JavaScript--详解typeof的用法

typeof定义

typeof是一元运算符，用来返回操作参数的类型(不是值)

检查一个变量是否存在,是否有值

typeof在两种情况下会返回"undefined":

一个变量没有被声明的时候,
一个变量没有赋值的时候

<!DOCTYPE html>
<html>

    <head>
        <meta charset="UTF-8">
        <title></title>
        <script>
            function testTypeOf() {
                //返回boolean
                console.log(typeof(true));
                
                //返回number
                console.log(typeof(1.2));
                
                //返回string
                console.log(typeof("foo"));
                
                //返回object
                console.log(typeof(new String("foo")));
                console.log(typeof(new Date()));
                console.log(typeof(null));
                console.log(typeof(new Number(1.2)));
                console.log(typeof(new Boolean(true)));
                console.log(typeof(new Array(1, 2, 3)));
                console.log(typeof(new Error()));
                console.log(typeof([1,2,3]));
                
                console.log(typeof(/abc/g));
                console.log(typeof(new RegExp("meow")));
                console.log(typeof({}));
                console.log(typeof(new Object()));
                
                //返回function
                console.log(typeof(new Function("")));
                
                //返回undefined
                var str;
                console.log(typeof(str));
                console.log(typeof(strLen));
            }
        </script>
    </head>

    <body>
        <button onclick="testTypeOf()">点击</button>
    </body>

</html>