Rubyの概念でecmascript6の新機能をまとめる

##式展開

Ruby

cat = "maru"
p "こんにちは#{cat}ちゃん" #=> "こんにちはmaruちゃん"

「`」（バッククオート）で文字列を囲うことで、文字列で変数を使用

JavaScript

var cat = 'maru';
console.log(`こんにちは${cat}ちゃん`); //=> こんにちはmaruちゃん

##Enumerator#each

Ruby

[1, 2, 3].each do |val|
  p val
end #=> 1, 2, 3

JavaScript

//es6
for (let v of [1, 2, 3]) {
  console.log(v) //=> 1, 2, 3
}

//es5
[1, 2, 3].forEach(val => {
  console.log(val); //=> 1,2,3
});

##アロー記法(Rubyのラムダ式)

Ruby

foo = ->(x) { x * x }
foo[5] #=> 25

JavaScript

var foo = ( x ) => { return x * x };
console.log(foo(5)) // => 25

##デフォルト引数

Ruby

def even(value = 10)
  (1..value).each do |val|
    if val.even?
      p val #=> 2,4,6,8,10
    end
  end
end

even

JavaScript

function even(count = 10) {
  for (var i = 1; i < count; i++) {
    if(i % 2 === 0){
      console.log(i) //=> 2,4,6,8,10
    }
  }
}

even();

##可変長引数

Ruby

def cat(*nya)
  p nya
end

cat(["punch", "punch", "punch"])
#=> "punch", "punch", "punch"

JavaScript

function cat(...nya) {
  console.log(nya);
}

cat('punch', 'punch', 'punch');
//=> [ 'punch', 'punch', 'punch' ]

##ブロックスコープ

Ruby

integer = "cat"

[1, 2, 3, 4, 5].each do |val|
  integer = val
  p integer #=> 1,2,3,4,5
end

puts integer #=> "cat"

JavaScript

var integer = 'cat';

[1,2,3,4,5].forEach(val => {
  let integer = val;
  console.log(integer); //=> 1,2,3,4,5
});

console.log(integer); //=> 'cat'

##与えられたオブジェクトを要素とする集合

Ruby

require 'set'

set = Set[1,2,3,5,1]
p set #=> #<Set: {1, 2, 3, 5}>

set.add(3)
set.add(10)
p set #=> #<Set: {1, 2, 3, 5, 10}>

JavaScript

var set = new Set([1,2,3,5,1]);

console.log(set) //=> Set {1, 2, 3, 5}

##Hashオブジェクト

Ruby

my_map = { one: "lion", two: "gorilla", three: "rabbit" }

p my_map #=> { :one => "lion", :two => "gorilla", :three => "rabbit" }

JavaScript

var myMap = new Map();
myMap.set('one', "lion")
myMap.set('two', "gorilla")
myMap.set('three', "rabbit")
console.log(myMap); //=> Map {"one" => "lion", "two" => "gorilla", "three" => "rabbit"}

##定数

Ruby

HOGE = 7
p HOGE #=> 7

JavaScript

const hoge = 7;
var hoge = 11; //=> Duplicate declaration "hoge"
console.log(hoge); //=> 7