Schema Composition with Merge, Extend, Pick, and Omit

Why Schema Composition Matters

In real-world projects, you often reuse and reshape your schemas:

• Share a base schema across multiple variations
• Exclude fields before exposing them in the frontend
• Split logic between database, form validation, and API

Zod provides several helpful tools for this: merge(), extend(), pick(), and omit().

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Merging Schemas with merge()

Combines two schemas of type z.object() into one:

const Base = z.object({ id: z.string() });
const Profile = z.object({ name: z.string() });

const User = Base.merge(Profile);

The result:

User.parse({ id: "123", name: "Alice" }); // ✅

> ⚠️ Note: Conflicts will favor the second schema (Profile in this case).

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Adding Fields with extend()

Use extend() to add more fields to an existing schema:

const User = z.object({ name: z.string() });

const Admin = User.extend({
  role: z.literal("admin"),
});

This preserves the original schema and creates a new one with additional fields.

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Selecting Fields with pick()

If you only want part of a schema (e.g. just id and email), use pick():

const FullUser = z.object({
  id: z.string(),
  name: z.string(),
  email: z.string(),
});

const PublicUser = FullUser.pick({ id: true, email: true });

Now PublicUser only accepts { id, email }.

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Removing Fields with omit()

The opposite of pick(). Remove sensitive/internal fields like passwords:

const DBUser = z.object({
  id: z.string(),
  email: z.string(),
  password: z.string(),
});

const SafeUser = DBUser.omit({ password: true });

SafeUser no longer allows password to appear.

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Summary

• Use .merge() to combine schemas
• Use .extend() to add fields cleanly
• Use .pick() and .omit() to reshape schemas for different use cases

Next up: Lesson 7 – Refinement, superRefine, and Custom Validators for powerful validation logic beyond types.

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