Creating extendable Zod schemas with refine

I was working on a project recently where I was using Zod to validate my schemas. In one schema I had, for password validation, I found I was repeating it in a few different places. I decided to take to try and extract this schema to a shared location.

I didn’t need to just create a shared schema, but an expandable one as well. Some schemas also had properties that always weren’t needed in every area. Each schema was also using custom validation logic via refine, so that had to be taken into account as well.

In this article, we’ll look at how you can create expandable Zod schemas that also apply custom refinements.

The issue with ‘extend()‘

The simplest answer here would be to create the base schema and use Zod’s extend method to build on top of it. This is what I tried originally, but it didn’t work exactly as I’d hoped.

Let’s say I have the following schema for password validation

export const passwordValidationSchema = z.object({
  password: z.string(),
  confirmPassword: z.string(),
});

I could then build another schema on it using extend

export const userRegisterSchema = passwordValidationSchema.extend({
  username: z.string(),
  email: z.string().email(),
  // etc
});

This works great, until I want to add a refinement to passwordValidationSchema

export const passwordValidationSchema = z.object({
  password: z.string(),
  confirmPassword: z.string(),
}).refine(
  (data) => data.password === data.confirmPassword, {
    message: "Passwords do not match",
    path: ["confirmPassword"],
});

The userRegisterSchema now breaks. This is because transform and refine are considered effects, not objects. The result we get back from extend is ZodObject<T>, while the result from refine is ZodEffect<ZodObject<T>>. Zod validates types first, then passes it through any refinements/transformations.

This means once an object has been refined/transformed, we can no longer extend it.

Extending a refined schema

Luckily, there is a simple solution for this. We can create a helper function that accepts any Zod schema and use expand. After that, we can then refine and return the resulting schema.

export const passwordValidationSchema = z.object({
  password: z.string(),
  confirmPassword: z.string(),
});

export function refinePasswordValidationSchema(schema: ZodRawShape) {
  return passwordValidationSchema
    .extend(schema)
    .refine((data) => data.password === data.confirmPassword, {
      message: "Passwords do not match",
      path: ["confirmPassword"],
    });
}

With this, we are now able to expand our original schema and apply a refinement to it.

Resouces

• .extend after .refine
• Zod