lodash/transform.js

import arrayEach from './_arrayEach';
import baseCreate from './_baseCreate';
import baseForOwn from './_baseForOwn';
import baseIteratee from './_baseIteratee';
import getPrototype from './_getPrototype';
import isArray from './isArray';
import isFunction from './isFunction';
import isObject from './isObject';
import isTypedArray from './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. 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 {Array|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) || isTypedArray(object);
  iteratee = baseIteratee(iteratee, 4);

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

export default transform;