JavaScript中的null和undefined

<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
<html xmlns="http://www.w3.org/1999/xhtml" xml:lang="en">
<head>
<meta http-equiv="Content-Type" content="text/html;charset=UTF-8" />
<title>null undefined</title>
</head>
<body>
<script type="text/javascript">
        //<![CDATA[


        var a;
        var b = null;


        document.write( a + "<br/>" );  //因为a未定义具体是值，所以是undefined
        document.write( b + "<br/>" );


        document.write( undefined == true );
        document.write( "<br/>" );
        document.write( undefined == "" );
        document.write( "<br/>" );
        document.write( undefined == 0 );
        document.write( "<br/>" );
        document.write( undefined == -0 );
        document.write( "<br/>" );
        document.write( undefined == NaN );
        document.write( "<br/>" );
        document.write( undefined == null ); //在某些时候他们两个比较是相等的
        document.write( "<br/>" );
        document.write( 1985 === "1985" );


        //]]>
    </script>
</body>
</html>
posted @ 2012-02-15 23:17  夏至冬末  阅读(114)  评论(0)  编辑  收藏  举报