Merge mocked GraphQL resolvers

mergeResolvers.js

// @flow

type ResolvedScalar = string | number | boolean | null;
type ResolvedValue =
  | ResolvedScalar
  | Array<ResolvedValue>
  | {[key: string]: ResolvedValue};
type ResolverFunction = (...args: Array<any>) => ResolvedValue;

export type ResolverMap = {
  [key: string]: () => {[key: string]: ResolvedValue | ResolverFunction} | null,
};

/**
 * Given a map of mock GraphQL resolver functions, merge in a map of
 * desired mocks. Generally, `target` will be the default mocked values,
 * and `input` will be the values desired for a portal example or Jest tests.
 */
const mergeResolvers = (target: ResolverMap, input: ResolverMap) => {
  const inputTypenames = Object.keys(input);
  const merged: ResolverMap = inputTypenames.reduce(
    (accum, key) => {
      const inputResolver = input[key];
      if (target.hasOwnProperty(key)) {
        const targetResolver = target[key];
        const resolvedInput = inputResolver();
        const resolvedTarget = targetResolver();
        if (
          !!resolvedTarget &&
          !!resolvedInput &&
          typeof resolvedTarget === 'object' &&
          typeof resolvedInput === 'object' &&
          !Array.isArray(resolvedTarget) &&
          !Array.isArray(resolvedInput)
        ) {
          const newValue = {...resolvedTarget, ...resolvedInput};
          return {
            ...accum,
            [key]: () => newValue,
          };
        }
      }
      return {...accum, [key]: inputResolver};
    },
    {...target},
  );
  return merged;
};

export default mergeResolvers;

would appreciate your review of the above and whether you're okay to merge this!

This snippet of code seriously saved my day !!