var myObject = {
  name: '@interview-questions'
};


if (myObject.name) { ... }

var myObject = {
  name: '@interview-questions'
};

myObject.hasOwnProperty('name'); // true
'name' in myObject; // true


myObject.hasOwnProperty('valueOf'); // false, valueOf 继承自原型链
'valueOf' in myObject; // true

var myFunc = function() {
  this.name = '@interview-questions';
};
myFunc.prototype.age = '10 days';

var user = new myFunc();

user.hasOwnProperty('name'); // true

user.hasOwnProperty('age'); // false, 因为age来自于原型链

const dateTime = Date.now();
const timestamp = Math.floor(dateTime / 1000);

const dateTime = new Date().getTime();
const timestamp = Math.floor(dateTime / 1000);

const dateTime = new Date('2012-06-08').getTime();
const timestamp = Math.floor(dateTime / 1000);

const dateTime = +new Date();
const timestamp = Math.floor(dateTime / 1000);

const dateTime = +new Date('2012-06-08');
const timestamp = Math.floor(dateTime / 1000);

// 当前的购物清单
var items = [{price: 10}, {price: 120}, {price: 1000}];

// reducer函数
var reducer = function add(sumSoFar, nextPrice) { return sumSoFar + nextPrice.price; };

// 开始运行
var total = items.reduce(reducer, 0);

console.log(total); // 1130

var total = items.reduce(reducer,-20);

console.log(total); // 1110

var reducers = {
  totalInDollar: function(state, item) {
    state.dollars += item.price;
    return state;
  },
  totalInEuros : function(state, item) {
    state.euros += item.price * 0.897424392;
    return state;
  },
  totalInPounds : function(state, item) {
    state.pounds += item.price * 0.692688671;
    return state;
  },
  totalInYen : function(state, item) {
    state.yens += item.price * 113.852;
    return state;
  }
  // more...
};

var combineTotalPriceReducers = function(reducers) {
  return function(state, item) {
    return Object.keys(reducers).reduce(
      function(nextState, key) {
        reducers[key](state, item);
        return state;
      },
      {}
    );
  }
};

var bigTotalPriceReducer = combineTotalPriceReducers(reducers);

var initialState = {dollars: 0, euros:0, yens: 0, pounds: 0};

var totals = items.reduce(bigTotalPriceReducer, initialState);

console.log(totals);

/*
Object {dollars: 1130, euros: 1015.11531904, yens: 127524.24, pounds: 785.81131152}
*/

if (color) {
  if (color === 'black') {
    printBlackBackground();
  } else if (color === 'red') {
    printRedBackground();
  } else if (color === 'blue') {
    printBlueBackground();
  } else if (color === 'green') {
    printGreenBackground();
  } else {
    printYellowBackground();
  }
}

switch(color) {
  case 'black':
    printBlackBackground();
    break;
  case 'red':
    printRedBackground();
    break;
  case 'blue':
    printBlueBackground();
    break;
  case 'green':
    printGreenBackground();
    break;
  default:
    printYellowBackground();
}

function printBackground(color) {
  if (!color || typeof color !== 'string') {
    return; // Invalid color, return immediately
  }
}

switch(true) {
  case (typeof color === 'string' && color === 'black'):
    printBlackBackground();
    break;
  case (typeof color === 'string' && color === 'red'):
    printRedBackground();
    break;
  case (typeof color === 'string' && color === 'blue'):
    printBlueBackground();
    break;
  case (typeof color === 'string' && color === 'green'):
    printGreenBackground();
    break;
  case (typeof color === 'string' && color === 'yellow'):
    printYellowBackground();
    break;
}

var colorObj = {
  'black': printBlackBackground,
  'red': printRedBackground,
  'blue': printBlueBackground,
  'green': printGreenBackground,
  'yellow': printYellowBackground
};


if (color in colorObj) {
  colorObj[color]();
}

// 单个 ~
console.log(~1337)    // -1338

// 数字输入
console.log(~~47.11)  // -> 47
console.log(~~1.9999) // -> 1
console.log(~~3)      // -> 3

a = 2147483647.123  // 比32位最大正数，再多一点
console.log(~~a)    // ->  2147483647     (ok)
a += 10000          // ->  2147493647.123 (ok)
console.log(~~a)    // -> -2147483648     (huh?)

