Understanding the double exclamation point

Question

I'm trying to understand what exactly the double exclamation mark does. Yes, I saw this question, with lots of answers. So I know in principle what it does, but I don't know why one would ever need to use it.

From what I understand, it converts the value to a boolean. So let's say I have the following code:

var myBool = !!(index === 0 || index > len);

Can't I just leave out the !! and I will get the same result:

var myBool = (index === 0 || index > len);

What do I gain by adding !!? Is't it already a boolean vaule?

Okay, what you didn't understand in the other question? If it's already a boolean value, it doesn't make any sense. — Praveen Kumar Purushothaman, Nov 11 '16 at 21:18
@Barmar, I just closed it and reopened. Did you read what OP has written? — Praveen Kumar Purushothaman, Nov 11 '16 at 21:20
@PraveenKumar So it would only make sense to use it if I do this: `var myBool = !!index`? — Horay, Nov 11 '16 at 21:20
You would use it when you want to force the coercion. Eg `var myBool = !!something;` or `var myBool = !!(something || something_else);`. You wouldn't use it on an expression that is already guaranteed to be a boolean, you would use it on an expression that isn't guaranteed. — dsh, Nov 11 '16 at 21:20
@Horay yes, in general, if you're not _sure_ what the value is, you can use double negation to convert it. By the way, the alternative is to use `Boolean(value)` instead of `!!value` which will do the same conversion. — VLAZ, Nov 11 '16 at 21:21

score 6 · Accepted Answer · answered Nov 11 '16 at 21:22

The purpose of !! is to canonicalize any type of truthy or falsey value to the corresponding boolean value.

If the value is already known to be a boolean, such as the result of a comparison operator, there's no point in it and it's redundant. So it's useless in the example you gave.

Understanding the double exclamation point

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