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Conditional Type Checks

npm version stable

As TypeScript's type system becomes more complex it's useful to be able to write tests for what a type should be.

This library offers reusable conditional types to do these checks.

Type Checks

These will resolve to the type true when they match and false otherwise.

  • IsNullable<T> - Checks if T is possibly null or undefined.
  • IsExact<T, U> - Checks if T exactly matches U.
  • Has<T, U> - Checks if T has U.
  • NotHas<T, U> - Checks if T does not have U.
  • IsAny<T> - Checks if T is the any type.
  • IsNever<T> - Checks if T is the never type.
  • IsUnknown<T> - Checks if T is the unknown type.
  • More to come...

Ways to Test

Use what you prefer:

  1. The AssertTrue, AssertFalse, or Assert types.
  2. The assert function.

Use with AssertTrue, AssertFalse, and Assert

Doing a test:

import { AssertTrue, AssertFalse, Has, IsNever, IsNullable } from "conditional-type-checks";

const result = someFunction(someArg);

type doTest = AssertTrue<Has<typeof result, string> | IsNullable<typeof result>>
    | AssertFalse<IsNever<typeof result>>
    | Assert<Has<typeof result, number>, true>;

Warning: Do not use an intersection type between checks (ex. Has<string | number, string> & IsNever<never>) because it will cause everything to pass if only one of the checks passes.

Use with assert

Doing a test:

import { assert, IsExact } from "conditional-type-checks";

const result = someFunction(someArg);

// compile error if the type of `result` is not exactly `string | number`
assert<IsExact<typeof result, string | number>>(true);

Failure:

// causes a compile error that `true` is not assignable to `false`
assert<IsNullable<string>>(true); // string is not nullable

Install

npm install --save-dev conditional-type-checks

About

Types for testing TypeScript types.

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