code: function mergeHook (
    parentVal: ?Array<Function>,
    childVal: ?Function | ?Array<Function>
  ): ?Array<Function> {
    return childVal
      ? parentVal
        ? parentVal.concat(childVal)
        : Array.isArray(childVal)
          ? childVal
          : [childVal]
      : parentVal
  }
docs: Hooks and props are merged as arrays.