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;

FYI, my colleague has submitted this to graphql-tools in ardatan/graphql-tools#1084

Thanks for sharing it with the world.

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

This snippet of code seriously saved my day !!