nest.js

import map from "./map";

export default function() {
  var keys = [],
      sortKeys = [],
      sortValues,
      rollup,
      nest;

  function apply(array, depth, createResult, setResult) {
    if (depth >= keys.length) return rollup != null
        ? rollup(array) : (sortValues != null
        ? array.sort(sortValues)
        : array);

    var i = -1,
        n = array.length,
        key = keys[depth++],
        keyValue,
        value,
        valuesByKey = map(),
        values,
        result = createResult();

    while (++i < n) {
      if (values = valuesByKey.get(keyValue = key(value = array[i]) + "")) {
        values.push(value);
      } else {
        valuesByKey.set(keyValue, [value]);
      }
    }

    valuesByKey.each(function(values, key) {
      setResult(result, key, apply(values, depth, createResult, setResult));
    });

    return result;
  }

  function entries(map, depth) {
    if (++depth > keys.length) return map;
    var array, sortKey = sortKeys[depth - 1];
    if (rollup != null && depth >= keys.length) array = map.entries();
    else array = [], map.each(function(v, k) { array.push({key: k, values: entries(v, depth)}); });
    return sortKey != null ? array.sort(function(a, b) { return sortKey(a.key, b.key); }) : array;
  }

  return nest = {
    object: function(array) { return apply(array, 0, createObject, setObject); },
    map: function(array) { return apply(array, 0, createMap, setMap); },
    entries: function(array) { return entries(apply(array, 0, createMap, setMap), 0); },
    key: function(d) { keys.push(d); return nest; },
    sortKeys: function(order) { sortKeys[keys.length - 1] = order; return nest; },
    sortValues: function(order) { sortValues = order; return nest; },
    rollup: function(f) { rollup = f; return nest; }
  };
}

function createObject() {
  return {};
}

function setObject(object, key, value) {
  object[key] = value;
}

function createMap() {
  return map();
}

function setMap(map, key, value) {
  map.set(key, value);
}