each, extend, and map in one function

invert.js

function invert( index, fn ) {
  var key = index.length, val

  if ( key > 0 ) while ( key-- ) this[ val = index[ key ] ] = fn ? fn( val, key ) : key
  else for ( key in index ) this[ val = index[ key ] ] = fn ? fn( val, key ) : key

  return this
}