<!DOCTYPE html>
<html>
<head>
<meta charset="UTF-8">
<title></title>
</head>
<body>
<script src="js/browser.min.js" type="text/javascript" charset="utf-8"></script>
<script type="text/javascript">
{
let a = 10;
var b = 1;
}
//console.log(a); //报错
console.log(b); //1
for(let i = 0; i < 10 ; i++){}
//报错console.log(i); //报错
var a = [];
for(var i = 0 ; i < 10 ; i++){
a[i] = function(){
console.log(i);
}
}
a[6](); //10
//let 版本
for(let i = 0 ; i < 10; i++){
a[i] = function(){
console.log(i);
}
}
a[6](); //6
//由于i是let生命的，当前的i只在本路循环有效。所以每一次循环的i其实都是一个新的值，所以输出6；
//不存在变量提升
//console.log(foo); //referunceError;
//typeof foo; //typof 不在安全
let foo = 2;
//暂时性死区
//只要块级作用域存在let命令，他声明的变量就绑定这个区域，不受外界影响
var tmp = 123;
if(true){
//tmp = ‘abc‘; //referernceError
let tmp;
}
//暂时性死区
if(true){
//死区开始
//tmp2 = ‘abc‘; //ReferenceError
//console.log(tmp2); //ref
let tmp2; //死区结束
console.log(tmp2);
tmp2 = 123;
console.log(tmp2);
}
function bar(x = y , y = 2){
return [x , y];
}
//由于x = y，而y没有声明，属于死区
//bar(); //y is not defined
function bar2(x = 2 , y = x){
return [x , y];
}
console.log(bar2()); //[2,2]
//不允许重复声明
if(true){
let a = 10;
//var a = 1; //has oready been declared
}
if(true){
let a = 10;
//let a = 10; //baocuo
}
//不许再函数内部重新声明参数
var func = function(args){
let args; //报错
}
func(1)
var func2 = function(args){
{
let args; //不报错
}
}
func2(1);
</script>
</body>
</html>
时间： 2024-08-25 05:44:31