myVar = !!someOtherVar
This question already has an answer here:
如果你需要一个布尔值传递给函数，或者是肛门约在条件语句仅评估布尔值，即蒙上someOtherVar由双否定它为你一个布尔值。
In non-strictly typed languages, the ! operator converts a value to a boolean. Doing it twice would be equivalent to saying
myVar = (boolean)someOtherVar
Note that this is not recommended for code clarity.
(Rewritten to clarify, simplify)
That statement performs a couple different actions:
myVar = // This portion is a regular assignment, it will store the value of the suffix
!!someOtherVar; // This portion is evaluated to a boolean result
The !!someOtherVar , I assume, is what you're really asking about. The answer is simple: it performs two logical NOT operations against the truthiness (a Javascript'ism) of someOtherVar .
In other words, if you understand the ! operator, this just combines two of them ( !! isn't a different operator). By doing this it essentially returns the boolean evaluation of someOtherVar --in other words, it's a cast from whatever type someOtherVar is to boolean .
So... to walk through this, and pay attention to the result of myVar :
myVar = someOtherVar; // myVar will be whatever type someOtherVar is
myVar = !someOtherVar; // myVar will *always be boolean, but the inverse of someOtherVar's truthiness
myVar = !!someOtherVar; // myVar will *always be boolean, and be the equivalent of someOtherVar's truthiness
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