Difference between !! and ! in JavaScript

 

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  • What is the !! (not not) operator in JavaScript? 31 answers

    • A single bang ( ! ) is used to negate a boolean.

    Double bang ( !! ) is used to coerce a truthy/falsey value to a boolean true or false.

    for example 

    var x = 0; // a falsey value
console.log(x); // logs 0
console.log(!x)// logs true
console.log(!!x)// logs false

var y = "Hello world"; // a truthy value
console.log(y); // logs "Hello world"
console.log(!y)// logs false
console.log(!!y)// logs true

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

    From the JS wiki on SO

    Regarding !! :

    "Coerces oObject to boolean. If it was falsey (eg 0, null, undefined, etc.), it will be false, otherwise, true."

    "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)"

    "So !! is not an operator, it's just the ! operator twice."

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