Converts a value to a true boolean: if `a` is truthy (is assimilated to `true` in boolean contexts), `!!a` will be `true` rather than the original value. It's equivalent to but much shorter than `Boolean(a)`, and just as readable if you know the idiom.

Double exclamation marks force a true value, as opposed to a truthy one. For example:

    SomethingTruthy => TruthyThing
    !SomethingTruthy => False
    !!SomethingTruthy => True

Thanks for explanations. Also worth pointing out that that code is part of Three.js - an amazing library that makes it very straightforward to do things like this.

Essentially, it returns whether a value exists or not