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Example codes for three scenarios of boolean type in JavaScript

高洛峰
Release： 2016-12-06 13:22:51
Original
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Examples are as follows: 

<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
<html xmlns="http://www.w3.org/1999/xhtml">
<head>
<meta http-equiv="Content-Type" content="text/html; charset=gb2312" />
<title>无标题文档</title>
</head>
 
<body>
    <script language="javascript">
        /*
        * 一般性的true,false
        */
        var flag = true;
        if(flag){
            alert(&#39;truetrue.............&#39;);
        }else{
            alert(&#39;falsefalse...........&#39;);
        }
    </script>
     
    <script language="javascript">
        /*
        *   一个对象不存在，为null时的true,false
        */
        var obj = document.getElementById(&#39;id&#39;);
        if(obj){ //虽然这里的obj为null，但是可以解读为false
            alert(&#39;obj不为null表示true&#39;);
        } else{
            alert(&#39;obj为null时可识别为false&#39;);
        }
    </script>
     
    <script language="javascript">
        /*
        * 一个对象不存在为undefined时的true,false
        */
        var a;
        if(a){//虽然a的值为undefined,但是这里可以解读为false
            alert(&#39;a的值不为undefined解读为true&#39;);
        } else {
            alert(&#39;a的值为undefined解读为false&#39;);
        }
    </script>
</body>
</html>
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