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.

Furnishing

it IS the NOT operator. MInd to show the whole code? I guess this isn't a function declaration but rather an IIFE... – Boot

Are you sure that it's not something like !function(){...return bool;}() – Inviting

@p.s.w.g: it shouldn't return boolean – Totem

Guess that's meant to be pseudo-code, but trueor falsewould do better, yes. – Boot

For context, the code is on github. If you understand what minification does, then I wouldn't question seemingly obscure syntax. – Rech

I don't understand why this question got a down vote. – Furnishing

@progo wasn't me, but most likely because you dind't specify the full code and, additionally, left out the most important part at the end - and the downvoter didn'T return after the edits. – Boot

Answer 1

!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. */

Lotuseater

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

yupp, you're missing the paranthesis at the end in your first line. – Boot

It's worth pointing out the function doesn't have a name. – Rech

Looks like there's also a performance increase to using bang functions: jsperf.com/bang-function – Grandmotherly

@Wiggs right, meant to do that. thanks for pointing that out ... – Lotuseater

@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 () { ; }. – Boot

@JohannesH.: Why shoudn't (function Name () { ; }) () be valid? It is. – Orlantha

It is? Oh, well, I must admit, I never tested it. It isn't something I'd considered usefull yet. Well, then ignore that part of my comment. Rest stays valid though ;) – Boot

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. – Orlantha

@JohannesH.: For function expressions, the name is optional (that's why it's omitted most of the time), for function declarations it is mandatory. – Orlantha

Hm. That does imply that var func1 = function func2 () { ; } assigns the function to func1 as well as func2, and both are visible inside the current scope, doesn't it? – Boot

@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. – Orlantha

@JohannesH.: You're welcome! Good resource: kangax.github.io/nfe – Orlantha

+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. – Reest

Answer 2

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?

Kaiserdom

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. – Boot