assertion-types

assertion-types is a tiny, fast, and, easy-to-learn type-testing library that requires no extra build/test setup.

Introduction

The assertion-types library provides four simple tests for inspecting types: Equals, Extends, NotEquals, and NotExtends. Along with the included Assert type, these utilities allow you to test that the results of your types are what you expect them to be. Because this method requires no other tools besides the TypeScript compiler, errors will show up right in your editor and you won't be able to build your project if your tests fail.

Usage is simple:

import {Assert, Equals, Extends, NotEquals, NotExtends} from "assertion-types";

// Using `export` to avoid unused type errors

// Equals:
export type EqualsExample = Assert<Equals<123, 123>>;

// NotEquals:
export type NotEqualsExample = Assert<NotEquals<123, 456>>;

// Extends:
export type ExtendsExample = Assert<Extends<123, number>>;

// NotExtends:
export type NotExtendsExample = Assert<NotExtends<number, 123>>;

Getting Started

Install via NPM:

npm install --save-dev assertion-types

After that, just import into your test files:

import * as assertionTypes from "assertion-types";
// OR
import {Assert, Equals, Extends, NotEquals, NotExtends} from "assertion-types";

No need to run any commands to run the tests - if they error, they will error when the TypeScript compiler runs.

Purpose

Any sufficiently complex TypeScript project will eventually require the development of utility types, which are often left untested because unit tests cannot be written for types. However, testing these types ensures that your type system is sound and therefore you can rely on type errors to prevent the cases that you assume cannot happen based on your understanding of your types.

Documentation

Assert<Test>

Simply asserts that the type of Test is true:

export type Good = Assert<true>; // ✔️

// Type 'false' does not satisfy the constraint 'true'. ts(2344)
export type Bad = Assert<false>; // ❌

Tests

For all of the following tests, the resulting type is true if the test passes (and false if it does not). Since Assert tests that the type is true, they will pass if used with Assert.

If any test fails, the error will be ts(2344):

Type 'false' does not satisfy the constraint 'true'.

Equals<A, B>

Tests that A and B are equal (by checking that A extends B and B extends A).

export type GoodA = Assert<Equals<string, string>>; // ✔️
export type GoodB = Assert<Equals<1 | 2, 1 | 2>>; // ✔️

export type BadA = Assert<Equals<"abc", string>>; // ❌
export type BadB = Assert<Equals<1, 1 | 2>>; // ❌

NotEquals<A, B>

Tests that A is not equal to B (by checking that A does not extend B or B does not extend A).

export type GoodA = Assert<NotEquals<"abc", string>>; // ✔️
export type GoodB = Assert<NotEquals<1, 1 | 2>>; // ✔️

export type BadA = Assert<NotEquals<string, string>>; // ❌
export type BadB = Assert<NotEquals<1 | 2, 1 | 2>>; // ❌

Extends<A, B>

Tests that A extends B (A is assignable to B).

export type GoodA = Assert<Extends<"abc", string>>; // ✔️
export type GoodB = Assert<Extends<1, 1 | 2>>; // ✔️

export type BadA = Assert<Extends<"abc", number>>; // ❌
export type BadB = Assert<Extends<string, "abc">>; // ❌

NotExtends<A, B>

Tests that A does not extend B (A cannot be assigned to B).

export type GoodA = Assert<NotExtends<"abc", number>>; // ✔️
export type GoodB = Assert<NotExtends<string, "abc">>; // ✔️

export type BadA = Assert<NotExtends<"abc", string>>; // ❌
export type BadB = Assert<NotExtends<1, 1 | 2>>; // ❌

Limitations

Type equality is checked by examining whether the types are both assignable to each other. It is possible (though rare) that two types could be different but assignable to each other:

export type Limitation = Assert<Equals<any, unknown>>; // ✔️!

Comparison With Alternatives

dtslint

The standard alternative for this is dtslint, which uses $ExpectType comments to test types with an external tool. This tool is in wide usage and works well, however it comes with some disadvantages that led me to create this library:

• It adds an extra step: you always have to run dtslint to test your types.
• It doesn't take advantage of the compiler's inherent ability to test types.
• It is very strict about what a type is. The following should pass, but instead it errors:

  // $ExpectType X<string | number>
  type Example = X<number | string>; // ❌!

tsd

Linked on the dtslint page is an alternative library just for testing types: tsd. tsd appears to solve many problems and is a great contendor for testing types, however:

• It still doesn't take advantage of the compiler's abilities (adds an additional testing step).
• It has a not-insignificant learning curve and doesn't always have the outcomes you'd expect (https://github.com/SamVerschueren/tsd/issues/65).
• Setup requires some extra steps, whereas this library is drop-in.
• It's not actively maintained (pull requests are open from September 2019).