This correctly returns an array with the elements 'a', 'b' and 'c':

$(['a','b','c']).map(function(elem,index) { return this; });

This incorrectly returns the indices:

$(['a','b','c']).map(function(elem,index) { return elem; });

And this incorrectly returns the elements:

$(['a','b','c']).map(function(elem,index) { return index; });

So the arguments to the callback are reversed, if the documentation of $.map is to be believed.