epoch_int = ~~(+new Date() / 1000)  // Date() 以毫秒计量，所以我们缩小它

// 2040年1月1日 00:00:00.123 UTC的时间戳
epoch = +new Date('2040-01-01') / 1000 + 0.123  // ->  2208988800.123

// 回到未来!
epoch_int = ~~epoch                                 // -> -2085978496
console.log(new Date(epoch_int * 1000))             // ->  Wed Nov 25 1903 17:31:44 UTC

// 这很搞笑，让我们来取得正确答案
epoch_flr = Math.floor(epoch)                       // ->  2208988800
console.log(new Date(epoch_flr * 1000))             // ->  Sun Jan 01 2040 00:00:00 UTC

console.log(~~[])   // -> 0
console.log(~~NaN)  // -> 0
console.log(~~null) // -> 0

function printUpperCase(words) {
  var elements = [].concat(words || []);
  for (var i = 0; i < elements.length; i++) {
    console.log(elements[i].toUpperCase());
  }
}

printUpperCase("cactus");
// => CACTUS
printUpperCase(["cactus", "bear", "potato"]);
// => CACTUS
//  BEAR
//  POTATO

var test = true;
var isTrue = function(){
  console.log('Test is true.');
};
var isFalse = function(){
  console.log('Test is false.');
};

// 普通的if语句
if(test){
  isTrue();    // Test is true
}

// 上面的语句可以使用 '&&' 写为：

( test && isTrue() );  // Test is true

test = false;
if(!test){
  isFalse();    // Test is false.
}

( test || isFalse());  // Test is false.

function theSameOldFoo(name){
    name = name || 'Bar' ;
    console.log("My best friend's name is " + name);
}
theSameOldFoo();  // My best friend's name is Bar
theSameOldFoo('Bhaskar');  // My best friend's name is Bhaskar

var dog = {
  bark: function(){
     console.log('Woof Woof');
   }
};

// 调用 dog.bark();
dog.bark(); // Woof Woof.

// 但是当dog未定义时，dog.bark() 将会抛出"Cannot read property 'bark' of undefined." 错误
// 防止这种情况，我们可以使用 &&.

dog&&dog.bark();   // This will only call dog.bark(), if dog is defined.

var someText = 'javascript rules';
if (someText.indexOf('javascript') !== -1) {
}

// or
if (someText.indexOf('javascript') >= 0) {
}

for (var key in obj) {
  // "reserved" exports
  if (~['name', 'prefix', 'engine', 'before'].indexOf(key)) continue;

exports.normalizeType = function(type){
  return ~type.indexOf('/')
    ? acceptParams(type)
    : { value: mime.lookup(type), params: {} };
};

// key is invalid
if (!~apiKeys.indexOf(key)) return next(error(401, 'invalid api key'));

var someText = 'text';
!!~someText.indexOf('tex'); // someText contains "tex" - true
!~someText.indexOf('tex'); // someText NOT contains "tex" - false
~someText.indexOf('asd'); // someText doesn't contain "asd" - false
~someText.indexOf('ext'); // someText contains "ext" - true

'something'.includes('thing'); // true

!!~[1, 2, 3].indexOf(1); // true
[1, 2, 3].includes(1); // true

// 定义一个数组
var list = [1, 2, 3, 4];
function empty() {
    //清空数组
    list = [];
}
empty();

var list = [1, 2, 3, 4];
function empty() {
    //empty your array
    list.length = 0;
}
empty();

var foo = [1,2,3];
var bar = [1,2,3];
var foo2 = foo;
var bar2 = bar;
foo = [];
bar.length = 0;
console.log(foo, bar, foo2, bar2);

