JavaScript 中的undefined and null 学习

时间 2019-12-14

标签 javascript undefined null 学习栏目 JavaScript 繁體版

原文原文链接

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,javascript

在读取未赋值的变量或试图读取对象没有的属性时获得的就是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:html

prop: undefined

null 表示已经赋值给了一个值，可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：html5

<!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：java

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：ide

<!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：ui

equality:true 
identity:false