not_not_operator.md

!! Operator

What is the !! (not not) operator?

Remember it by "bang, bang you're boolean"

It’s all about “type coercion”, which is fancy words for converting a value from one type to another. A horribly obscure way to do a type conversion. ! is NOT. So !true is false, and !false is true. !0 is true, and !1 is false.

So you're converting a value to a boolean, then inverting it, then inverting it again.

> const x = { a: 1 }
undefined
> x
{ a: 1 }
> !x
false
> !!x
true

More examples:

	!!false === false
	!!true === true

	!!0 === false
	!!parseInt("foo") === false // NaN is falsy
	!!1 === true
	!!-1 === true  // -1 is truthy

	!!"" === false // empty string is falsy
	!!"foo" === true  // non-empty string is truthy
	!!"false" === true  // ...even if it contains a falsy value

	!!new Boolean(false)  // true
	!!Boolean(false) // false

	!!window.foo === false // undefined is falsy
	!!null === false // null is falsy

	!!{} === true  // an (empty) object is truthy
	!![] === true  // an (empty) array is truthy; PHP programmers beware!

You could write Boolean(req.session.keystoneItemId) instead which is more idiot proof.

Stackoverflow