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javascript中为5种为假的情况测试

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Javascript假的5种情况：
Falsy values in Javascript

Here's a complete list of falsy values in Javascript:

1> false (the boolean false is also considered falsy) 
2> "" (an empty string)
3> 0 (zero)
4> null
5> undefined
6> NaN (a property that represents the "Not-a-Number" value - 
indicating that a value is not a legal number)


为真的： 0 == 0  、"" == "" 、 null == null 、 undefined == undefined  (注：NaN != NaN，NaN不等于他自己 )
        0 == "" 、0 == false 、"" == false 、 undefined == null 。
其他情况都为假！ 

<!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=utf-8" />
<title>javascript_false_condition</title>
<style type="text/css"></style>
<script type="text/javascript">
/* 
假的5种情况：
Falsy values in Javascript

Here's a complete list of falsy values in Javascript:

1> false (the boolean false is also considered falsy) 
2> "" (an empty string)
3> 0 (zero)
4> null
5> undefined
6> NaN (a property that represents the "Not-a-Number" value - 
indicating that a value is not a legal number)
*/
console.log("------------0------------");
console.log("0 == 0  || "        + (0 == 0) );         // true
console.log("0 == false  || "    + (0 == false) );     // true
console.log("0 == \"\" || "      + (0 == "") );        // true
console.log("0 == null  || "     + (0 == null) );      // false
console.log("0 == undefined || " + (0 == undefined) ); // false
console.log("0 == NaN  || "      + (0 == NaN) );       // false

console.log("------------\"\"------------");
console.log("\"\" == \"\" || "      + ("" == "" ));        // true
console.log("\"\" == false || "     + ("" == false ));     // true
console.log("\"\" == 0 || "         + ("" == 0 ));         // true
console.log("\"\" == null || "      + ("" == null ));      // false
console.log("\"\" == undefined || " + ("" == undefined )); // false
console.log("\"\" == NaN || "       + ("" == NaN ));       // false

console.log("------------null------------");
console.log("null == null || "      + (null == null) );      // true
console.log("null == undefined || " + (null == undefined )); // true
console.log("null == false || "     + (null == false) );     // false
console.log("null == 0 || "         + (null == 0 ));         // false
console.log("null == \"\" || "      + (null == "") );        // false
console.log("null == NaN || "       + (null == NaN ));       // false

console.log("------------undefined------------");

console.log("undefined == undefined || " + (undefined == undefined) );// true
console.log("undefined == null || "      + (undefined == null) );     // true
console.log("undefined == false || "     + (undefined == false) );    // false
console.log("undefined == 0 || "         + (undefined == 0) );        // false
console.log("undefined == \"\" || "      + (undefined == "") );       // false
console.log("undefined == NaN || "       + (undefined == NaN) );      // false

console.log("------------NaN------------");
console.log("NaN == NaN || "       + (NaN == NaN) );      // false
console.log("NaN == false || "     + (NaN == false) );    // false
console.log("NaN == null || "      + (NaN == null) );     // false
console.log("NaN == 0 || "         + (NaN == 0) );        // false
console.log("NaN == \"\" || "      + (NaN == "") );       // false
console.log("NaN == undefined || " + (NaN == undefined) );// false

/*
为真的： 0 == 0  、"" == "" 、 null == null 、 undefined == undefined  (注：NaN != NaN，NaN不等于他自己 )
        0 == "" 、0 == false 、"" == false 、 undefined == null 。
其他情况都为假！

*/

</script>
</head>

<body>

</body>
</html>


参考资料：
http://www.programmerinterview.com/index.php/javascript/falsy-in-javascript/
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