JavaScript !function(){} [duplicate]

Question

when looking at the minified Sizzle code, I noticed that it begins like this:

!function(a){//... }(window)

Why is there an exclamation point at the beginning?

I thought that ! was the not operator.

Thank you.

Edit:

Full Code.

it IS the NOT operator. MInd to show the whole code? I guess this isn't a function declaration but rather an IIFE... — Johannes H.
– Johannes H., Commented Feb 5, 2014 at 3:51
Are you sure that it's not something like !function(){...return bool;}() — p.s.w.g
– p.s.w.g, Commented Feb 5, 2014 at 3:51
Guess that's meant to be pseudo-code, but trueor falsewould do better, yes. — Johannes H.
– Johannes H., Commented Feb 5, 2014 at 3:53
For context, the code is on github. If you understand what minification does, then I wouldn't question seemingly obscure syntax. — user1508519
– user1508519, Commented Feb 5, 2014 at 3:54

Alex · Accepted Answer · 2014-02-05 03:55:30Z

6

!function(a){/* ... */}();

Using an unary operator to invoke an IIFE is common practice. That's a common shorthand for:

(function(a){/* ... */}());

or:

(function(a){/* ... */})();

You can also substitute the not unary operator with any other unary operator:

-function(a){ /* ... */ }(); +function(a){ /* ... */ }(); /* ... etc. */

answered Feb 5, 2014 at 3:55

Alex

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14 Comments

megawac Over a year ago

You mean !function() {}() doing !function(){} won't call it

Johannes H. Over a year ago

@remyabel Closures usually don't have one, and almost never when used as an IIFE. Actually, (function Name () { ; }) ()isn't valid, the "normal" funciton statement (function name () { ; }) most people use is just a shortcut for var name = function () { ; }.

Felix Kling Over a year ago

Please note that the ! does not invoke the function. It's purpose is to make the parser treat the function definition like an expression, not a statement.

Felix Kling Over a year ago

@JohannesH.: In older IE versions it does (it's a bug). The specification dictates that the name of a function expression is only visible within the function itself. It's great for debugging and recursive functions.

cookie monster Over a year ago

+1 But be wary of using the + and - operators for this since they're overloaded as binary operators as well. So if you omit a semicolon on the previous line, it can give an undesired result. The unary operators that are not overloaded are best, like !, void, ~, etc.

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Community · Accepted Answer · 2017-05-23 11:51:32Z

gives a good explaination for function invocation https://github.com/airbnb/javascript/issues/44#issuecomment-13063933

!function () {}();

is equivalent to

(function(){})();

except the author is saving 1 byte of code.

In many cases, it's about saving bytes.

!function aaa(){}() !function bbb(){}();

is the same as this:

!function aaa(){}() ;(function bbb(){})();

notice the ";" in that last bit. That is defensive, as it protects your code a bit from runaway js that might preceed it.

funny, I asked this same question some time ago:

Came across a convention I've never seen. What does it do? !function

great reference on it: What does the exclamation mark do before the function?

Just for reference: in other programming languages, similar things are common. In bash scripts for example, the "do nothing command" (:) is often used to evaluate variable expansions.

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