The easiest way to test your TypeScript types with Jest.

• Zero-config
• Works with your current Jest setup
• Write your regular tests in either JavaScript or TypeScript
• Jest outputs descriptive, helpful error messages when tests fail

# Install with npm
npm i -D jest-tsd @tsd/typescript

# Or install with yarn
yarn add --dev jest-tsd @tsd/typescript

Note: @tsd/typescript will be used to compile your type tests. If you have compiling issues, adjust its version to match the version of typescript you have installed.

Type tests are written in a separate .test-d.ts file from the rest of your tests, and then run from within your test file by calling expectTypeTestsToPassAsync().

1. Call expectTypeTestsToPassAsync() in your Jest test file

   // src/dir/foo.test.[jt]s
   
   import {expectTypeTestsToPassAsync} from 'jest-tsd'
   
   it('should not produce static type errors', async () => {
     await expectTypeTestsToPassAsync(__filename)
   })

   If for some reason your .test-d.ts type definition test file is not co-located to your Jest test file, you can pass the definition test file's absolute path to expectTypeTestsToPassAsync() (instead of __filename).

2. Create a type definition test file .test-d.ts in the same directory with the same name as your Jest test file

   • e.g. If your Jest test is located at src/dir/foo.test.js, create a src/dir/foo.test-d.ts file

   • In your type definition test you can import the assertion functions from jest-tsd

     // src/dir/foo.test-d.ts
     
     import {
       expectType,
       expectError,
       expectNotType,
       expectAssignable,
       expectNotAssignable,
       expectDeprecated,
       expectNotDeprecated,
     } from 'jest-tsd'
     
     test('Array.from() can be called with a variety of types', () => {
       Array.from('foo')
       Array.from(new Set())
       Array.from([1, 2, 3])
       Array.from({length: 3}, (_, i) => i)
     })
     
     test('Adding two numbers should produce a number', () => {
       expectType<number>(1 + 1)
       expectType<number>(2 + 2)
     })
     
     test('A plain object should not have a filter function', () => {
       expectError({}.filter((x: any) => x))
     })
     
     test('Partial<T> should make all keys optional', () => {
       expectAssignable<Partial<{a: string; b: string}>>({})
     })

Your type definition tests can also use JSX!

The only requirement is that both your Jest test file and your type definition test file end with x.

For example:

src/dir/foo.test.jsx
src/dir/foo.test-d.tsx

These assertions are re-exported from tsd-lite:

Asserts that the type of expression is identical to type T.

Asserts that expression throws an error.

Asserts that the type of expression is not identical to type T.

Asserts that the type of expression is assignable to type T.

Asserts that the type of expression is not assignable to type T.

Asserts that expression is marked as @deprecated.

Asserts that expression is not marked as @deprecated.