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js中，只有三种情况变量值是fasle
- WBOYOriginal
- 2016-06-01 09:55:071331browse
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.
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