What does !1 and !0 mean in Javascript?

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return !1 in javascript

In a JavaScript file I had to read today, there was a line where a variable was declared like a factorial, like this :

var myVariable = !1;

and then something similar was used as parameter in a function like this :

return variable.myFunction(!0);

Can anyone explain me what the exclamation mark means in this context and eventually, why this is generally used for (benefits) ?

Thank you in advance !

It's not a factorial, it's a boolean [negation](http://en.wikipedia.org/wiki/Negation) operator. Turns `true` into `false`, and vice-versa. Probably turns anything else non-zero into zero? — Rob I, Jul 18 '12 at 21:32
That's horrible code. Does it throw a [`ViolentPsychopathKnowsWhereYouLiveException`](http://www.codinghorror.com/blog/2008/06/coding-for-violent-psychopaths.html)? — Robert Harvey, Jul 18 '12 at 21:35
I laughed uncontrollably upon seeing the title of this question. I had to click it. SMH to whoever codes like this. — Marlon, Jul 18 '12 at 21:46
I guess the reason is that the programmer cared a lot about those few bytes that can be saved this way - but this is not a good practice. — kapa, Jul 19 '12 at 06:38

score 119 · Accepted Answer · answered Jul 18 '12 at 21:34

119

The ! is the boolean NOT operator.

NOT (!): toggles a statement from true to false or from false to true.

!0 = true
!1 = false

This is a brilliant introduction to boolean operators and their use in javascript.

answered Jul 18 '12 at 21:34

Thank you a lot for your quick answer ! I will check this question as answered when I can (in 10 minutes). – m_vdbeek Jul 18 '12 at 21:36
1

Good answer, when you don't know the result on a variable you can always try 'alert(!1)', this will show a pop up with thw word false – Mark E Jul 18 '12 at 21:40
1

or console.log(variable), so you don't have [object] info – Grzegorz Kaczan Jul 18 '12 at 21:43
85

this a technique to minify values true or false – Kronos Mar 25 '14 at 21:59
3

why not use `!true` or `!false`, because it is saving download size of the script ? – Legends Nov 19 '16 at 18:11
2

@Legends, yes, because JS is transferred over the wire as it is and run at the client. – Anirudh Ramanathan Nov 20 '16 at 01:25
@Kronos: Mind blown! It makes sense! – Jose A Jun 02 '17 at 13:33
26

Additional clarification: why not simply use `0` and `1`? Is it to make sure that after the minification process it is still clear that they are `boolean` and not `int`? – don Jul 27 '17 at 13:31
19

@don Correct--adding the `!` operator coerces the 0 and 1 from an `int` to a `boolean`. – Jake Reece Jul 10 '19 at 10:30
1

@JakeReece thanks for this comment. helps clarify this a lot – rpivovar Nov 28 '19 at 15:06

1 Answers1