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 help test your types.

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...

Use what you prefer:

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

Doing a test:

import type {
  AssertFalse,
  AssertTrue,
  Has,
  IsNever,
  IsNullable,
} from "https://deno.land/x/conditional_type_checks/mod.ts";

const result = someFunction(someArg);

type _test =
  | 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.

Doing a test:

import {
  assert,
  IsExact,
} from "https://deno.land/x/conditional_type_checks/mod.ts";

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

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