Why use if(!!obj){} in javascript instead of if(obj){}

Question

I brow some code in javascript, found some code of function like that

function isPromise(obj) {
  return !!obj && (typeof obj === 'object' || typeof obj === 'function') && typeof obj.then === 'function';
}

As what I say to the title, why this function use !!obj" ??

Agree, it looks pointless in your code (though a better test would always return a boolean) — CertainPerformance, Dec 07 '18 at 04:39
You can absolutely avoid using `!!` because you're checking it with typeof. So, it's pointless to use. — Bhojendra Rauniyar, Dec 07 '18 at 04:49
@BhojendraRauniyar: Yes and no. `!!` is not necessary but the boolean check for `obj` is. Without it, `typeof null === 'object'` is `true`, and then `typeof null.then` will throw an error. — Felix Kling, Dec 07 '18 at 04:53

score 1 · Answer 1 · answered Dec 07 '18 at 04:45

1

If you are referring to "double bang" in JavaScript it is a force coercion falsely or truthy to its boolean true or false

answered Dec 07 '18 at 04:45

Jacob

1 Answers1