transform.js

var arrayEach = require('./_arrayEach'),
    baseCreate = require('./_baseCreate'),
    baseForOwn = require('./_baseForOwn'),
    baseIteratee = require('./_baseIteratee'),
    getPrototype = require('./_getPrototype'),
    isArray = require('./isArray'),
    isBuffer = require('./isBuffer'),
    isFunction = require('./isFunction'),
    isObject = require('./isObject'),
    isTypedArray = require('./isTypedArray');

/**
 * An alternative to `_.reduce`; this method transforms `object` to a new
 * `accumulator` object which is the result of running each of its own
 * enumerable string keyed properties thru `iteratee`, with each invocation
 * potentially mutating the `accumulator` object. If `accumulator` is not
 * provided, a new object with the same `[[Prototype]]` will be used. The
 * iteratee is invoked with four arguments: (accumulator, value, key, object).
 * Iteratee functions may exit iteration early by explicitly returning `false`.
 *
 * @static
 * @memberOf _
 * @since 1.3.0
 * @category Object
 * @param {Object} object The object to iterate over.
 * @param {Function} [iteratee=_.identity] The function invoked per iteration.
 * @param {*} [accumulator] The custom accumulator value.
 * @returns {*} Returns the accumulated value.
 * @example
 *
 * _.transform([2, 3, 4], function(result, n) {
 *   result.push(n *= n);
 *   return n % 2 == 0;
 * }, []);
 * // => [4, 9]
 *
 * _.transform({ 'a': 1, 'b': 2, 'c': 1 }, function(result, value, key) {
 *   (result[value] || (result[value] = [])).push(key);
 * }, {});
 * // => { '1': ['a', 'c'], '2': ['b'] }
 */
function transform(object, iteratee, accumulator) {
  var isArr = isArray(object),
      isArrLike = isArr || isBuffer(object) || isTypedArray(object);

  iteratee = baseIteratee(iteratee, 4);
  if (accumulator == null) {
    var Ctor = object && object.constructor;
    if (isArrLike) {
      accumulator = isArr ? new Ctor : [];
    }
    else if (isObject(object)) {
      accumulator = isFunction(Ctor) ? baseCreate(getPrototype(object)) : {};
    }
    else {
      accumulator = {};
    }
  }
  (isArrLike ? arrayEach : baseForOwn)(object, function(value, index, object) {
    return iteratee(accumulator, value, index, object);
  });
  return accumulator;
}

module.exports = transform;