Why use '!!' before an expression?

Question

I'm on VSCode with ESLint installed and I tried to type that expression to get a "true" or "false" output

const result = goodGuys.includes(guy) ?

but it didn't worked and with a quick fix (from ESLint that I don't understand, so here I am lol) of removing the "?" and adding two exclamation marks at the beginning, it work just well.

const result = !!goodGuys.includes(guy);

Can someone explain this to me please? Thx

The above snippet isn’t syntactically correct. What exactly is the code supposed to do? — Sebastian Simon, Aug 04 '19 at 13:50
`includes` always returns a boolean. So, just `const result = goodGuys.includes(guy) ` should be enough — adiga, Aug 04 '19 at 13:51

Jack Bashford · Accepted Answer · 2019-08-04T14:17:25.823

0

That's just an ESLint thing. includes returns a Boolean, so it actually functions like that in code. ESLint sometimes wants to enforce Boolean rules, so !! does that for you. It works by using double negation - once to get an inverted Boolean, then again to un-invert it, giving you the Boolean representation of your expression. It's basically the same as this:

const result = Boolean(goodGuys.includes(guy));

edited Aug 04 '19 at 14:17

answered Aug 04 '19 at 13:50

Jack Bashford

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But, `includes` always returns a boolean. Why does ESLint complain about this? – adiga Aug 04 '19 at 13:53
I honestly do not know @adiga - I don't use linters. – Jack Bashford Aug 04 '19 at 13:54
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But, `new Boolean()` returns an object – adiga Aug 04 '19 at 13:54
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Just `Boolean()` is equivalent to `!!`. – Sebastian Simon Aug 04 '19 at 14:16
Oh, of course @SebastianSimon...thanks! I completely forgot about that! – Jack Bashford Aug 04 '19 at 14:17

Why use '!!' before an expression?

1 Answers1