', 'cool=true', '-s'); // true
hasFlags('special'); // falseCreates a hash for a value using the SHA-256 algorithm. Returns a promise.
Use crypto API to create a hash for the given value, setTimeout to prevent blocking on a long operation, and a Promise to give it a familiar interface.
const crypto = require('crypto');
const hashNode = val =>
new Promise(resolve =>
setTimeout(
() =>
resolve(
crypto
.createHash('sha256')
.update(val)
.digest('hex')
),
0
)
);示例
hashNode(JSON.stringify({ a: 'a', b: [1, 2, 3, 4], foo: { c: 'bar' } })).then(console.log); // '04aa106279f5977f59f9067fa9712afc4aedc6f5862a8defc34552d8c7206393'Checks if the given argument is a duplex (readable and writable) stream.
Check if the value is different from null, use typeof to check if a value is of type object and the pipe property is of type function.
Additionally check if the typeof the _read, _write and _readableState, _writableState properties are function and object respectively.
const isDuplexStream = val =>
val !== null &&
typeof val === 'object' &&
typeof val.pipe === 'function' &&
typeof val._read === 'function' &&
typeof val._readableState === 'object' &&
typeof val._write === 'function' &&
typeof val._writableState === 'object';示例
const Stream = require('stream');
isDuplexStream(new Stream.Duplex()); // trueChecks if the given argument is a readable stream.
Check if the value is different from null, use typeof to check if the value is of type object and the pipe property is of type function.
Additionally check if the typeof the _read and _readableState properties are function and object respectively.
const isReadableStream = val =>
val !== null &&
typeof val === 'object' &&
typeof val.pipe === 'function' &&
typeof val._read === 'function' &&
typeof val._readableState === 'object';示例
const fs = require('fs');
isReadableStream(fs.createReadStream('test.txt')); // trueChecks if the given argument is a stream.
Check if the value is different from null, use typeof to check if the value is of type object and the pipe property is of type function.
const isStream = val => val !== null && typeof val === 'object' && typeof val.pipe === 'function';示例
const fs = require('fs');
isStream(fs.createReadStream('test.txt')); // trueChecks if the current environment is Travis CI.
Checks if the current environment has the TRAVIS and CI environment variables (reference).
const isTravisCI = () => 'TRAVIS' in process.env && 'CI' in process.env;示例
isTravisCI(); // true (if code is running on Travis CI)Checks if the given argument is a writable stream.
Check if the value is different from null, use typeof to check if the value is of type object and the pipe property is of type function.
Additionally check if the typeof the _write and _writableState properties are function and object respectively.
const isWritableStream = val =>
val !== null &&
typeof val === 'object' &&
typeof val.pipe === 'function' &&
typeof val._write === 'function' &&
typeof val._writableState === 'object';示例
const fs = require('fs');
isWritableStream(fs.createWriteStream('test.txt')); // trueWrites a JSON object to a file.
Use fs.writeFile(), template literals and JSON.stringify() to write a json object to a .json file.
const fs = require('fs');
const JSONToFile = (obj, filename) =>
fs.writeFile(`${filename}.json`, JSON.stringify(obj, null, 2));示例
JSONToFile({ test: 'is passed' }, 'testJsonFile'); // writes the object to 'testJsonFile.json'Returns an array of lines from the specified file.
Use readFileSync function in fs node package to create a Buffer from a file.
convert buffer to string using toString(encoding) function.
creating an array from contents of file by spliting file content line by line (each \n).
const fs = require('fs');
const readFileLines = filename =>
fs
.readFileSync(filename)
.toString('UTF8')
.split('\n');示例
/*
contents of test.txt :
line1
line2
line3
___________________________
*/
let arr = readFileLines('test.txt');
console.log(arr); // ['line1', 'line2', 'line3']Converts a tilde path to an absolute path.
Use String.prototype.replace() with a regular expression and OS.homedir() to replace the ~ in the start of the path with the home directory.
const untildify = str => str.replace(/^~($|\/|\\)/, `${require('os').homedir()}$1`);示例
untildify('~/node'); // '/Users/aUser/node'Generates a UUID in Node.JS.
