flarnie / js-notes

Notes on JavaScript

js-notes

##Ruby to JavaScript

Here are some key differences between our beloved Ruby and it's best friend, JavaScript.

(Note: There are more similarities than differences! For example, they deal with scope in very similar ways.
The 'Scope of variables' concept noted in this table is one of the only differences in the way they deal with scope.)

Concepts	in Ruby	in JavaScript
Is everything an object?	you have Strings, Numbers, booleans, etc., and they are all objects	strings, numbers, booleans, 'undefined' and 'null' are primitives, not objects
Truthy and Falsy	Only 'nil' and 'false' are falsy	'undefined', 'null', 'NaN' ("not a number"), 0, and '' are all falsy values in JS
Local vs. Global Variables	Globals require special '$' sign	Globals happen when you forget 'var'
Referencing current instance	'self' is current instance anywhere within the class and it's methods	'this' refers to the thing that the method was called on.
Type comparisons	"5" == 5 returns false	"5" == 5 returns true .. to "strictly compare" try "5" === 5 (returns false) or "5" !== 5 (returns true)
Implicit return?	Without a 'return' statement, a function will return the last value listed.	Without 'return statement, a function will return 'undefined'.
Scope of variables	Referencing a variable that is not defined inside the method (or it's containing method) will raise an error.	You may use variables defined in the global scope, (outside of the function and it's containing function),without passing them in as arguments. (a.k.a. Lexical Scoping)
Wrong number of Arguments	Raises an ArgumentError	Silently ignores extra arguments or gives missing arguments the value 'undefined', arguments can also be access with 'arguments' keyword.
Storing key/value pairs	You call this a 'hash' in ruby.	'Object' type variables do this in JS. Even better, build your own dictionary type constructor.
String Interpolation	Easy string interpolation	No string interpolation- you must concatenate.
Conventions for Naming/Capitalization	Classes are CamelCase, and most other things are snake_case	Most everything is camelCase, and constructors are Capitalized

###Syntax for similar operations:

####String Slicing

In Ruby:

word = "Hello World."
word[2..3]
# OR
word[2, 2]
#returns "ll"

In JavaScript:

var word = "Hello World.";
//(Yes I do put a ; at the end of every line)
word.slice(2,4);
//returns "ll"

####Defining a 'block' of code a.k.a. anonymous functions

In Ruby:

Proc.new { |n| n * n }

In JavaScript:

function (n) {
  return n * n;
};

####Defining and calling a named method/function

In Ruby:

#define it
def the_name(n)
  n * n
end

#call it
the_name(12);

In JavaScript:

//define it    
var the_name = function(n) {
  return n * n;
};

//OR
function the_name(n) {
  return n * n;
};


//call it
the_name(12);

####If/Else

In Ruby:

if 2 + 2 == 4
  result = "Sane world."
elsif 2 + 2 == 5
  result = "Very large values of 2."
else
  result = "Not sane world."
end

In JavaScript:

if (2 + 2 == 4) {
  var result = "Sane world."; }
else if (2 + 2 == 5) {
  var result = "Very large values of 2."; }
else {
  result = "Not sane world."; }
end

####String and Int conversion

In Ruby:

"5".to_i
#becomes the number 5

5.to_s
#becomes the string "5"

In JavaScript:

parseInt("5", 10);
//becomes the number 5
//the 10 specifies base 10

(5).toString();
//becomes the string "5"

####Push value to an array

In Ruby:

[] << "something"

#OR

[].push("something")

In JavaScript:

var arr = [];
arr.push("something");
// no "shovel" operator : (

####Default values for arguments

In Ruby:

def add_n_or_1(x, n = 1)
  x + n
end

In JavaScript:

//you must specifically check that it is undefined
function add_n_or_1(x, n) {
  n = typeof n !== 'undefined' ? n : 1;
  return x + n;
};

//OR in some cases this works
function add_n_or_1(x, n) {
  if (arguments.length == 1) {
    n = 1;
  return x + n; }
};

####Print statements for debugging

In Ruby:

puts "Help!"
#prints to the terminal or (in RoR) the session log

In JavaScript:

console.log("Help!");
//prints to the console in the browser; use devtools to see it.

####The semicolon

Use it to end every statement in JavaScript. What qualifies as a statement in Javascript? If you would put it on a newline in Ruby, OR if it is the end of a 'block' of code then it should probably end with a semicolon.

"In some cases, JavaScript allows you to omit the semicolon at the end of a statement. In other cases, it has to be there or strange things will happen. The rules for when it can be safely omitted are complex and weird. In this book, I won't leave out any semicolons, and I strongly urge you to do the same in your own programs." -EloquentJS, ch. 2

####Style Guide for JS

JavaScript style conventions are listed in this guide by Douglas Crockford.