zod-fast-check

zod-fast-check

A small library to automatically derive fast-check arbitraries from schemas defined using the validation library Zod. These enables easy and thorough property-based testing.

The tool works with Zod 3.5 or later.

Usage

Here is a complete example using Jest.

import * as z from "zod";
import * as fc from "fast-check";
import { ZodFastCheck } from "zod-fast-check";

// Define a Zod schema
const User = z.object({
  firstName: z.string(),
  lastName: z.string(),
});

// Define an operation using the data type
function fullName(user: unknown): string {
  const parsedUser = User.parse(user);
  return `${parsedUser.firstName} ${parsedUser.lastName}`;
}

// Create an arbitrary which generates valid inputs for the schema
const userArbitrary = ZodFastCheck().inputOf(User);

// Use the arbitrary in a property-based test
test("User's full name always contains their first and last names", () =>
  fc.assert(
    fc.property(userArbitrary, (user) => {
      const name = fullName(user);
      expect(name).toContain(user.firstName);
      expect(name).toContain(user.lastName);
    })
  ));

The main interface is the ZodFastCheck class, which has the following methods:

inputOf

inputOf<Input>(zodSchema: ZodSchema<unknown, ZodTypeDef, Input>): Arbitrary<Input>

Creates an arbitrary which will generate values which are valid inputs to the schema. This should be used for testing functions which use the schema for validation.

outputOf

outputOf<Output>(zodSchema: ZodSchema<Output, ZodTypeDef, unknown>): Arbitrary<Output>

Creates an arbitrary which will generate values which are valid outputs of parsing the schema. This means any transformations have already been applied to the values. This should be used for testing functions which do not use the schema directly, but use data parsed by the schema.

override

override<Input>(schema: ZodSchema<unknown, ZodTypeDef, Input>, arbitrary: Arbitrary<Input>): ZodFastCheck

Returns a new ZodFastCheck instance which will use the provided arbitrary when generating inputs for the given schema. This includes if the schema is used as a component of a larger schema.

For example, if we have a schema which validates that a string has a prefix, we can define an override to produce valid values.

const WithFoo = z.string().regex(/^foo/);

const zodFastCheck = ZodFastCheck().override(
  WithFoo,
  fc.string().map((s) => "foo" + s)
);

const arbitrary = zodFastCheck.inputOf(z.array(WithFoo));

Schema overrides are matched based on object identity, so you need to define the override using the exact schema object, rather than an equivalent schema.

If you need to use zod-fast-check to generate the override, it is easy to end up with a circular dependency. You can avoid this by defining the override lazily using a function. This function is called with the ZodFastCheck instance as an argument.

const WithFoo = z.string().regex(/^foo/);

const zodFastCheck = ZodFastCheck().override(WithFoo, (zfc) =>
  zfc.inputOf(z.string()).map((s) => "foo" + s)
);

const arbitrary = zodFastCheck.inputOf(z.array(WithFoo));

Supported Zod Schema Features

Data types

✅ string (including email, UUID and URL)
✅ number
✅ nan
✅ bigint
✅ boolean
✅ date
✅ undefined
✅ null
✅ array
✅ object
✅ union
✅ discriminated union
✅ tuple
✅ record
✅ map
✅ set
✅ function
✅ literal
✅ enum
✅ nativeEnum
✅ promise
✅ any
✅ unknown
✅ void
✅ optional
✅ nullable
✅ default
✅ branded types
✅ transforms
✅ refinements (see below)
❌ intersection
❌ lazy
❌ never

Refinements

Refinements are supported, but they are produced by filtering the original arbitrary by the refinement function. This means that for refinements which have a very low probability of matching a random input, it will not be able to generate valid values. This is most common when using refinements to check that a string matches a particular format. If this occurs, it will throw a ZodFastCheckGenerationError.

In cases like this, it is recommended to define an override for the problematic subschema.