【代码笔记】Web-Javascript-Javascript typeof

一，效果图。

二，代码。

 

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<!DOCTYPE html>
<html>

<head>
    <meta charset="utf-8">
    <title>JavaScript typeof, null, 和 undefined</title>
</head>

<body>
    <p> typeof 操作符返回变量或表达式的类型。</p>
    <p id="demo"></p>
    <script>
    document.getElementById("demo").innerHTML =
        typeof "john" + "<br>" +
        typeof 3.14 + "<br>" +
        typeof false + "<br>" +
        typeof [1, 2, 3, 4] + "<br>" +
        typeof {
            name: 'john',
            age: 34
        };
    </script>
    <p>对象可以通过设置为<b>null</b>来清空</p>
    <p id="demo"></p>
    <script>
    var person = {
        firstName: "john",
        lastName: "Doe",
        age: 50,
        eyeColor: "blue"
    };
    var person = null;
    document.getElementById("demo").innerHTML = typeof person;
    </script>
    <p>变量的值如果不存在则改变量的值为<b>undefined</b></p>
    <p id="demo"></p>
    <script>
    var person;
    document.getElementById("demo").innerHTML = person + "<br>" + typeof person;
    </script>
    <p id="demo"></p>
    <script>
    document.getElementById("demo").innerHTML =
        typeof undefined + "<br>" +
        typeof null + "<br>" +
        (null === undefined) + "<br>" +
        (null == undefined);
    </script>
</body>

</html>
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参考资料：《菜鸟教程》

posted @ 2018-12-25 09:19  花儿迎风笑  阅读(167)  评论(0)  编辑  收藏  举报