What is the difference between !!variable and variable [duplicate]

The double not operator !! coerces a (potentially non-boolean) value to a boolean.

In your specific example:

var someVariable = (someOtherVariable === 'true'); if (!!someVariable) { // do some stuff here } 

someVariable is already guaranteed to be a Boolean (since the result of an === comparison is always a Boolean) so coercing it to a Boolean does not change the operation in any way and is pretty much wasted code. Even if it wasn't already a Boolean, you don't need to coerce it to a Boolean just to test it like if (someVariable) either so there's yet another reason not to use the !! here.

When !! is useful is when you want to store a true Boolean somewhere, but you may only have a truthy or falsey value, not necessarily a true Boolean. Then you can coerce it to a Boolean with the !!.

So, suppose you had some value that is not necessarily a Boolean and you wanted to set some other value to a true Boolean based on the truthy-ness or falsey-ness of the first variable. You could do this:

var myVar; if (someVar) { myVar = true; } else { myVar = false; } 

or this:

myVar = someVar ? true : false; 

or this:

myVar = !!someVar;