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Conditional Type Checks

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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.

Type Checks

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

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 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.

Use with assert

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

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