Use crypto API to generate a UUID, compliant with RFC4122 version 4.
const crypto = require('crypto');
const UUIDGeneratorNode = () =>
([1e7] + -1e3 + -4e3 + -8e3 + -1e11).replace(/[018]/g, c =>
(c ^ (crypto.randomBytes(1)[0] & (15 >> (c / 4)))).toString(16)
);示例
UUIDGeneratorNode(); // '79c7c136-60ee-40a2-beb2-856f1feabefc'Binds methods of an object to the object itself, overwriting the existing method.
Use Array.prototype.forEach() to return a function that uses Function.prototype.apply() to apply the given context (obj) to fn for each function specified.
const bindAll = (obj, ...fns) =>
fns.forEach(
fn => (
(f = obj[fn]),
(obj[fn] = function() {
return f.apply(obj);
})
)
);示例
var view = {
label: 'docs',
click: function() {
console.log('clicked ' + this.label);
}
};
bindAll(view, 'click');
jQuery(element).on('click', view.click); // Logs 'clicked docs' when clicked.Creates a deep clone of an object.
Use recursion.
Use Object.assign() and an empty object ({}) to create a shallow clone of the original.
Use Object.keys() and Array.prototype.forEach() to determine which key-value pairs need to be deep cloned.
const deepClone = obj => {
let clone = Object.assign({}, obj);
Object.keys(clone).forEach(
key => (clone[key] = typeof obj[key] === 'object' ? deepClone(obj[key]) : obj[key])
);
return Array.isArray(obj) && obj.length
? (clone.length = obj.length) && Array.from(clone)
: Array.isArray(obj)
? Array.from(obj)
: clone;
};示例
const a = { foo: 'bar', obj: { a: 1, b: 2 } };
const b = deepClone(a); // a !== b, a.obj !== b.objDeep freezes an object.
Calls Object.freeze(obj) recursively on all unfrozen properties of passed object that are instanceof object.
const deepFreeze = obj =>
Object.keys(obj).forEach(
prop =>
!(obj[prop] instanceof Object) || Object.isFrozen(obj[prop]) ? null : deepFreeze(obj[prop])
) || Object.freeze(obj);示例
'use strict';
const o = deepFreeze([1, [2, 3]]);
o[0] = 3; // not allowed
o[1][0] = 4; // not allowed as wellDeep maps an object keys.
Creates an object with the same values as the provided object and keys generated by running the provided function for each key.
Use Object.keys(obj) to iterate over the object's keys.
Use Array.prototype.reduce() to create a new object with the same values and mapped keys using fn.
const deepMapKeys = (obj, f) =>
Array.isArray(obj)
? obj.map(val => deepMapKeys(val, f))
: typeof obj === 'object'
? Object.keys(obj).reduce((acc, current) => {
const val = obj[current];
acc[f(current)] =
val !== null && typeof val === 'object' ? deepMapKeys(val, f) : (acc[f(current)] = val);
return acc;
}, {})
: obj;示例
const obj = {
foo: '1',
nested: {
child: {
withArray: [
{
grandChild: ['hello']
}
]
}
}
};
const upperKeysObj = deepMapKeys(obj, key => key.toUpperCase());
/*
{
"FOO":"1",
"NESTED":{
"CHILD":{
"WITHARRAY":[
{
"GRANDCHILD":[ 'hello' ]
}
]
}
}
}
*/Assigns default values for all properties in an object that are undefined.
Use Object.assign() to create a new empty object and copy the original one to maintain key order, use Array.prototype.reverse() and the spread operator ... to combine the default values from left to right, finally use obj again to overwrite properties that originally had a value.
const defaults = (obj, ...defs) => Object.assign({}, obj, ...defs.reverse(), obj);示例
defaults({ a: 1 }, { b: 2 }, { b: 6 }, { a: 3 }); // { a: 1, b: 2 }Returns the target value in a nested JSON object, based on the given key.