//[] [] [1, 2, 3] []

['Shanghai', 'New York', 'Mumbai', 'Buenos Aires'].sort();
// ["Buenos Aires", "Mumbai", "New York", "Shanghai"]

// 西班牙语
['único','árbol', 'cosas', 'fútbol'].sort();
// ["cosas", "fútbol", "árbol", "único"] // bad order

// 德语
['Woche', 'wöchentlich', 'wäre', 'Wann'].sort();
// ["Wann", "Woche", "wäre", "wöchentlich"] // bad order

['único','árbol', 'cosas', 'fútbol'].sort(function (a, b) {
  return a.localeCompare(b);
});
// ["árbol", "cosas", "fútbol", "único"]

['Woche', 'wöchentlich', 'wäre', 'Wann'].sort(function (a, b) {
  return a.localeCompare(b);
});
// ["Wann", "wäre", "Woche", "wöchentlich"]

['único','árbol', 'cosas', 'fútbol'].sort(Intl.Collator().compare);
// ["árbol", "cosas", "fútbol", "único"]

['Woche', 'wöchentlich', 'wäre', 'Wann'].sort(Intl.Collator().compare);
// ["Wann", "wäre", "Woche", "wöchentlich"]

var arr = [1,2,3,4,5];
var arr2 = [];

arr.push(6);
arr[arr.length] = 6;
arr2 = arr.concat([6]);

最快的为

1. arr[arr.length] = 6; // 平均 5 632 856 ops/sec
2. arr.push(6); // 慢35.64%
3. arr2 = arr.concat([6]); // 慢62.67%

最快的为

1. arr[arr.length] = 6; // 平均42 345 449 ops/sec
2. arr.push(6); // 慢34.66%
3. arr2 = arr.concat([6]); // 慢85.79%

var arr = [1,2,3,4,5];

arr.unshift(0);

[0].concat(arr);

最快的为

1. [0].concat(arr); // 平均4 972 622 ops/sec
2. arr.unshift(0); // 慢64.70%

最快的为

1. [0].concat(arr); // 平均6 032 573 ops/sec
2. arr.unshift(0); // 慢78.65%

var items = ['one', 'two', 'three', 'four'];
items.splice(items.length / 2, 0, 'hello');

var sayHello = function({ name, surname }) {
  console.log(`Hello ${name} ${surname}! How are you?`);
};

sayHello({ name: 'John', surname: 'Smith' })
// -> Hello John Smith! How are you?

var sayHello2 = function({ name = "Anony", surname = "Moose" } = {}) {
  console.log(`Hello ${name} ${surname}! How are you?`);
};

sayHello2()
// -> Hello Anony Moose! How are you?
sayHello2({ name: "Bull" })
// -> Hello Bull Moose! How are you?

var sayHelloTimes = function({ name, surname }, times) {
  console.log(`Hello ${name} ${surname}! I've seen you ${times} times before.`);
}

sayHelloTimes({ name: "Pam" }, 5678)
// -> Hello Pam undefined! I've seen you 5678 times before.
sayHelloTimes(5678)
// -> Hello undefined undefined! I've seen you undefined times before.

sayHelloTimes()
// -> Uncaught TypeError: Cannot match against 'undefined' or 'null'...

var sayHelloTimes2 = function({ name = "Anony", surname = "Moose" } = {}, times) {
  console.log(`Hello ${name} ${surname}! I've seen you ${times} times before.`);
};

sayHelloTimes2({ name: "Pam" }, 5678)
// -> Hello Pam Moose! I've seen you 5678 times before.
sayHelloTimes2(5678)
// -> Hello Anony Moose! I've seen you undefined times before.
sayHelloTimes2()
// -> Hello Anony Moose! I've seen you undefined times before.

