这是什么! ！ (不是) JavaScript 中的操作符？

Question

I saw some code that seems to use an operator I don't recognize, in the form of two exclamation points, like so: !!. Can someone please tell me what this operator does?

The context in which I saw this was,

this.vertical = vertical !== undefined ? !!vertical : this.vertical;

转载于:https://stackoverflow.com/questions/784929/what-is-the-not-not-operator-in-javascript

Answer 1

Coerces 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 null  

but if you ⇒

console.log(!!navigator.userAgent.match(/MSIE 8.0/));  
// returns true or false