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JavaScript变量提升 面试题 

<!DOCTYPE html>
<html lang="zh">

    <head>
        <meta charset="UTF-8" />
        <meta name="viewport" content="width=device-width, initial-scale=1.0" />
        <meta http-equiv="X-UA-Compatible" content="ie=edge" />
        <title>JavaScript变量提升 面试题</title>
    </head>

    <body>
        <script type="text/javascript">
            //函数提示优于变量
            console.log(a);//function a(){}
            console.log(b);//undefined
            var a = 1;
            function a(){}
            var b= function(){};
            console.log(a)//1
        </script>
    </body>

</html>

 面试题2：

var a = 10;
(function () {
    console.log(a)
    a = 5
    console.log(window.a)
    var a = 20;
    console.log(a)
})()

 

解释：

执行解析步骤：
var a = undefined;
a = 10;
(function () {
// 变量提升（预解析）
var a = undefined;
console.log(a); // 输出undefined
a = 5;
console.log(window.a); // 找window(全局)对象的a, 输出10
a = 20;
console.log(a); // 输出20
})()

 

posted @ 2018-03-29 16:17  孟繁贵  阅读(3170)  评论(0)    收藏  举报
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