Question

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What is the !! operator in JavaScript?

Sorry if this one is obvious but I can’t google it.

What is the “!!” operator in Javascript? e.g.

if (!!window.EventSource) {
  var source = new EventSource('stream.php');
} else {
  // Result to xhr polling :(
}

Did the author just use “!” twice i.e. a double negation? I’m confused because this is in the official doc.

Answer 1

It will convert anything to true or false:

!!0    // => false
!!1    // => true 
!!'a'  // => true
!!''   // => false
!!null // => false

Technically, !! is not an operator, it is just two ! operators next to each other. But a double-negation is pointless unless you are using !! like an operator to convert to Boolean type.