var sayHelloTimes2a = function({ name = "Anony", surname = "Moose" }, times) {
  console.log(`Hello ${name} ${surname}! I've seen you ${times} times before.`);
};

sayHelloTimes2a({ name: "Pam" }, 5678)
// -> Hello Pam Moose! I've seen you 5678 times before.
sayHelloTimes2a(5678)
// -> Hello Anony Moose! I've seen you undefined times before.
sayHelloTimes2a()
// -> Uncaught TypeError: Cannot match against 'undefined' or 'null'.

const _err = function( message ){

  throw new Error( message );

}

const getSum = (a = _err('a is not defined'), b = _err('b is not defined')) => a + b

getSum( 10 ) // throws Error, b is not defined

getSum( undefined, 10 ) // throws Error, a is not defined

(function() {
 // Do something​
 }
)()

(someNamedFunction = function(msg) {
	console.log(msg || "Nothing for today !!")
	}) (); // 输出 --> Nothing for today !!​
​
someNamedFunction("Javascript rocks !!"); // 输出 --> Javascript rocks !!
someNamedFunction(); // 输出 --> Nothing for today !!​

function callback() {
  console.log('Hi human');
}

document.getElementById('someelem').addEventListener('click', callback);

function callback(a, b) {
  return function() {
    console.log('sum = ', (a+b));
  }
}

var x = 1, y = 2;
document.getElementById('someelem').addEventListener('click', callback(x, y));

var alertText = function(text) {
  alert(text);
};

document.getElementById('someelem').addEventListener('click', alertText.bind(this, 'hello'));

for (var i=0; i<5; i++) {
	setTimeout(function(){
		console.log(i);
	}, 1000 * (i+1));
}

> 5
> 5
> 5
> 5
> 5

for (var i=0; i<5; i++) {
	var temp = i;
 	setTimeout(function(){
		console.log(temp);
	}, 1000 * (i+1));
}

> 4
> 4
> 4
> 4
> 4

var temp;
for (var i=0; i<5; i++) {
 	temp = i;
	setTimeout(function(){
		console.log(temp);
  	}, 1000 * (i+1));
}

for (var i=0; i<5; i++) {
	(function(num){
		setTimeout(function(){
			console.log(num);
		}, 1000 * (i+1));
	})(i);
}

for (let i=0; i<5; i++) {
	var temp = i;
 	setTimeout(function(){
		console.log(i);
	}, 1000 * (i+1));
}

let values = [2, 56, 3, 41, 0, 4, 100, 23];

let values = [2, 56, 3, 41, 0, 4, 100, 23];
let sum = values.reduce((previous, current) => current += previous);
let avg = sum / values.length;
// avg = 28

let values = [2, 56, 3, 41, 0, 4, 100, 23];
let count = values.length;
values = values.reduce((previous, current) => current += previous);
values /= count;
// avg = 28

let values = [2, 56, 3, 41, 0, 4, 100, 23];
values.sort((a, b) => a - b);
let lowMiddle = Math.floor((values.length - 1) / 2);
let highMiddle = Math.ceil((values.length - 1) / 2);
let median = (values[lowMiddle] + values[highMiddle]) / 2;
// median = 13,5

let values = [2, 56, 3, 41, 0, 4, 100, 23];
values.sort((a, b) => a - b);
let median = (values[(values.length - 1) >> 1] + values[values.length >> 1]) / 2
// median = 23

var y, x = y = 1 //== var x; var y; x = y = 1
console.log('--> 1:', `x = ${x}, y = ${y}`)

// 将会输出
//--> 1: x = 1, y = 1

;(() => {
  var x = y = 2 // == var x; y = 2;
  console.log('2.0:', `x = ${x}, y = ${y}`)
})()
console.log('--> 2.1:', `x = ${x}, y = ${y}`)

// 将会输出
//2.0: x = 2, y = 2
//--> 2.1: x = 1, y = 2

;(() => {
  var x, y = 3 // == var x; var y = 3;
  console.log('3.0:', `x = ${x}, y = ${y}`)
})()
console.log('--> 3.1:', `x = ${x}, y = ${y}`)

