🦆 @gentleduck/ttest

TypeScript Type-Level Test Framework. Assert your types like you mean it, at compile time.

💡 What is this?

ttest is a zero-runtime library for writing type-level tests in TypeScript.

You spend countless hours designing complex types and systems, ttest gives you a way to validate them with confidence. No console.log, no runtime hacks, just pure type assertion at compile-time.

✨ Features

• ✅ Zero-runtime: Fully erased at compile time, no emitted JS.
• 📐 Precise assertions: Verify assignability, equality, and inference.
• 🧠 IDE-friendly: Fails type-checking with helpful messages.
• 🧪 Works with your favorite setup (e.g. tsc, tsd, vitest + @ts-expect-error)
• 🦆 Designed for duck-typed systems, schema builders, type-safe APIs, and ORMs.

📦 Installation

pnpm add -D @gentleduck/ttest
# or
npm install --save-dev @gentleduck/ttest

🚀 Quick Start

type Schema = InferSchema<"CREATE TABLE users (id INT PRIMARY KEY, email TEXT NOT NULL)">;

type Test_Column = AssertTrue<
  Equal<Schema['email'], string>,
  'Expected SQL "TEXT" to infer as string'
>;

❓ Why Not tsd or expect-type?

@gentleduck/ttest is inspired by them but designed for framework authors and type-heavy codebases:

• Cleaner API for grouped tests.
• More powerful inference testing.
• Easier integration with custom toolchains.
• It's a duck thing 🦆, it fits your ecosystem.

🧙 Advanced Patterns

type X1 = XOR<{ a: number; common: string }, { b: boolean; common: string }>

// expected: { a: number } | { b: boolean }
type Test_X1 = AssertTrue<
  Equal<X1, { a: number } | { b: boolean }>,
  'Expected XOR to produce mutually exclusive union of properties'
>;

🧠 Philosophy

Runtime tests test behavior. Type tests test contracts.

If you’re building a schema system, compiler, or framework, types are your architecture. You can’t trust what you can’t test. @gentleduck/ttest exists to close the feedback loop between your types and your logic.

🦆 License

MIT © GentleDuck