jQuery $.map vs $.fn.map different arguments

Question

I'm just wondering if anyone knows why $.map and $.fn.map pass arguments in flipped order from one-another. Is there a valid reason (e.g. ECMA specs somewhere) for it or was it just a poorly-planned API that is now impossible to fix due to the amount of code relying on jQuery?

$.map([ 'a', 'b', 'c' ], function(){ console.log(arguments); })
// ['a', 0], ['b', 1], ['c', 2]

$.fn.map.call([ 'a', 'b', 'c' ], function(){ console.log(arguments); })
// [0, 'a'], [1, 'b'], [2, 'c']

.each doesn't act like this

Answer 1

Indeed, that might be considered as an oversight, but there is a very good reason for the arguments to be passed in that order.

Of course, on the surface, that's because the map() / each() methods and $.map() are not supposed to be equivalent, and each side is meant to be used in its own way (dealing with jQuery objects and arrays/hashes, respectively).

But the main point is that the parameter position is optimized for the case your callback function only wants to take a single argument. Consider both cases:

• map() and each() are meant to be called on jQuery objects, and the callback function will be called in the context of each DOM element involved in the mapping/loop (the context object is available through the this keyword). So, it makes more sense to pass the index first, then the element, because this already designates the element.

• $.map() deals with arrays and Javascript objects (hashes), and the callback function is invoked in the "global" context (this is always the window object). So, it makes more sense to pass the value first, then the index, because mapping is about avoiding loop index variables in the first place. If the caller is interested in those, he can use a "standard" for loop (or the second argument passed to the callback function).