JavaScript !! operators [duplicate]

Question

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

I've seen operator ! used like !!. For example

var filter = !!(document.body.filters);

If I'm not wrong it's equivalent var filters = typeof document.body.filters != 'undefined'?

Is it a good practice to use !!?

Answer 1

It's up to you. All !! does is "cast" its argument to a Boolean.

Answer 2

It's a common way to convert any return type to boolean (usually to avoid compilation warnings). And second: no, checking if type is "undefined" is mandatory anyway and "!!" can not cover it.