Question

I saw some javaScript, like Backbone-relational written in this way:

( function( undefined ) {
    "use strict";
        // some code
})();

My question is: When should undefined parameter be used?
Always or in special circumstances?

Answer 1

Most of the time when you see that pattern they are ensuring that ‘undefined’ is truly undefined. See how at the bottom they don’t pass any arguments.

This is a self executing function and you can pass parameters to it.

(function(undefined) {  // no parameters are passed in so undefined is undefined.
 "use strict";

})(/*no parameters passed*/)

If your code is using undefined and you need to ensure that some other javascript library doesn’t alter the meaning of undefined then you would want to use this pattern. However, this is not something that commonly occurs, it is just a safety net. For some reason in javascript ‘undefined’ can be set a value just like any other variable.