javascript当中null和undefined的==和===的比较_if(z1 == '' || z1 == null || z1 == 'null' || z1 ==-优快云博客

本文链接：https://blog.youkuaiyun.com/mark_to_win/article/details/88602850

例 3.1.3（null和undefined的==和===的比较）

<head>
    <meta http-equiv="content-type" content="text/html; charset=utf-8"/>
</head>
<script>
    //马克-to-win：声明变量x
    /* if a value is not set, its typeof is undefined, its value is undefined, if a value= null, its typeof is object, its value
is null,but when you use == to test, they are the same, but when to
use === to test, they are not the same,== means as long as value is the same, ok, but === means type also must be equal. */
    var x;
    var z1 = "d";
    var y = null;

    //      document.writeln("z1 is"+z1);
    /*if you cancel commenting out the following statement, it will give the blank page, which means it has error.*/
    //        document.writeln(zyx);
    document.writeln(x + " is x");
    document.writeln(typeof(x) + " is typeof(x)");
    document.writeln(y + " is y");
    document.writeln(typeof(y) + " is typeof(y)");
    var z = undefined;
    document.writeln(z + " is z");
    document.writeln(typeof(z) + " is typeof(z)");
    if (y == undefined)
    {
        document.writeln('null and undefined is interchangable');
    }
    if (z1 != null)
    {
        document.writeln('z1 != null');
    }
    if (y === undefined)
    {
        document.writeln('null and undefined is exactly the same');
    }
    if (x == undefined)
    {
        document.writeln('声明变量后默认值为undefined');
    }

    if (x === undefined)
    {
        document.writeln('声明变量后默认值为exactly the same as undefined');
    }

    if (x == null)
    {
        document.writeln('声明变量后默认值为null');
    }