let 和 var 的区别

1、作用域只局限于当前代码块

if(true){
    var str = 1
    let str1 = 2
    console.log(str)
    console.log(str1)
}
console.log(str)
console.log(str1)// str1 is not defined

2、let声明的作用于不会被提升

console.log(str)
var str = 1
console.log(str1) // str1 is not defined
let str1 = 2

3、同一作用域下不能定义相同的变量

var str = 1
var str = 11
console.log(str)//覆盖上一个定义的变量
let str1 = 2
let str1 = 22
console.log(str1)//'str1' has already been declared

4、for循环体现let的父子作用域

<body>
<button>按钮1</button>
<button>按钮2</button>
<button>按钮3</button>
<button>按钮4</button>
<button>按钮5</button>
<script>
    window.onload = function () {
        var btns = document.querySelectorAll("button")
        for(var i = 0; i<btns.length; i++){
            btns[i].onclick= function(){
                console.log('第'+i+'个按钮');
            }
        }
        /*解决方法1
        var btns = document.querySelectorAll("button")
        for (var i = 0; i < btns.length; i++) {
            (function (i) {
                btns[i].onclick = function () {
                    console.log('第' + i + '个按钮');
                }
            })(i)
        }*/
        /*解决方法2
        let btns = document.querySelectorAll("button")
        for(let i = 0; i<btns.length; i++){
            btns[i].onclick= function(){
                console.log('第'+i+'个按钮');
            }
        }*/
    }
</script>
</body>
posted @ 2023-06-28 10:01  路暝月  阅读(28)  评论(0)    收藏  举报  来源