Question

20

Javascript functions run automatically vs only when called

rated 0 times [ 26] [ 6] / answers: 1 / hits: 18015 / 11 Years ago, sun, june 16, 2013, 12:00:00

I'm working on some code where a given page has many .js files associated with it (utilizing them like libraries). Each .js file looks like this inside:

(function() {

    .... all the lib functions and objects ....

})();

After some playing, I see that functions of the format (function() {...})(); get called automatically. If I strip away the outer paren's to have function() {...} then the code is invalid. If I add a function name then the code is valid but does not run till called function foo() { ... }.

Is there a special reason the lib has been written in this way? I would guess it would encapsulate variable names and such. What is it about the syntax of this that allows it to be run automatically on page load?

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cody

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2 Years ago

answered 11 Years ago cruzs · Accepted Answer

That's called an IIFE, an Immediately-Invoked Function Expression.

It lets you define variables, including functions, which aren't visible from the outer scope and doesn't encumber the global name space.

(function() {

    var v = ... // this variable can be used in the IIFE but not from outside

})();

The reason why you need the outer parenthesis is because a statement starting with function something is interpreted as a function declaration, which here would be invalid because a function declaration needs a name. You have to use a trick to make it an expression. Parenthesis do that, but you could have used other tricks, for example

+function(){

  ...

}();

but the outer parenthesis is the clearest and probably the less surprising solution.