// 将会输出
//3.0: x = undefined, y = 3
//--> 3.1: x = 1, y = 2

;(() => {
  var y, x = y = 4 // == var x; var y; x = y = 4
  console.log('4.0:', `x = ${x}, y = ${y}`)
})()
console.log('--> 4.1:', `x = ${x}, y = ${y}`)

// 将会输出
//4.0: x = 4, y = 4
//--> 4.1: x = 1, y = 2

x = 5 // == x = 5
console.log('--> 5:', `x = ${x}, y = ${y}`)

// 将会输出
//--> 5: x = 5, y = 2

var one = 1;
var two = 2;
var three = '3';

var result = ''.concat(one, two, three); //"123"

var one = 1;
var two = 2;
var three = '3';

var result = one + two + three; //"33" instead of "123"

var one = 1;
var two = 2;
var three = '3';

var result = `${one}${two}${three}`; //"123"

[12,4,16,3].sort(function() {
    return 0.5 - Math.random();
});
//输出：[16, 12, 3, 4] 每次都不一样

function shuffle(arr) {
    var i,
        j,
        temp;
    for (i = arr.length - 1; i > 0; i--) {
        j = Math.floor(Math.random() * (i + 1));
        temp = arr[i];
        arr[i] = arr[j];
        arr[j] = temp;
    }
    return arr;
};

var a = [1, 2, 3, 4, 5, 6, 7, 8];
var b = shuffle(a);
console.log(b);
// [2, 7, 8, 6, 5, 3, 1, 4]

var me = {					// 1
	'partOf' : 'A Team'
};

function myTeam(me) {		// 2

	me = {					// 3
		'belongsTo' : 'A Group'
	};
}

myTeam(me);
console.log(me);			// 4  : {'partOf' : 'A Team'}

var me = {					// 1
	'partOf' : 'A Team'
};

function myGroup(me) { 		// 2
	me.partOf = 'A Group';  // 3
}

myGroup(me);
console.log(me);			// 4  : {'partOf' : 'A Group'}

var obj = {
    'prop1': 'value1',
    'prop2': 'value2',
    'prop3': 'value3'
};

var selectedProperties = ['prop1', 'prop2'];

var str = JSON.stringify(obj, selectedProperties);

// str
// {"prop1":"value1","prop2":"value2"}

function selectedProperties(key, val) {
    // the first val will be the entire object, key is empty string
    if (!key) {
        return val;
    }

    if (key === 'prop1' || key === 'prop2') {
        return val;
    }

    return;
}

var str = JSON.stringify(obj, selectedProperties, '\t\t');

/* str output with double tabs in every line.
{
        "prop1": "value1",
        "prop2": "value2"
}
*/

console.time("Array initialize");
var arr = new Array(100),
    len = arr.length,
    i;

for (i = 0; i < len; i++) {
    arr[i] = new Object();
};
console.timeEnd("Array initialize"); // Outputs: Array initialize: 0.711ms

!!"" // false
!!0 // false
!!null // false
!!undefined // false
!!NaN // false

!!"hello" // true
!!1 // true
!!{} // true
!![] // true

// 全脚本严格模式
"use strict";
var v = "Hi!  I'm a strict mode script!";

function f()
{

  // 方法级严格模式
  'use strict';
  function nested() { return "And so am I!"; }
  return "Hi!  I'm a strict mode function!  " + nested();
}
function f2() { return "I'm not strict."; }

// bind polyfill 示例
function bind(fn) {
  var prev = Array.prototype.slice.call(arguments, 1);
  return function bound() {
    var curr = Array.prototype.slice.call(arguments, 0);
    var args = Array.prototype.concat.apply(prev, curr);
    return fn.apply(null, args);
  };
}


// unapply攻击
function unapplyAttack() {
  var concat = Array.prototype.concat;
  Array.prototype.concat = function replaceAll() {
    Array.prototype.concat = concat; // restore the correct version
    var curr = Array.prototype.slice.call(arguments, 0);
    var result = concat.apply([], curr);
    return result;
  };
}

