What does the double exclamation !! operator mean? [duplicate]

Question

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What is the !! operator in JavaScript?
What does !! (double exclamation point) mean?

I am going through some custom JavaScript code at my workplace and I am not able to understand the following construct.

var myThemeKey = (!!$('row') && $('row').hasClassName('green-theme')) ? 'green' : 'white'; 

I understand everything on the above line except !! operator. I assume that it is a NOT operator and NOT of NOT is the original value but why would someone do a NOT of NOT?

Can someone please help me understand what is happening on the above line of code?

Answer 1

The !! ensures the resulting type is a boolean (true or false).

javascript:alert("foo") --> foo

javascript:alert(!"foo") --> false

javascript:alert(!!"foo") --> true

javascript:alert(!!null) --> false

They do this to make sure $('row') isn't null.

It's shorter to type than $('row') != null ? true : false.