Experiments to deeply merge nested objects in Zod library.

zod-deep-merge-experiment.ts

// mkdir zod-deep-merge-experiement
// Copy and paste this code to your editor, save as `index.ts`
// yarn init --yes
// yarn add ts-node zod typescript @types/node
// yarn run tsc --init
// yarn run ts-node .
import { strict as assert } from "node:assert";

import { z } from "zod";

/**
 * All schemas
 */

const personSchema = z.object({
  name: z.string(),
  hobbies: z.array(z.string()),
  attributes: z.record(z.string()),
  company: z.object({
    name: z.string(),
    address: z.string(),
    train: z.enum(["Yamanote", "Fukutoshin", "Tobu-Tojo", "Seibu Shinjuku"]),
  }),
});

const alternativePersonSchema = z.object({
  address: z.string(),
  age: z.number(),
  description: z.string(),
  company: z.object({
    profession: z.string(),
    description: z.string(),
  }),
});

/**
 * First solution, dirty solution.
 */

const coalescedPerson = z
  .object({})
  .merge(personSchema)
  .merge(alternativePersonSchema)
  .deepPartial()
  .and(personSchema.pick({ company: true }).deepPartial());

type CoalescedPerson = z.infer<typeof coalescedPerson>;

/**
 * Second solution, a bit clearer because there's no `pick` and `deepPartial` in one line. We just
 * create a new schema based on what is required to be deeply merged, so we can reuse this in the next step.
 */

const nestedCompany = z.object({
  company: z
    .object({})
    .merge(personSchema.shape.company)
    .merge(alternativePersonSchema.shape.company),
});

const alternativeCoalescedPerson = z
  .object({})
  .merge(personSchema)
  .merge(alternativePersonSchema)
  .merge(nestedCompany)
  .deepPartial();

type AlternativeCoalescedPerson = z.infer<typeof alternativeCoalescedPerson>;

if (require.main === module) {
  const T: CoalescedPerson = { company: { profession: "Building Management" } };
  const K: AlternativeCoalescedPerson = { company: { train: "Yamanote" } };

  assert.deepStrictEqual(T, { company: { profession: "Building Management" } });
  assert.deepStrictEqual(K, { company: { train: "Yamanote" } });

  console.log(T, K);
}