Zod transform and pipe

Quick answer: what does Zod transform do?

`.transform()` runs after validation and maps the parsed value into a new shape, which also changes the schema's inferred output type. Use `.pipe()` to validate the transformed value with a second schema, and use `z.codec()` in Zod 4 when the transform must be reversible.

Transform validated input

`.transform()` receives the already-validated value and returns whatever you map it to. The inferred output type follows the return value, so downstream code sees the reshaped data.

import * as z from "zod";

const TrimmedName = z.string().min(1).transform((value) => value.trim());

TrimmedName.parse("  Jane  "); // "Jane"

const Slug = z.object({ title: z.string() }).transform((value) => ({
  ...value,
  slug: value.title.toLowerCase().replace(/\s+/g, "-"),
}));

Validate the transformed value with pipe

When a transform produces a value you still need to validate, `.pipe()` passes the output into a second schema. This is the clean way to parse a string into a number and then enforce numeric rules.

const PositiveFromString = z
  .string()
  .transform((value) => Number(value))
  .pipe(z.number().int().positive());

PositiveFromString.parse("42"); // 42
PositiveFromString.safeParse("-3").success; // false

Reversible transforms with z.codec()

A plain transform is one-way: it cannot turn the output back into the input. Zod 4 adds `z.codec()` for two-way conversions, with `decode` and `encode` so you can serialize back to the original form.

const DateCodec = z.codec(z.iso.datetime(), z.date(), {
  decode: (isoString) => new Date(isoString),
  encode: (date) => date.toISOString(),
});

const parsed = DateCodec.decode("2026-06-17T00:00:00.000Z"); // Date
const serialized = DateCodec.encode(parsed); // ISO string

transform vs refine: which to use

These solve different problems. Reach for the one that matches your intent so the inferred type and error behavior stay predictable.

• Use `.refine()` or `.superRefine()` to validate without changing the value.
• Use `.transform()` to reshape the value when validation passes.
• Use `.pipe()` when a transformed value needs another validation pass.
• Use `z.codec()` when you need both decode and encode directions.

Reshaping data in Zod

Use FrameworkKit to generate the starter code, then review the output before shipping it in production.

Zod validation resources

FAQ

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