a group-by implementation

groupby.js

function groupBy( array, key ) {
  var result = {};
  var by, property;
  var item, i=0;
  var isAlreadyFunction = typeof( key ) === 'function';
  if( isAlreadyFunction ) {
    by = key;
  } else {
    by = function( item ) {
      return item[ key ];
    };
  }
  while( item = array[ i ] ) {
    property = by( item );
    ( result[ property ] || ( result[ property ] = [] ) ).push( item );
    i++;
  }
  return result;
}