code: function assignMergeValue(object, key, value) {
    if ((value !== undefined && !eq_1(object[key], value)) ||
        (value === undefined && !(key in object))) {
      _baseAssignValue(object, key, value);
    }
  }
docs: This function is like `assignValue` except that it doesn't assign
  `undefined` values.
  
  @private
  @param {Object} object The object to modify.
  @param {string} key The key of the property to assign.
  @param {*} value The value to assign.