why some people write a function like $(function( $ )

Question

I know that code like $(function( $ ) does not make any sense, but I have find this sort of code at various places including todomvc.

There is a reason writing functions like jQuery(function( $ ) to resolve any potential conflict of $ used by any other library, but not $function($).

Answer 1

There is no reason to use

$(function($))...

If you use the dollar sign in the beginning of the line you rely on that it's a jQuery object, so if you pass the jQuery object later on as a parameter to avoid conflicts, why didn't you use it on the first place? It's too later for that now...

The right way to use it is with:

(function($){ // The dollar variable is a parameter.
   $(function(){ // You use the $ variable only inside the function.
   });
})(jQuery); // jQuery is passed as a parameter.

$.somePrototypeFunctionHere(); // Here the dollar variable can be something else.

Answer 2

It's useless

This form is not useful at all:

$(function($) {
});

It will work ONLY if there are no other libraries that could override $ (e.g. prototype).

How it could be useful

Since jQuery passes itself as the first argument to the DOM ready event handler function, this is how you could make good use of it:

jQuery(function($) {
    // this === document
    // $ === jQuery
});

The proof of this behaviour can also be found in the source:

readyList.resolveWith( document, [ jQuery ] );

It maps this to document and the first argument to your ready function to jQuery. The reason why the code looks a bit non-obvious is because the ready event is handled using Deferred.

The equivalent

The somewhat equivalent notation is this:

(function($) {
  $(function() {
  }
}(jQuery));

This would be preferred if you're doing more things outside of the ready handler, which actually happens more often than you think.