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<html xmlns="http://www.w3.org/1999/xhtml" xml:lang="en">
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<meta http-equiv="Content-Type" content="text/html;charset=UTF-8" />
<title>null undefined</title>
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<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>