var,let,const三种声明方式的区别

//使用var定义
    var a = ‘hello World‘;
    function bb(){
    var a = ‘hello Bill‘;
    console.log(a); 
    }
    bb() // ‘hello Bill‘
    console.log(a); // ‘hello world‘

var a = 100
function aa() {
    a = 10
    console.log(a)
}
aa()  

var temp = 100;
console.log(temp, window.temp); // 100 100


let temp1 = 10;
console.log(temp1, window.temp1); // 10 undefined


const temp2 = 1;
console.log(temp2, window.temp2); // 1 undefined

console.log(s); // undefined ===> a已声明还没赋值，默认得到undefined值
var a = 100;


console.log(b); // 报错：b is not defined ===> 找不到b这个变量
let b = 10;

if(1){
var a = 100;
let b = 10;
}
console.log(a); // 100
console.log(b) // 报错：b is not defined ===> 找不到b这个变量


if(1){
var a = 100;
const c = 1;
}
console.log(a); // 100
console.log(c) // 报错：c is not defined ===> 找不到c这个变量

const a = 100;

const list = [];
list[0] = 10;
console.log(list);　　// [10]


const obj = {a:100};
obj.name = ‘apple‘;
obj.a = 10000;
console.log(obj);　　// {a:10000,name:‘apple‘}