@skarab/tssert

Micro TypeScript assertion (test) library.

Add your assertions to your usual test suite and run tsc --noEmit, enjoy!

📌 Please note that this library does not perform any check at runtime, but only at compile time (or in your IDE).

Why ?

An attempt to reduce the verbosity of type testing.

Install

pnpm add @skarab/tssert typescript

Usages

With a full description of the error

import { expectType } from '@skarab/tssert';

expectType<boolean>().toAccept(true);
expectType<true>().toAccept(false); // ts-error with description

Without error description, but stricter and more flexible

expectType('Hello').toExtend('42').toBe(true);
expectType<string>().toExtend('42').toBe(true);
expectType('Hello').toExtend<string>().toBe(true);
expectType<string>().toExtend<string>().toBe(true);

expectType<boolean>().toExtend<true>().toBe(true); // ts-error
expectType<true>().toExtend<boolean>().toBe(true);

When you move the mouse over toAccept, toExtend and toEqual you can see the expected and received values.

API

toExtend()

interface TypeA {
	a: 42;
	b: string | number;
}

interface TypeB {
	a: 42;
	b: number;
}

expectType<TypeB>().toExtend<TypeA>().toBe(true);
expectType<TypeA>().toExtend<TypeB>().toBe(true); // ts-error

toEqual()

expectType<TypeA>().toEqual<TypeA>().toBe(true);
expectType<TypeA>().toEqual<TypeB>().toBe(true); // ts-error
expectType<TypeB>().toEqual<TypeA>().toBe(true); // ts-error

Types ...

import type { ExpectExtend, ExpectEqual } from '@skarab/tssert';

type A = ExpectExtend<TypeB, TypeA>; // true
type B = ExpectEqual<TypeA, TypeB>; // false

TypeScript ~~god~~ strict mode

It is strongly recommended to activate the strict mode of TypeScript which will activate all checking behaviours that results in stronger guarantees of the program's correctness.

Contributing 💜

See CONTRIBUTING.md