TypeFuzz

Property-based testing for TypeScript that drops straight into your existing test runner. No wrappers, no config — write fuzz tests like you write unit tests.

Why TypeFuzz?

• Test runner integration — fuzzIt works like it/test inside Vitest and Jest. No separate CLI.
• Zero dependencies — ships nothing you don't need. Vitest, Jest, and Zod are optional peer deps.
• TypeScript-first — full type inference from generators to predicates. No casting.
• Deterministic — seeded RNG means every failure is reproducible with a single seed value.
• Built-in shrinking — automatic counterexample minimisation using binary search, length reduction, and delta debugging.

Install

npm install -D typefuzz

pnpm add -D typefuzz
yarn add -D typefuzz
bun add -D typefuzz

Quick example

import { fuzzIt } from 'typefuzz/vitest';
import { gen } from 'typefuzz';

fuzzIt('sort is idempotent', gen.array(gen.int(-100, 100), { minLength: 0, maxLength: 50 }), (arr) => {
  const sorted = [...arr].sort((a, b) => a - b);
  const sortedTwice = [...sorted].sort((a, b) => a - b);
  return JSON.stringify(sorted) === JSON.stringify(sortedTwice);
});

If the property fails, TypeFuzz shrinks the input to the smallest counterexample and prints the seed for replay.

TypeFuzz vs fast-check

| | TypeFuzz | fast-check | | --- | --- | --- | | Test runner integration | fuzzIt — one function, same signature as it | @fast-check/vitest with test.prop | | API surface | Small, opinionated | Large, highly configurable | | Shrinking | Binary search + delta debugging | Shrink-on-generate | | Model-based testing | Built-in | Built-in | | Zod integration | typefuzz/zod adapter | Community zod-fast-check | | Async support | Native | Native |

Features

Model-based testing

Test stateful systems against a simplified model. TypeFuzz generates random command sequences, runs them against both your system and the model, and shrinks failing sequences to the shortest reproduction. Read more →

Zod adapter

Already have Zod schemas? Generate test data directly from them with zodArbitrary. Supports objects, arrays, unions, discriminated unions, transforms, and more. Read more →

Shrinking

When a property fails, TypeFuzz automatically minimises the counterexample — binary search for numbers, length halving for strings and arrays, delta debugging for model-based command sequences. Read more →

Documentation

• Documentation Index
• Getting Started
• Generators
• Core API
• Test Runner Integrations
• Model-Based Testing
• Zod Adapter
• Shrinking and Replay
• FAQ
• Releasing
• AI Testing Playbook
• AI Agent Compatibility
• Agent Instructions
• LLM Index

Development

bun install
bun run lint
bun run typecheck
bun run test

Licence

MIT