map properties for arrays and objects

annotated.js

function(
  a, // source object or array
  b, // map function
  c, // (placeholder)
  d  // (placeholder)
){
  d = a;         // copy the object to the result, keys will be overwritten shortly
  for (        // for
    c          // each property
    in a       // in the source object.. works on both arrays and objects
  ) d[c] =     // set the same on the target object with
    b(a[c],c); // the results of the map function
  return d       // finally, return the target object
}

index.js

function(a,b,c,d){d=a;for(c in a)d[c]=b(a[c],c);return d}

LICENSE.txt

            DO WHAT THE FUCK YOU WANT TO PUBLIC LICENSE
                    Version 2, December 2004

 Copyright (C) 2011 Mike Sherov <http://mike.sherov.com>

 Everyone is permitted to copy and distribute verbatim or modified
 copies of this license document, and changing it is allowed as long
 as the name is changed.

            DO WHAT THE FUCK YOU WANT TO PUBLIC LICENSE
   TERMS AND CONDITIONS FOR COPYING, DISTRIBUTION AND MODIFICATION

  0. You just DO WHAT THE FUCK YOU WANT TO.

package.json

{
  "name": "map",
  "keywords": ["map", "array", "arrays", "functional"]
}

test.js

// Define the map function
var map = function(a,b,c,d){d=a;for(c in a)d[c]=b(a[c],c);return d}

// Define an array and a object
var arr = ["a", "b", "c", "d", "e"]
var obj = {a:0, b:1, c:2, d:3, e:4}

// Map each, returing the key/val at each
arr = map( arr, function( value, index ){ return index + value } )
obj = map( obj, function( value, index ){ return index + value } )

// Log the results:
console.log( arr ) // => [ '0a', '1b', '2c', '3d', '4e' ]
console.log( obj ) // => { a: 'a0', b: 'b1', c: 'c2', d: 'd3', e: 'e4' }

well, ideally, i'd like to have something that would roll map and filter into one for arrays and objects, where members could be mapped to a number of members from 0 to 1 for the object case and 0 to n for the array case.

@mathiasbynens, do you have any thoughts on this?