Testing Types ​

Vitest allows you to write tests for your types, using expectTypeOf or assertType syntaxes. By default all tests inside *.test-d.ts files are considered type tests, but you can change it with typecheck.include config option.

Under the hood Vitest calls tsc or vue-tsc , depending on your config, and parses results. Vitest will also print out type errors in your source code, if it finds any. You can disable it with typecheck.ignoreSourceErrors config option.

Keep in mind that Vitest doesn't run these files, they are only statically analyzed by the compiler. Meaning, that if you use a dynamic name or test.each or test.for , the test name will not be evaluated - it will be displayed as is.

Using CLI flags, like --allowOnly and -t are also supported for type checking.

import { assertType, expectTypeOf } from 'vitest'
import { mount } from './mount.js'
test('my types work properly', () => {
  expectTypeOf(mount).toBeFunction()
  expectTypeOf(mount).parameter(0).toMatchTypeOf<{ name: string }>()
  // @ts-expect-error name is a string
  assertType(mount({ name: 42 }))
})

Any type error triggered inside a test file will be treated as a test error, so you can use any type trick you want to test types of your project.

You can see a list of possible matchers in API section .

Reading Errors ​

If you are using expectTypeOf API, refer to the expect-type documentation on its error messages .

When types don't match, .toEqualTypeOf and .toMatchTypeOf use a special helper type to produce error messages that are as actionable as possible. But there's a bit of an nuance to understanding them. Since the assertions are written "fluently", the failure should be on the "expected" type, not the "actual" type ( expect<Actual>().toEqualTypeOf<Expected>() ). This means that type errors can be a little confusing - so this library produces a MismatchInfo type to try to make explicit what the expectation is. For example:

expectTypeOf({ a: 1 }).toEqualTypeOf<{ a: string }>()

Is an assertion that will fail, since {a: 1} has type {a: number} and not {a: string} . The error message in this case will read something like this:

test/test.ts:999:999 - error TS2344: Type '{ a: string; }' does not satisfy the constraint '{ a: \\"Expected: string, Actual: number\\"; }'.
  Types of property 'a' are incompatible.
    Type 'string' is not assignable to type '\\"Expected: string, Actual: number\\"'.
999 expectTypeOf({a: 1}).toEqualTypeOf<{a: string}>()

Note that the type constraint reported is a human-readable messaging specifying both the "expected" and "actual" types. Rather than taking the sentence Types of property 'a' are incompatible // Type 'string' is not assignable to type "Expected: string, Actual: number" literally - just look at the property name ( 'a' ) and the message: Expected: string, Actual: number . This will tell you what's wrong, in most cases. Extremely complex types will of course be more effort to debug, and may require some experimentation. Please raise an issue if the error messages are actually misleading.

The toBe... methods (like toBeString , toBeNumber , toBeVoid etc.) fail by resolving to a non-callable type when the Actual type under test doesn't match up. For example, the failure for an assertion like expectTypeOf(1).toBeString() will look something like this:

test/test.ts:999:999 - error TS2349: This expression is not callable.
  Type 'ExpectString<number>' has no call signatures.
999 expectTypeOf(1).toBeString()
                    ~~~~~~~~~~

The This expression is not callable part isn't all that helpful - the meaningful error is the next line, Type 'ExpectString<number> has no call signatures . This essentially means you passed a number but asserted it should be a string.

If TypeScript added support for "throw" types these error messages could be improved significantly. Until then they will take a certain amount of squinting.

Concrete "expected" objects vs typeargs ​

Error messages for an assertion like this:

expectTypeOf({ a: 1 }).toEqualTypeOf({ a: '' })

Will be less helpful than for an assertion like this:

expectTypeOf({ a: 1 }).toEqualTypeOf<{ a: string }>()

This is because the TypeScript compiler needs to infer the typearg for the .toEqualTypeOf({a: ''}) style, and this library can only mark it as a failure by comparing it against a generic Mismatch type. So, where possible, use a typearg rather than a concrete type for .toEqualTypeOf and toMatchTypeOf . If it's much more convenient to compare two concrete types, you can use typeof :

const one = valueFromFunctionOne({ some: { complex: inputs } })
const two = valueFromFunctionTwo({ some: { other: inputs } })
expectTypeOf(one).toEqualTypeof<typeof two>()

If you find it hard working with expectTypeOf API and figuring out errors, you can always use more simple assertType API:

const answer = 42
assertType<number>(answer)
// @ts-expect-error answer is not a string
assertType<string>(answer)

// @ts-expect-error answer is not a string
assertType<string>(answr) //

Run Typechecking ​

To enable typechecking, just add --typecheck flag to your Vitest command in package.json :

{
  "scripts": {
    "test": "vitest --typecheck"