Question

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What do parentheses surrounding a JavaScript object/function/class declaration mean?

I see functions inside parentheses in jQuery plugins and the like.

For example,

(function(args) {
   // ...
})(localVariable);

What does this do, exactly?

Answer 1

You mean something like the following?:

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

This basically ensures that any “var” declarations are kept private (they are scoped to the anonymous function in which they have been placed) rather than global. For example, consider:

 var counter = 0;
 window['inc'] = function() {
    return counter++;
 };

vs.

 (function() {
    var counter = 0;
    window['inc'] = function() {
       return counter++;
    };
 })();

Both of these have the effect of defining a function window.inc that returns a counter that gets incremented; however, in the first version, counter is actually visible to all other modules, because it is in the global scope, whereas the latter ensures that only window.inc can access counter.

Note that the code creates a function and immediately invokes it (that’s what the last parens are for).