Use the in operator to check if target exists in obj.
If found, return the value of obj[target], otherwise use Object.values(obj) and Array.prototype.reduce() to recursively call dig on each nested object until the first matching key/value pair is found.
const dig = (obj, target) =>
target in obj
? obj[target]
: Object.values(obj).reduce((acc, val) => {
if (acc !== undefined) return acc;
if (typeof val === 'object') return dig(val, target);
}, undefined);示例
const data = {
level1: {
level2: {
level3: 'some data'
}
}
};
dig(data, 'level3'); // 'some data'
dig(data, 'level4'); // undefinedPerforms a deep comparison between two values to determine if they are equivalent.
Check if the two values are identical, if they are both Date objects with the same time, using Date.getTime() or if they are both non-object values with an equivalent value (strict comparison).
Check if only one value is null or undefined or if their prototypes differ.
If none of the above conditions are met, use Object.keys() to check if both values have the same number of keys, then use Array.prototype.every() to check if every key in the first value exists in the second one and if they are equivalent by calling this method recursively.
const equals = (a, b) => {
if (a === b) return true;
if (a instanceof Date && b instanceof Date) return a.getTime() === b.getTime();
if (!a || !b || (typeof a !== 'object' && typeof b !== 'object')) return a === b;
if (a === null || a === undefined || b === null || b === undefined) return false;
if (a.prototype !== b.prototype) return false;
let keys = Object.keys(a);
if (keys.length !== Object.keys(b).length) return false;
return keys.every(k => equals(a[k], b[k]));
};示例
equals({ a: [2, { e: 3 }], b: [4], c: 'foo' }, { a: [2, { e: 3 }], b: [4], c: 'foo' }); // trueReturns the first key that satisfies the provided testing function. Otherwise undefined is returned.
Use Object.keys(obj) to get all the properties of the object, Array.prototype.find() to test the provided function for each key-value pair. The callback receives three arguments - the value, the key and the object.
const findKey = (obj, fn) => Object.keys(obj).find(key => fn(obj[key], key, obj));示例
findKey(
{
barney: { age: 36, active: true },
fred: { age: 40, active: false },
pebbles: { age: 1, active: true }
},
o => o['active']
); // 'barney'Returns the last key that satisfies the provided testing function.
Otherwise undefined is returned.
Use Object.keys(obj) to get all the properties of the object, Array.prototype.reverse() to reverse their order and Array.prototype.find() to test the provided function for each key-value pair.
The callback receives three arguments - the value, the key and the object.
const findLastKey = (obj, fn) =>
Object.keys(obj)
.reverse()
.find(key => fn(obj[key], key, obj));示例
findLastKey(
{
barney: { age: 36, active: true },
fred: { age: 40, active: false },
pebbles: { age: 1, active: true }
},
o => o['active']
); // 'pebbles'Flatten an object with the paths for keys.
Use recursion.
Use Object.keys(obj) combined with Array.prototype.reduce() to convert every leaf node to a flattened path node.
If the value of a key is an object, the function calls itself with the appropriate prefix to create the path using Object.assign().
Otherwise, it adds the appropriate prefixed key-value pair to the accumulator object.
You should always omit the second argument, prefix, unless you want every key to have a prefix.
const flattenObject = (obj, prefix = '') =>
Object.keys(obj).reduce((acc, k) => {
const pre = prefix.length ? prefix + '.' : '';
if (typeof obj[k] === 'object') Object.assign(acc, flattenObject(obj[k], pre + k));
else acc[pre + k] = obj[k];
return acc;
}, {});示例
flattenObject({ a: { b: { c: 1 } }, d: 1 }); // { 'a.b.c': 1, d: 1 }Iterates over all own properties of an object, running a callback for each one.
Use Object.keys(obj) to get all the properties of the object, Array.prototype.forEach() to run the provided function for each key-value pair. The callback receives three arguments - the value, the key and the object.
const forOwn = (obj, fn) => Object.keys(obj).forEach(key => fn(obj[key], key, obj));示例
forOwn({ foo: '
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