Question

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

I have encountered this piece of code:

function printStackTrace(options) {
    options = options || {guess: true};
    var ex = options.e || null, guess = !!options.guess;
    var p = new printStackTrace.implementation(), result = p.run(ex);
    return (guess) ? p.guessAnonymousFunctions(result) : result;
}

And I couldn’t help to wonder why the double negation? And is there an alternative way to achieve the same effect?

(The code is from https://github.com/eriwen/javascript-stacktrace/blob/master/stacktrace.js.)

Answer 1

It casts to boolean. The first ! negates it once, converting values like so:

• undefined to true
• null to true
• +0 to true
• -0 to true
• '' to true
• NaN to true
• false to true
• All other expressions to false

Then the other ! negates it again. A concise cast to boolean, exactly equivalent to ToBoolean simply because ! is defined as its negation. It’s unnecessary here, though, because it’s only used as the condition of the conditional operator, which will determine truthiness in the same way.