_.map.js

function map(collection, callback, thisArg) {
  var index = -1,
      length = collection ? collection.length : 0,
      result = isArray(collection) ? Array(length) : {};

  callback = createCallback(callback, thisArg);
  if (isArray(result)) {
    while (++index < length) {
      result[index] = callback(collection[index], index, collection);
    }
  } else {
    each(collection, function(value, key, collection) {
      result[key] = callback(value, key, collection);
    });
  }
  return result;
}

example.map.js

var fruitList = [
  { id:1,  name:'lime',     color:'green',  size:'small' },
  { id:2,  name:'banana',   color:'yellow', size:'large' },
  { id:3,  name:'lemon',    color:'yellow', size:'small' },
  { id:4,  name:'papaya',   color:'green',  size:'large' },
  { id:5,  name:'kiwi',     color:'green',  size:'small' },
  { id:6,  name:'apple',    color:'green',  size:'small' },
  { id:7,  name:'pear',     color:'yellow', size:'small' },
  { id:8,  name:'grape',    color:'green',  size:'small' },
  { id:9,  name:'mango',    color:'yellow', size:'large' },
  { id:10, name:'honeydew', color:'green',  size:'large' }
];

var fruitByColorThenSize = _.map( _.groupBy(fruitList, 'color'), function(byColor) {
  return _.map( _.groupBy(byColor, 'size'), function(bySize) {
    return _.object(_.pluck(bySize, 'id'), _.pluck(bySize, 'name'));
  });
});

console.dir(fruitByColorThenSize);

/*{
  "green":{
    "large":{
      "4":"papaya",
      "10":"honeydew"
    },
    "small":{
      "1":"lime",
      "5":"kiwi",
      "6":"apple",
      "8":"grape"
    }
  },
  "yellow":{
    "large":{
      "2":"banana",
      "9":"mango"
    },
    "small":{
      "3":"lemon",
      "7":"pear"
    }
  }
}*/