(function freezePrototypes() {
  if (typeof Object.freeze !== 'function') {
    throw new Error('Missing Object.freeze');
  }
  Object.freeze(Object.prototype);
  Object.freeze(Array.prototype);
  Object.freeze(Function.prototype);
}());

var args = Array.prototype.slice.call(arguments);

var args = [].slice.call(arguments);

var args = new Array(arguments.length);
for(var i = 0; i < args.length; ++i) {
  args[i] = arguments[i];
}

  typeof variable === "undefined"

  variable === null

  null == undefined // true

  null === undefined // false

function Person(name) {
  this.name = name;

  this.sayName = function() {
    console.log("Hello my name is: ", this.name);
    return this;
  };

  this.changeName = function(name) {
    this.name = name;
    return this;
  };
}

var person = new Person("John");
person.sayName().changeName("Timmy").sayName();

var firstName = 'Jake';
var lastName = 'Rawr';
console.log('My name is ' + firstName + ' ' + lastName);
// My name is Jake Rawr

var firstName = 'Jake';
var lastName = 'Rawr';
console.log(`My name is ${firstName} ${lastName}`);
// My name is Jake Rawr

var val1 = 1, val2 = 2;
console.log(`${val1} is ${val1 < val2 ? 'less than': 'greater than'} ${val2}`)
// 1 is less than 2

x += 23; // x = x + 23;
y -= 15; // y = y - 15;
z *= 10; // z = z * 10;
k /= 7; // k = k / 7;
p %= 3; // p = p % 3;
d **= 2; // d = d ** 2;
m >>= 2; // m = m >> 2;
n <<= 2; // n = n << 2;
n ++; // n = n + 1;
n --; n = n - 1;

var a = 2;
var b = a++;
// 现在 a == 3  b == 2

var a = 2;
var b = ++a;
// 现在a和b都是3

var newValue;
if(value > 10)
  newValue = 5;
else
  newValue = 2;

var newValue = (value > 10) ? 5 : 2;

if (variable1 !== null || variable1 !== undefined || variable1 !== '') {
     var variable2 = variable1;
}

var variable2 = variable1  || '';

var a = new Array();
a[0] = "myString1";
a[1] = "myString2";

var a = ["myString1", "myString2"];

var skillSet = new Array();
skillSet['Document language'] = 'HTML5';
skillSet['Styling language'] = 'CSS3';

var skillSet = {
    'Document language' : 'HTML5',
    'Styling language' : 'CSS3'
};

const handler = function () {
  console.log("Tada!")
}
element.addEventListener("click", handler)
// 之后
element.removeEventListener("click", handler)

element.addEventListener('click', function click(e) {
  if (someCondition) {
    return e.currentTarget.removeEventListener('click', click);
  }
});

function handleEvent (eventName, {onElement, withCallback, useCapture = false} = {}, thisArg) {
  const element = onElement || document.documentElement

  function handler (event) {
    if (typeof withCallback === 'function') {
      withCallback.call(thisArg, event)
    }
  }

  handler.destroy = function () {
    return element.removeEventListener(eventName, handler, useCapture)
  }

  element.addEventListener(eventName, handler, useCapture)
  return handler
}

// 你需要的时候
const handleClick = handleEvent('click', {
  onElement: element,
  withCallback: (event) => {
    console.log('Tada!')
  }
})

// 你想删除它的时候
handleClick.destroy()

if (!module.parent) {
    // 通过 `node something.js` 启动
    app.listen(8088, function() {
        console.log('app listening on port 8088');
    })
} else {
    // 通过 `require('/.something.js')` 被引用
    module.exports = app;
}

Array.apply(null, {length: N}).map(Function.call, Number);

 Array.from(new Array(N),(val,index)=>index);

Array.apply(null, {length: N}).map(function(value, index){
  return index + 1;
});

