! preceding function in javascript?

Question

Possible Duplicate:
What does the exclamation mark do before the function?

I saw a function formatted like this today for the first time:

!function(){}();

What is the preceding exclamation mark for? I assume it functions the same as:

(function(){})();

But... what's going on here?

Mike Lewis · Accepted Answer · 2011-03-24T16:58:54.303

46

The preceding ! takes the un-parseable statement, and allows it to to be parsed by the JS engine, which in turn returns true.

function(){}();
SyntaxError: Unexpected token (

!function(){}();
>>true

edited Mar 24 '11 at 16:58

answered Mar 24 '11 at 16:53

Mike Lewis

38

This is so misleading, it doesn't make anything parseable, it makes a function declaration be parsed as an expression, that evaluates to the function itself, allowing it to be called. – michelpm Jun 08 '13 at 18:29
I actually think "unparseable" is accurate - the js vm will not parse the first example. – Gershom Maes Feb 19 '20 at 20:59
@JeffHykin Your comment is interesting and entertaining. I will, however, point out that it should say "unary" ;) – valid Dec 05 '20 at 22:13
1

To add some clarity (adding to @michelpm's point) both `+function(){}();` and `-function(){}();` also work because `+` and `-` can be unary operators. However `!` is preferable since it is only a unary operator, while + and - can be both binary and unary operators. – Jeff Hykin Dec 07 '20 at 11:43
Wow that's embarrassing on my part, thank you @[valid](https://stackoverflow.com/users/1467307/valid). It's a shame Stack Overflow doesn't let comments be edited after 5 min. That one was worth deleting and fixing. – Jeff Hykin Dec 07 '20 at 11:47

score 14 · Answer 2 · answered Mar 24 '11 at 16:55

14

It simply makes the JavaScript parser parse it as an expression, which is necessary to execute it.

answered Mar 24 '11 at 16:55

ThiefMaster

1

That may be the motivation. But the resulting expression still behaves as any expression with `!` does. – Matthew Flaschen Mar 24 '11 at 17:00
5

@Matthew: I can't speak for gaoshan88, but it looks like he's interested in the motivation, since he compared it (correctly) to another common way of writing self-executing functions. – Matthew Crumley Mar 24 '11 at 17:05

score 13 · Answer 3 · answered Mar 24 '11 at 16:55

I've tried it, it returned true. The function returns undefined, and !undefined is true.

!function(){}();
^          ^ ^
C          A  B

I think they used that trick for a code golf or an obfuscated code. It is a bad practice to practially use that

Try javascript:alert(!function(){}()) in your browser address bar

3 Answers3