javascript 的boolean 类型
If a number is used where a boolean value is expected, the number is converted to TRue unless the number is 0 or NaN, which are converted to false. If a string is used where a boolean value is expected, it is converted to true except for the empty string, which is converted to false. null and the undefined value convert to false, and any non-null object, array, or function converts to true.页:
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