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

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.