ES6可以替代ES5的方法

let 声明

• for 循环，使用 let 声明变量在块级作用域内

for (let i = 0; i < a.length; i++) { let x = a[i] … } 
for (let i = 0; i < b.length; i++) { let y = b[i] … } 
let callbacks = [] 
for (let i = 0; i <= 2; i++) { callbacks[i] = function () { return i * 2 } }
callbacks[0]() === 0 
callbacks[1]() === 2 
callbacks[2]() === 4
ECMAScript 5 
var i, x, y; 
for (i = 0; i < a.length; i++) { x = a[i]; … } 
for (i = 0; i < b.length; i++) { y = b[i]; … } 
var callbacks = []; 
for (var i = 0; i <= 2; i++) { (function (i) { callbacks[i] = function() { return i * 2; }; })(i); } 
callbacks[0]() === 0; 
callbacks[1]() === 2; 
callbacks[2]() === 4;

箭头函数

• 箭头函数 =>

odds  = evens.map(v => v + 1) 
pairs = evens.map(v => ({ even: v, odd: v + 1 })) 
nums  = evens.map((v, i) => v + i)
ECMAScript 5
odds  = evens.map(function (v) { return v + 1; }); 
pairs = evens.map(function (v) { return { even: v, odd: v + 1 }; }); 
nums  = evens.map(function (v, i) { return v + i; });

扩展运算符

• 扩展运算符替换 concat

var params = [ "hello", true, 7 ]
var other = [ 1, 2, ...params ] // [ 1, 2, "hello", true, 7 ]

var params = [ "hello", true, 7 ];
var other = [ 1, 2 ].concat(params); // [ 1, 2, "hello", true, 7 ]

• 扩展运算符 … 为 function 赋值

function f (x, y, ...a) { return (x + y) * a.length }
f(1, 2, ...params) === 9

function f (x, y) {
var a = Array.prototype.slice.call(arguments, 2);
return (x + y) * a.length; };
f.apply(undefined, [ 1, 2 ].concat(params)) === 9;

• 扩展运算符 … 代替 split

var str = "foo";
var chars = str.split(""); // [ "f", "o", "o" ]

const chars = [ ...str ] // [ "f", "o", "o" ]

const chars = Array.from(str);  // ["f", "o", "o"]

• 找出最大、最小值

const arr = [2, 8, 15, 4];
Math.max(...arr); // 15
Math.min(...arr); // 2

解构赋值

function f (x, y = 7, z = 42) { return x + y + z }
f(1) === 50

function f (x, y, z) {
if (y === undefined) y = 7; if (z === undefined) z = 42;
return x + y + z; };
f(1) === 50;

let x, y = 20;

let x;
let y = 20;

let [a, b, c] = [5, 8, 12];

let a, b, c;
a = 5;
b = 8;
c = 12;

obj = { x, y }

obj = { x: x, y: y };

var customer = { name: "Foo" }

var card = { amount: 7, product: "Bar", unitprice: 42 }

var message = `Hello ${customer.name}, want to buy ${card.amount} ${card.product} for a total of ${card.amount * card.unitprice} bucks?`

var message = "Hello " + customer.name + ",\n" + "want to buy " + card.amount + " " + card.product + " for\n" + "a total of " + (card.amount * card.unitprice) + " bucks?";