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JavaScript 中的undefined and null learn record from the definitive guide to html5

JavaScript 中有两个特殊值:undefined and null,

在读取未赋值的变量或试图读取对象没有的属性时得到的就是undefined值。

<!DOCTYPE HTML>
<html>
	<head>
		<title>Example</title>
	</head>
	<body>
		<script type="text/javascript">
			var myData = {
				name: "admin",
				weather: "sunny"
			};
			document.writeln("prop:" + myData.doesntexist);
		</script>
	</body>
</html>

输出:

prop: undefined

null 表示已经赋值给了一个值,但是该值不是一个有效的object,string,number,boolean 值(也就是说所定义的是一个无值[no value]).

例子:

<!DOCTYPE HTML>
<html>
    <head>
        <title>Example</title>
    </head>
    <body>
        <script type="text/javascript">
            var myData = {
                name: "admin"
            };
            document.writeln("var:" + myData.weather);
            document.writeln("prop:" + ("weather" in myData));

            
            document.writeln("</br>");
            myData.weather = "sunny";
            document.writeln("var:" + myData.weather);
            document.writeln("prop:" + ("weather" in myData));
            
            document.writeln("</br>");
            myData.weather = null;
            document.writeln("var:" + myData.weather);
            document.writeln("prop:" + ("weather" in myData));


        </script>
    </body>
</html>

输出:

var:undefined prop:false 
var:sunny prop:true 
var:null prop:true

undefined and null的相等和等同比较

例子:

<!DOCTYPE HTML>
<html>
    <head>
        <title>Example</title>
    </head>
    <body>
        <script type="text/javascript">
            var firstVal = null;
            var secondVal ;

            var equality = firstVal == secondVal;
            var identity = firstVal === secondVal;


            document.writeln("equality:" + equality);
            document.writeln("</br>");
            document.writeln("identity:" + identity);

        </script>
    </body>
</html>

输出:

equality:true 
identity:false
posted on 2016-04-09 03:15  chensss2008  阅读(178)  评论(0编辑  收藏  举报