Front-end JS classic: differences between let, const, and var

1. Scope

if (true) {
  // var出来的变量是全局的，但是不能跨函数访问
  var a = 0;
  let b = 0;
  const c = 0;
}
console.log(a); // 0
console.log(b); // b is not defined
console.log(c); // c is not defined

2. Variable promotion

console.log(a); // 1
console.log(b); // b is not defined
console.log(c); // c is not defined

var a = 1;
let b = 2;
const c = 3;

3. Global properties

var a = 1;
let b = 2;
const c = 3;

console.log(window.a); // 1
console.log(window.b); // undefined
console.log(window.c); // undefined

4. Initial value

// var, let 不用设置初始值
// const 必须设置初始值