12.【原創】ES6常用的新語法

一、變量聲明let、const

/**
 * @變量聲明
 */
"use strict"
// 1.變量越域
{
    var a = 1;
    let b = 2;
}
console.log(a);// 1
console.log(b);// b is not defined

// 2.不可多次聲明同一變量
var x = 0;
var x = 1;

let y = 2;
let y = 3;
console.log(x);// Identifier 'y' has already been declared

// 3.var變量提升
console.log(m);
var m = 10;// undefined
console.log(n); // Cannot access 'n' before initialization
let n = 10;

// 4.常量const,必須初始化，且不可改值
const k = 5;
k = 6;// Assignment to constant variable.

　　

二、解構表達式

三、函數優化

四、對象優化

五、Promise