code: function customDefaultsAssignIn(objValue, srcValue, key, object) {
        if (objValue === undefined ||
            (eq(objValue, objectProto[key]) && !hasOwnProperty.call(object, key))) {
          return srcValue;
        }
        return objValue;
      }
docs: 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.