<!DOCTYPE HTML>
<html lang="en-US">
<head>
<meta charset="UTF-8">
<title>null和undefined</title>
</head>
<body>
<script type="text/javascript">
var a;//定义变量但没有类型所以输出为undefined
var b = null ;//定义变量并制定值为null
document.write( a + "<br/>");//
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);
document.write( "<br/>" );
document.write( 1988 == "1988");
document.write( "<br/>" );
document.write( 1988 === "1988");
document.write( "<br/>" );
</script>
</body>
</html>
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