# var、let、const区别？

#### 重复声明

```javascript
var a = 1;
var a = 2;

let b = 1;
let b = 2; //Identifier 'b' has already been declared

const c = 1;
const c = 2; //Identifier 'c' has already been declared
```

#### 变量提升

```javascript
console.log(a);
var a = 1;

console.log(b);  //Uncaught ReferenceError:Cannot access 'b' before initialization
let b = 1;

console.log(c);  //Uncaught ReferenceError:Cannot access 'c' before initialization
const c = 2;
```

#### 顶层对象

```javascript
var a = 1;
console.log(window.a)  //1

let b = 1;
console.log(window.b)  //undefined

const c = 1;
console.log(window.c)  //undefined
```

#### 重新赋值

```javascript
var a = 'hello';
a = 'world';
console.log(a)   //'world'

let a = 'hello';
a = 'world';
console.log(a)   //'world'

const a = 'hello';
a = 'world';   //Uncaught TypeError: Assignment to constant variable.

const a = {x:'hello'}
a.y = 'world'
console.log(a)   //{x:'hello',y:'world'}
```

#### 块级作用域

```javascript
{ 
  var i = 9;
} 
console.log(i);  // 9

{ 
  let i = 9;     // i变量只在 花括号内有效！！！
} 
console.log(i);  // Uncaught ReferenceError: i is not defined
```

最常见使用场景

```javascript
for (var i = 0; i <10; i++) {  
  setTimeout(function() {  // 同步注册回调函数到 异步的 宏任务队列。
    console.log(i);        // 执行此代码时，同步代码for循环已经执行完成
  }, 0);
}
// 输出结果
10   共10个

// i虽然在全局作用域声明，但是在for循环体局部作用域中使用的时候，变量会被固定，不受外界干扰。
for (let i = 0; i < 10; i++) { 
  setTimeout(function() {
    console.log(i);    //  i 是循环体内局部作用域，不受外界影响。
  }, 0);
}
// 输出结果：
0  1  2  3  4  5  6  7  8 9
```

### 总结

|       | var | let | const            |
| ----- | --- | --- | ---------------- |
| 重复声明  | ✓   | ✕   | ✕                |
| 变量提升  | ✓   | ✕   | ✕                |
| 顶层对象  | ✓   | ✕   | ✕                |
| 重新赋值  | ✓   | ✓   | ✕（基础类型不可，引用类型可以） |
| 块级作用域 | ✕   | ✓   | ✓                |


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