var let const

var price = 100;
price = 200;
console.log(price);     //200,var声明的变量可重新赋值
var price = 300;
console.log(price);    //300,var声明的变量可重复声明

function getPrice(){
    var price = 100;
    console.log(price);
}
getPrice();       //100,函数内部访问变量
console.log(price);      //price is not defined,var声明的变量是function scope

var price = 100;
var count = 10;
if( count > 5 ){
    var discount = price * 0.6;
}
console.log(discount);        //60,没有function，discount是全局变量

var price = 100;
var count = 10;
if( count > 5 ){
    let discount = price * 0.6;     
}
console.log(discount);   //discount is not defined,let和const是block scope，这里使用const也是相同结果

let count = 10;
count = 20;
console.log(count);    //20,let声明的变量可重新赋值
let count = 20;     //Identifier 'count' has already been declared,let声明的变量不可以重复声明
let discount = 0.9;           //全局作用域下的变量
if( count > 5 ){
    let discount = 0.5;            //block中的变量
}
console.log(discount);     //0.9,获取全局变量

const key = '123';
key = '1234';      //Assignment to constant variable,const声明的变量不可重新赋值

const person = {
    name: 'Asher',
    age: 5
}
person = {
    name: 'allen',
    age: 2
}
cosnole.log(person);    //Assignment to constant variable,指针指向新的地方了，不可以
const person = {
    name: 'Asher',
    age: 5
}
person.age = 6;
console.log(person);        //{name: "Asher", age: 6},const晟敏的对象可以修改属性

//let使用场景
for(var i=0;i<5;i++){
    console.log(i);
    setTimeout(function(){
        console.log(i);          //输出5个5
    },1000)
}

for(let i=0;i<5;i++){
    console.log(i);
    setTimeout(function(){
        console.log(i);          //输出01234
    },100)
}

console.log(color);
var color = 'blue';         //undefined,变量提升

//ES6中建议使用var/let/const的规则
//默认使用const
//当变量需要重新绑定或更新时使用let
//尽量不使用var
console.log(color);
let color = 'blue';         //color is not defined,暂时性死区，let声明不会被提升到当前执行上下文的顶部，const也同样