es6中的Let和Const详解（文末有教程分享）

for (var i = 0; i < 5; i++) {
    console.log(i) // 0 1 2 3 4
}
console.log(i) // 5 i成了全局变量
 
==============
 
for (let j = 0; j < 5; j++) {
    console.log(j) // 0 1 2 3 4
}
// let定义的j变量只在for循环的块级作用域中生效，不存在变量提升
console.log(j) // Uncaught ReferenceError: j is not defined

var i = 5;
(function () {
  console.log(i) // undefined
  var i = 10
})()
 
===============
 
let j = 55;
(function () {
  console.log(j) // ReferenceError: j is not defined
  let j = 77
})()

(function hhh() {
  console.log(j) // j的临时性死区 Uncaught ReferenceError: j is not defined
  let j = 77 // 从这个函数的开始部分到这里，都是新的j的临时性死区
})()

let zxx = 'zxx is a great girl'
let zxx = 'zxx is 18 years old' // Uncaught SyntaxError: Identifier 'zxx' has already been declared
 
=================
 
let zxx3 = 'zxx is a great girl'
var zxx3 = 'zxx is 18 years old' // Uncaught SyntaxError: Identifier 'zxx3' has already been declared
 
==============
{
let zxx3 = 'zxx is a great girl'
var zxx4 = 'zxx is 18 years old'
}
 
{
let zxx3 = 'zxx is a good girl'
var zxx4 = 'zxx is 8 years old'
}
// 不会报错，各个块级作用域之间不互相影响

const b; // Uncaught SyntaxError: Missing initializer in const declaration

const ZXX = 'zxx is a great girl'
ZXX = 'NO' // Uncaught TypeError: Assignment to constant variable.

const ZXX = {
  age: 18,
  name: 'zxx'
}
ZXX ==> {age: 18, name: "zxx"}
ZXX.age = 8 {age: 8, name: "zxx"}
ZXX.weight = 90 {age: 8, name: "zxx", weight: 90}
 
ZXX = {} // Uncaught TypeError: Assignment to constant variable.
// 修改了ZXX的指针，这是不允许的