fuath / conditional-type-checks

Types for testing TypeScript types.

Conditional Type Checks

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 types to do these checks.

Type Checks

These types do the checks:

• IsNullableType<T> - Checks if T is possibly null or undefined.
• IsExactType<T, U> - Checks if T exactly matches U.
• HasType<T, U> - Checks if T has U.
• NotHasType<T, U> - Checks if T does not have U.
• IsNeverType<T> - Checks if T is the never type.
• More to come...

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

Ways to Test

Use either (whichever you prefer);

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

Use with AssertTrue and AssertFalse

Doing a test:

import { AssertTrue, AssertFalse, HasType, IsNeverType, IsNullableType } from "conditional-type-checks";

const result = someFunction(someArg);

type doTest = AssertTrue<HasType<typeof result, string> | IsNullableType<typeof result>>
    | AssertFalse<IsNeverType<typeof result>>;

Warning: Do not use an intersection type in the type parameter for AssertTrue and AssertFalse (ex. HasType<string | number, string> & IsNeverType<never>) because it will cause the assertion to pass if only one of the checks passes.

Use with assert

Doing a test:

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

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

Failure:

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

Install

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