let和var的区别

let和var的区别

1 let声明的变量有块作用域

if(!'a' in window){var a=1} console.log(a);
undefined //变量提升

if(!'a' in window){let a=1} console.log(a)
VM75:1 Uncaught ReferenceError: a is not defined
    at <anonymous>:1:41 //a只存在于块作用域内

2 let声明的全局变量不是全局对象的属性

let a=123
window.a
//undefined
var b=345
window.b
//345

3 重复定义let变量会报错

let x=123
var x=123
//VM217:1 Uncaught SyntaxError: Identifier 'x' has already been declared
    at <anonymous>:1:1
var y=345
let y=345
//VM245:1 Uncaught SyntaxError: Identifier 'y' has already been declared
    at <anonymous>:1:1
var z=678
var z=789
z
//789

4 for循环中的let每次迭代时都为i创建新的绑定。
var声明全局变量i，循环内过度共享i变量

for (var i = 0; i < 5; i++) {
  setTimeout(function () {
    console.log(i);
  }, i * 1500);
}

let声明的变量每次迭代都为i创建新的绑定

for (let i = 0; i < 5; i++) {
  setTimeout(function () {
    console.log(i);
  }, i * 1500);
}