关于javascript:varlet的区别

var申明晋升

console.log(name)  //土豆
var name = '土豆';

应用var申明的变量会主动晋升到函数作用域的顶部

function foo(){
    console.log(name);
    var name = "土豆";
}
foo()  //undefined

//等价于

function foo(){
    var name;
    console.log(name);
    name = "土豆";
}
foo(); //undefined

console.log(name)  //ReferenceError name没有被定义
let name ="土豆";

而应用let定义的name不会被晋升，报出ReferenceError的谬误；

var能够反复申明同一个变量

var name = "番茄";
var name = "西瓜";
var name = "土豆";
console.log(name);//土豆

而应用let反复申明同一个变量会报错。

var name = "土豆";
let name = "土豆";
console.log(name);//'name' has already been declared

let name = "土豆";
var name = "土豆";

console.log(name);//Cannot redeclare block-scoped variable 'name'.

var全局申明

var name = "土豆";
console.log(window.name);//土豆

let age = "24";
console.log(window.age);  //undefined

应用var定义的变量会被挂载到window上，成为window对象的属性，而应用let不会。

作用域

if(true){
    var name = "土豆";
    console.log(name);
}
console.log(name);
//土豆
//土豆

if(true){
    let age = 24;
    console.log(age);
}
console.log(age);
//24
//undefined

应用var申明的范畴是函数作用域，应用let申明的范畴是块作用域。