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