lodash/invert.js

define(['./internal/arrayReduce', './keys'], function(arrayReduce, keys) {

  /** Used for built-in method references. */
  var objectProto = Object.prototype;

  /** Used to check objects for own properties. */
  var hasOwnProperty = objectProto.hasOwnProperty;

  /**
   * Creates an object composed of the inverted keys and values of `object`.
   * If `object` contains duplicate values, subsequent values overwrite property
   * assignments of previous values unless `multiVal` is `true`.
   *
   * @static
   * @memberOf _
   * @category Object
   * @param {Object} object The object to invert.
   * @param {boolean} [multiVal] Allow multiple values per key.
   * @param- {Object} [guard] Enables use as an iteratee for functions like `_.map`.
   * @returns {Object} Returns the new inverted object.
   * @example
   *
   * var object = { 'a': 1, 'b': 2, 'c': 1 };
   *
   * _.invert(object);
   * // => { '1': 'c', '2': 'b' }
   *
   * // with `multiVal`
   * _.invert(object, true);
   * // => { '1': ['a', 'c'], '2': ['b'] }
   */
  function invert(object, multiVal, guard) {
    return arrayReduce(keys(object), function(result, key) {
      var value = object[key];
      if (multiVal && !guard) {
        if (hasOwnProperty.call(result, value)) {
          result[value].push(key);
        } else {
          result[value] = [key];
        }
      }
      else {
        result[value] = key;
      }
      return result;
    }, {});
  }

  return invert;
});