es2015(es6) Core features

• From var to const/let (var 声明的全局变量，会加入 window 对象，千万不要用 var！)
• From IIFEs(函数包裹作用域,立即执行函数) to blocks({})
• From concatenating strings(拼接字符串 +) to template literals(模版字符串`${}`)
• From function expressions to arrow functions
• Multiple return values via(通过) objects 解构赋值
• From for to forEach() to for-of 数组遍历优先级 for=>forEach=>for of
• Handling parameter default values 函数申明默认参数
• Handling named parameters

function selectEntries(options) {
    options = options || {}; // (A)
    var start = options.start || 0;
    var end = options.end || -1;
    var step = options.step || 1;
    ···
}
to 
function selectEntries({ start=0, end=-1, step=1 } = {}) {
    ···
}

• From arguments to rest parameters
• From apply() to the spread operator (…)
• From concat() to the spread operator (…)
• From function expressions in object literals to method definitions

var obj = {
    foo: function () {
        ···
    },
    bar: function () {
        this.foo();
    }, // trailing comma is legal in ES5
}
to
const obj = {
    foo() {
        ···
    },
    bar() {
        this.foo();
    },
}

• From constructors to classes
• From custom error constructors to subclasses of Error
• From objects to Maps
• New string methods
• New Array methods
• From CommonJS modules to ES6 modules

语言进化

• module and class default is use strict 类和模块隐式为严格模式;