Type Testing

Test TypeScript types without runtime execution.

Setup

Type tests use .test-d.ts extension:

// math.test-d.ts
import { expectTypeOf } from 'vitest'
import { add } from './math'

test('add returns number', () => {
  expectTypeOf(add).returns.toBeNumber()
})

Configuration

defineConfig({
  test: {
    typecheck: {
      enabled: true,
      
      // Only type check
      only: false,
      
      // Checker: 'tsc' or 'vue-tsc'
      checker: 'tsc',
      
      // Include patterns
      include: ['**/*.test-d.ts'],
      
      // tsconfig to use
      tsconfig: './tsconfig.json',
    },
  },
})

expectTypeOf API

import { expectTypeOf } from 'vitest'

// Basic type checks
expectTypeOf<string>().toBeString()
expectTypeOf<number>().toBeNumber()
expectTypeOf<boolean>().toBeBoolean()
expectTypeOf<null>().toBeNull()
expectTypeOf<undefined>().toBeUndefined()
expectTypeOf<void>().toBeVoid()
expectTypeOf<never>().toBeNever()
expectTypeOf<any>().toBeAny()
expectTypeOf<unknown>().toBeUnknown()
expectTypeOf<object>().toBeObject()
expectTypeOf<Function>().toBeFunction()
expectTypeOf<[]>().toBeArray()
expectTypeOf<symbol>().toBeSymbol()

Value Type Checking

const value = 'hello'
expectTypeOf(value).toBeString()

const obj = { name: 'test', count: 42 }
expectTypeOf(obj).toMatchTypeOf<{ name: string }>()
expectTypeOf(obj).toHaveProperty('name')

Function Types

function greet(name: string): string {
  return `Hello, ${name}`
}

expectTypeOf(greet).toBeFunction()
expectTypeOf(greet).parameters.toEqualTypeOf<[string]>()
expectTypeOf(greet).returns.toBeString()

// Parameter checking
expectTypeOf(greet).parameter(0).toBeString()

Object Types

interface User {
  id: number
  name: string
  email?: string
}

expectTypeOf<User>().toHaveProperty('id')
expectTypeOf<User>().toHaveProperty('name').toBeString()

// Check shape
expectTypeOf({ id: 1, name: 'test' }).toMatchTypeOf<User>()

Equality vs Matching

interface A { x: number }
interface B { x: number; y: string }

// toMatchTypeOf - subset matching
expectTypeOf<B>().toMatchTypeOf<A>()  // B extends A

// toEqualTypeOf - exact match
expectTypeOf<A>().not.toEqualTypeOf<B>()  // Not exact match
expectTypeOf<A>().toEqualTypeOf<{ x: number }>()  // Exact match

Branded Types

type UserId = number & { __brand: 'UserId' }
type PostId = number & { __brand: 'PostId' }

expectTypeOf<UserId>().not.toEqualTypeOf<PostId>()
expectTypeOf<UserId>().not.toEqualTypeOf<number>()

Generic Types

function identity<T>(value: T): T {
  return value
}

expectTypeOf(identity<string>).returns.toBeString()
expectTypeOf(identity<number>).returns.toBeNumber()

Nullable Types

type MaybeString = string | null | undefined

expectTypeOf<MaybeString>().toBeNullable()
expectTypeOf<string>().not.toBeNullable()

assertType

Assert a value matches a type (no assertion at runtime):

import { assertType } from 'vitest'

function getUser(): User | null {
  return { id: 1, name: 'test' }
}

test('returns user', () => {
  const result = getUser()
  
  // @ts-expect-error - should fail type check
  assertType<string>(result)
  
  // Correct type
  assertType<User | null>(result)
})

Using @ts-expect-error

Test that code produces type error:

test('rejects wrong types', () => {
  function requireString(s: string) {}
  
  // @ts-expect-error - number not assignable to string
  requireString(123)
})

Running Type Tests

# Run type tests
vitest typecheck

# Run alongside unit tests
vitest --typecheck

# Type tests only
vitest --typecheck.only

Mixed Test Files

Combine runtime and type tests:

// user.test.ts
import { describe, expect, expectTypeOf, test } from 'vitest'
import { createUser } from './user'

describe('createUser', () => {
  test('runtime: creates user', () => {
    const user = createUser('John')
    expect(user.name).toBe('John')
  })

  test('types: returns User type', () => {
    expectTypeOf(createUser).returns.toMatchTypeOf<{ name: string }>()
  })
})

Key Points

• Use .test-d.ts for type-only tests
• expectTypeOf for type assertions
• toMatchTypeOf for subset matching
• toEqualTypeOf for exact type matching
• Use @ts-expect-error to test type errors
• Run with vitest typecheck or --typecheck

<!-- Source references:

• https://vitest.dev/guide/testing-types.html
• https://vitest.dev/api/expect-typeof.html

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