Factory bot like mock API for js

create-mock-fn.ts

type Primitive = string | number | boolean | bigint | symbol | undefined | null;

// When the type is an array we want you to make your own createMockFn
// for the array elements
type CustomDeepPartial<T> = {
  [P in keyof T]?: T[P] extends unknown[]
    ? T[P]
    : T[P] extends Primitive
      ? T[P]
      : CustomDeepPartial<T[P]>;
};

export const delegateMockFn =
  <T>(defaults: T) =>
  (overrides: CustomDeepPartial<T> = {}): T => {
    const mocked: Record<string, unknown> = {};

    if (overrides === null) {
      // @ts-expect-error: we're only going to prove function boundaries (in->out)
      return null;
    }

    // @ts-expect-error: we're only going to prove function boundaries (in->out)
    for (const [key, value] of Object.entries(defaults)) {
      if (
        typeof value === 'object' &&
        value !== null &&
        !Array.isArray(value)
      ) {
        mocked[key] =
          key in overrides
            ? // @ts-ignore: we're only going to prove function boundaries (in->out)
              delegateMockFn(value)(overrides[key])
            : value;
      } else {
        // @ts-expect-error: we're only going to prove function boundaries (in->out)
        mocked[key] = key in overrides ? overrides[key] : value;
      }
    }

    return mocked as T;
  };

type TraitMap<T> = Record<string, () => CustomDeepPartial<T>>;

export const createMockFn =
  <T, M extends TraitMap<T> = TraitMap<T>>(
    makeDefaults: () => T,
    traitMap?: M
  ) =>
  (overrides: CustomDeepPartial<T> = {}, traits: (keyof M)[] = []): T => {
    const withTraits = traits.reduce(
      (current, trait) => delegateMockFn(current)(traitMap![trait]()),
      makeDefaults()
    );

    return delegateMockFn(withTraits)(overrides);
  };

usage like:

type MyResponse = { id: string; accepted: boolean, rejectionReason: string | null } 

const mockResponse = createMockFn(
  (): MyResponse => ({ id: 'ID', accepted: true, rejectionReason: null }),
  {
     rejected: () => ({ accepted: false, rejectionReason: 'trait rejected' })
  }
)

mockResponse() //  { id: 'ID', accepted: true, rejectionReason: null }
mockResponse({}, ['rejected']) //  { id: 'ID', accepted: false, rejectionReason: 'trait rejected' }
mockResponse({ rejectionReason: 'custom' }, ['rejected']) //  { id: 'ID', accepted: false, rejectionReason: 'custom' }