JS的undefined(未定义的)数据类型 

<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>Title</title>
    <script>
        // undefined场景1
        var a;
        console.log(a,typeof a) // undefined 'undefined' #未定义的

        // undefined场景2
        function foo() {

        }
        
        var ret = foo()
        console.log(ret) // undefined


    </script>
</head>
<body>
<h1>welcome to JS!</h1>
</body>
</html>

 

posted @ 2022-06-01 19:54  呼长喜  阅读(72)  评论(0)    收藏  举报