not-not-operator-in-javascript.md

Questions - two exclamation points, like so: !!. What does this operator does?

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

Ans

Converts Object 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":

const 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

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Further Explanations

It's just the logical NOT operator, twice - it's used to convert something to boolean, e.g.:

true === !!10

false === !!0

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Even more explanations

the !! operator results in a double negation.

var foo = "Hello World!";

!foo // Result: false
!!foo // Result: true

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More explanations

Sometime its quite obscure and unreadable 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.

// Maximum Obscurity:
val.enabled = !!userId;

// Partial Obscurity:
val.enabled = (userId != 0) ? true : false;

// And finally, much easier to understand:
val.enabled = (userId != 0);

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More explanations

!!expr returns a Boolean value (true or false) depending on the truthiness of the expression. It makes more sense when used on non-boolean types. Consider these examples, especially the 3rd example and onward:

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

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

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

     !!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!