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js中，只有三种情况变量值是fasle

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Release： 2016-06-01 09:55:07
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第一， 0是false，这个大家都知道。

注意：var a="0"; 变量a的值是true;

第二，当一个变量未定义时，是undefined类型，undefined类型是false.

第三，NaN是false.

js中，只有这三种情况是false，其他的全部是true.

Related labels：
false true variable
source：php.cn
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