Array.from(new Array(N),(val,index)=>index+1);

function doTheThing() {
  // ReferenceError: notDeclared is not defined
  console.log(notDeclared);

  // Outputs: undefined
  console.log(definedLater);
  var definedLater;

  definedLater = 'I am defined!'
  // Outputs: 'I am defined!'
  console.log(definedLater)

  // Outputs: undefined
  console.log(definedSimulateneously);
  var definedSimulateneously = 'I am defined!'
  // Outputs: 'I am defined!'
  console.log(definedSimulateneously)

  // Outputs: 'I did it!'
  doSomethingElse();

  function doSomethingElse(){
    console.log('I did it!');
  }

  // TypeError: undefined is not a function
  functionVar();

  var functionVar = function(){
    console.log('I did it!');
  }
}

var fibonacci = function(n){
    return n < 2 ? n : fibonacci(n-1) + fibonacci(n-2);
}

var fibonacci = (function() {
  var cache = [0, 1]; // cache the value at the n index
  return function(n) {
    if (cache[n] === undefined) {
      for (var i = cache.length; i <= n; ++i) {
        cache[i] = cache[i-1] + cache[i-2];
      }
    }
    return cache[n];
  }
})()

var memoize = function(func){
    var cache = {};
    return function(){
        var key = Array.prototype.slice.call(arguments).toString();
        return key in cache ? cache[key] : (cache[key] = func.apply(this,arguments));
    }
}
fibonacci = memoize(fibonacci);

var memoize = function(func){
    const cache = {};
    return (...args) => {
        const key = [...args].toString();
        return key in cache ? cache[key] : (cache[key] = func(...args));
    }
}
fibonacci = memoize(fibonacci);

var gcd = memoize(function(a,b){
    var t;
    if (a < b) t=b, b=a, a=t;
    while(b != 0) t=b, b = a%b, a=t;
    return a;
})
gcd(27,183); //=> 3

var factorial = memoize(function(n) {
    return (n <= 1) ? 1 : n * factorial(n-1);
})
factorial(5); //=> 120

const observeConfig = {
    attributes: true,
    childList: true,
    characterData: true,
    subtree: true
};

function initExtension(rootElement, targetSelector, subTargetSelector) {
    var rootObserver = new MutationObserver(function(mutations) {
        console.log("Inside root observer");
        targetElement = rootElement.querySelector(targetSelector);
        if (targetElement) {
            rootObserver.disconnect();
            var elementObserver = new MutationObserver(function(mutations) {
                console.log("Inside element observer")
                subTargetElement = targetElement.querySelector(subTargetSelector);
                if (subTargetElement) {
                    elementObserver.disconnect();
                    console.log("subTargetElement found!")
                }
            })
            elementObserver.observe(targetElement, observeConfig);
        }
    })
    rootObserver.observe(rootElement, observeConfig);
}

(function() {

    initExtension(document.body, "div.target", "div.subtarget")

    setTimeout(function() {
        del = document.createElement("div");
        del.innerHTML = "<div class='target'>target</div>"
        document.body.appendChild(del)
    }, 3000);


    setTimeout(function() {
        var el = document.body.querySelector('div.target')
        if (el) {
            del = document.createElement("div");
            del.innerHTML = "<div class='subtarget'>subtarget</div>"
            el.appendChild(del)
        }
    }, 5000);

})()

Math.max(1, 2, 3, 4); // 4
Math.min(1, 2, 3, 4); // 1

var numbers = [1, 2, 3, 4];
Math.max.apply(null, numbers) // 4
Math.min.apply(null, numbers) // 1

var numbers = [1, 2, 3, 4];
Math.max(...numbers) // 4
Math.min(...numbers) // 1

var myArray = [[1, 2],[3, 4, 5], [6, 7, 8, 9]];

