var vs let 与 const

var 与 let (ES6新增) 都是用来声明变量。

变量提升

var 存在变量提升

console.log(a); // 输出undefined
var a = 2;

let 不存在变量提升

console.log(b); // Uncaught ReferenceError: Cannot access 'a' before initialization
let b = 2;

总结:let 先声明在使用，var不受限制

作用域

var 全局作用域

if(true){
  var a = 12
}
console.log(a); //12

let 块级作用域

即：声明所在的作用域

let b =12
if(true){
  console.log(b,"全局作用域"); //输出12
  let a = 123
  console.log(a,"if 块级作用域") //输出123
}
console.log(a,"作用域外访问if块级作用域") // Uncaught ReferenceError: a is not defined

这里有一个问题就是“暂时性死区”,简称“TDZ”

f (true) {
  a = 'abc'; // ReferenceError
  console.log(a); // ReferenceError

  let a; // TDZ结束
  console.log(a); // undefined

  a = 123;
  console.log(a); // 123
}

重复声明

var 可以重复声明

var a = 123
var a = 456
//输出456

let 同一作用域不可以重复声明

let b = 123
let b = 456
//输出Uncaught SyntaxError: Identifier 'b' has already been declared

//不同作用域可以重复声明
if(true){
  let a = 12
  console.log(a); //12
  if(true){
    let a = 123
    console.log(a); //13
  }
}

const 声明只读常量,值不能改变。

不存在变量提升

console.log(a,"a==="); //输出Uncaught ReferenceError: Cannot access 'a' before initialization
const a = 12;

块级作用域

const b = 12; //全局作用域
if(true){
  const a = 12; //if 块级作用域
  console.log("b",b);
}
console.log("a",a); //Uncaught ReferenceError: a is not defined

同一作用域不可以重复声明

const a = 12; 
const a = 123
console.log("a",a); //Uncaught SyntaxError: Identifier 'a' has already been declared

声明必须赋值

const a  //Uncaught SyntaxError: Missing initializer in const declaration