lodash/_customDefaultsAssignIn.js

define(['./eq'], function(eq) {

  /** Used as a safe reference for `undefined` in pre-ES5 environments. */
  var undefined;

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

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

  /**
   * Used by `_.defaults` to customize its `_.assignIn` use to assign properties
   * of source objects to the destination object for all destination properties
   * that resolve to `undefined`.
   *
   * @private
   * @param {*} objValue The destination value.
   * @param {*} srcValue The source value.
   * @param {string} key The key of the property to assign.
   * @param {Object} object The parent object of `objValue`.
   * @returns {*} Returns the value to assign.
   */
  function customDefaultsAssignIn(objValue, srcValue, key, object) {
    if (objValue === undefined ||
        (eq(objValue, objectProto[key]) && !hasOwnProperty.call(object, key))) {
      return srcValue;
    }
    return objValue;
  }

  return customDefaultsAssignIn;
});