What is the !! (not not) operator in JavaScript?

解答1

Coerces强制 oObject to boolean. If it was falsey (e.g. 0, null, undefined, etc.), it will be false, otherwise, true.

!oObject  //Inverted boolean

!!oObject //Non inverted boolean so true boolean representation

So !! is not an operator, it's just the ! operator twice.

Real World Example "Test IE version":

let isIE8 = false;

isIE8 = !! navigator.userAgent.match(/MSIE 8.0/);

console.log(isIE8); // returns true or false 

If you ⇒

console.log(navigator.userAgent.match(/MSIE 8.0/));

// returns either an Array or null  

but if you ⇒

console.log(!!navigator.userAgent.match(/MSIE 8.0/));

// returns either true or false

It converts a nonboolean to an inverted boolean (for instance, !5 would be false, since 5 is a non-false value in JS), then boolean-inverts that so you get the original value as a boolean (so !!5 would be true).

解答2

! is "boolean not", which essentially typecasts the value of "enable" to its boolean opposite.

The second ! flips this value.

So, !!enable means "not not enable," giving you the value of enable as a boolean.