[1, 2, 3, 4, 5, 6, 7, 8, 9]

var myNewArray = [].concat.apply([], myArray);
// [1, 2, 3, 4, 5, 6, 7, 8, 9]

var myNewArray = myArray.reduce(function(prev, curr) {
  return prev.concat(curr);
});
// [1, 2, 3, 4, 5, 6, 7, 8, 9]

var myNewArray3 = [];
for (var i = 0; i < myArray.length; ++i) {
  for (var j = 0; j < myArray[i].length; ++j)
    myNewArray3.push(myArray[i][j]);
}
console.log(myNewArray3);
// [1, 2, 3, 4, 5, 6, 7, 8, 9]

var myNewArray4 = [].concat(...myArray);
console.log(myNewArray4);
// [1, 2, 3, 4, 5, 6, 7, 8, 9]

const nodelist = document.querySelectorAll('div');
const nodelistToArray = Array.apply(null, nodelist);


//之后 ..

nodelistToArray.forEach(...);
nodelistToArray.map(...);
nodelistToArray.slice(...);


//等...

const nodelist = document.querySelectorAll('div');
const nodelistToArray = Array.prototype.slice.call(nodelist); // or equivalently Array.prototype.slice.apply(nodelist);

//之后 ..

nodelistToArray.forEach(...);
nodelistToArray.map(...);
nodelistToArray.slice(...);

//等...

const nodelist = [...document.querySelectorAll('div')]; // 返回一个真正的数组

//之后 ..

nodelist.forEach(...);
nodelist.map(...);
nodelist.slice(...);


//等...

console.log(data.value) && false

function funcVariable(){
  var a = "码农题库";
}
console.log(funcVariable + '');

[10] ==  10      // 为 true
[10] === 10      // 为 false

'10' ==  10      // 为 true
'10' === 10      // 为 false

 []  ==  0       // 为 true
 []  === 0       // 为 false

 ''  ==  false   // 为 true 但 true == "a" 为false
 ''  === false   // 为 false

// 箭头函数的一般语法
param => expression

// 也可以用用小括号
// 多参数时小括号是必须的
(param1 [, param2]) => expression


// 使用functions
var arr = [5,3,2,9,1];
var arrFunc = arr.map(function(x) {
  return x * x;
});
console.log(arr)

// 使用箭头函数
var arr = [5,3,2,9,1];
var arrFunc = arr.map((x) => x*x);
console.log(arr)

// 全局定义 this.i
this.i = 100;

var counterA = new CounterA();
var counterB = new CounterB();
var counterC = new CounterC();
var counterD = new CounterD();

// 不好的例子
function CounterA() {
  // CounterA的`this`实例 (!!调用时忽略了此实例)
  this.i = 0;
  setInterval(function () {
    // `this` 指向全局(global)对象,而不是CounterA的`this`
    // 所以从100开始计数,而不是0 (本地的this.i)
    this.i++;
    document.getElementById("counterA").innerHTML = this.i;
  }, 500);
}

// 手动绑定 that = this
function CounterB() {
  this.i = 0;
  var that = this;
  setInterval(function() {
    that.i++;
    document.getElementById("counterB").innerHTML = that.i;
  }, 500);
}

// 使用 .bind(this)
function CounterC() {
  this.i = 0;
  setInterval(function() {
    this.i++;
    document.getElementById("counterC").innerHTML = this.i;
  }.bind(this), 500);
}

// 箭头函数
function CounterD() {
  this.i = 0;
  setInterval(() => {
    this.i++;
    document.getElementById("counterD").innerHTML = this.i;
  }, 500);
}

var myObject = {
	z: 1,
	'@': 2,
	b: 3,
	1: 4,
	5: 5
};
console.log(myObject) // Object {1: 4, 5: 5, z: 1, @: 2, b: 3}

for (item in myObject) {...
// 1
// 5
// z
// @
// b

var myObject = new Map();
myObject.set('z', 1);
myObject.set('@', 2);
myObject.set('b', 3);
for (var [key, value] of myObject) {
  console.log(key, value);
...
// z 1
// @ 2
// b 3