! preceding function in javascript? [duplicate]

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 · 2011-03-24 16:58:54Z

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, 2011 at 16:58

answered Mar 24, 2011 at 16:53

Mike Lewis

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5 Comments

michelpm Over a year ago

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.

Gershom Maes Over a year ago

I actually think "unparseable" is accurate - the js vm will not parse the first example.

valid Over a year ago

@JeffHykin Your comment is interesting and entertaining. I will, however, point out that it should say "unary" ;)

Jeff Hykin Over a year ago

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 Over a year ago

Wow that's embarrassing on my part, thank you @valid. It's a shame Stack Overflow doesn't let comments be edited after 5 min. That one was worth deleting and fixing.

ThiefMaster · Accepted Answer · 2011-03-24 16:55:20Z

14

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

answered Mar 24, 2011 at 16:55

ThiefMaster

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2 Comments

Matthew Flaschen Over a year ago

That may be the motivation. But the resulting expression still behaves as any expression with ! does.

Matthew Crumley Over a year ago

@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.

Ming-Tang · Accepted Answer · 2011-03-24 16:55:29Z

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

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

A. function(){} is an empty anonymous function
B. () executes the function (A), returning undefined
C. ! negates undefined, which becomes true

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

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