expect-type

What is expect-type?

The expect-type npm package is a TypeScript utility that allows developers to assert and validate types at compile time. It is particularly useful for ensuring that types are as expected during development, which can help catch type-related errors early.

What are expect-type's main functionalities?

Type Assertion

This feature allows you to assert that a given value matches a specific type. In this example, the `expectType` function is used to assert that the `user` object conforms to the `User` interface.

import { expectType } from 'expect-type';

interface User {
  name: string;
  age: number;
}

const user: User = { name: 'Alice', age: 30 };
expectType<User>(user);

Type Equality

This feature allows you to check if two types are equal. In this example, `expectType` is used to assert that types `A` and `B` are equal.

import { expectType } from 'expect-type';

type A = { a: string };
type B = { a: string };

expectType<A, B>(true);

Type Inference

This feature allows you to assert the inferred type of a value. In this example, `expectType` is used to assert that the result of the `add` function is a `number`.

import { expectType } from 'expect-type';

function add(a: number, b: number) {
  return a + b;
}

const result = add(1, 2);
expectType<number>(result);

Other packages similar to expect-type

tsd

The `tsd` package is a TypeScript type assertion library that allows you to write tests for your type definitions. It is similar to `expect-type` in that it helps ensure type correctness, but it is more focused on writing test cases for type definitions.

typescript-is

The `typescript-is` package provides runtime type checking for TypeScript. Unlike `expect-type`, which is used for compile-time type assertions, `typescript-is` allows you to perform type checks at runtime, making it useful for scenarios where you need to validate types dynamically.

io-ts

The `io-ts` package is a runtime type system for IO decoding/encoding with TypeScript. It allows you to define codecs that can validate and decode data at runtime. While `expect-type` focuses on compile-time type assertions, `io-ts` provides a way to ensure type safety at runtime.

README

expect-type

Compile-time tests for types. Useful to make sure types don't regress into being overly permissive as changes go in over time.

Similar to expect, but with type-awareness. Gives you access to several type-matchers that let you make assertions about the form of a reference or generic type parameter.

import {expectTypeOf} from 'expect-type'
import {foo, bar} from '../foo'

// make sure `foo` has type {a: number}
expectTypeOf(foo).toMatchTypeOf<{a: number}>()

// make sure `bar` is a function taking a string:
expectTypeOf(bar).parameter(0).toBeString()
expectTypeOf(bar).returns.not.toBeAny()

It can be used in your existing test files (and is actually built in to vitest). Or it can be used in any other type-checked file you'd like - it's built into existing tooling with no dependencies. No extra build step, cli tool, IDE extension, or lint plugin is needed. Just import the function and start writing tests. Failures will be at compile time - they'll appear in your IDE and when you run tsc.

See below for lots more examples.

Contents

• Contents
• Installation and usage
• Documentation
  • Features
  • Why is my assertion failing?
  • Where is .toExtend?
  • Internal type helpers
  • Error messages
    • Concrete "expected" objects vs type arguments
  • Overloaded functions
  • Within test frameworks
  • Vitest
    • Jest & eslint-plugin-jest
  • Limitations
• Similar projects
  • Comparison
• TypeScript backwards-compatibility
• Contributing
  • Documentation of limitations through tests

Installation and usage

npm install expect-type --save-dev

import {expectTypeOf} from 'expect-type'

Documentation

The expectTypeOf method takes a single argument or a generic type parameter. Neither it nor the functions chained off its return value have any meaningful runtime behaviour. The assertions you write will be compile-time errors if they don't hold true.

Features

Check an object's type with .toEqualTypeOf:

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

.toEqualTypeOf can check that two concrete objects have equivalent types (note: when these assertions fail, the error messages can be less informative vs the generic type argument syntax above - see error messages docs):

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

.toEqualTypeOf succeeds for objects with different values, but the same type:

expectTypeOf({a: 1}).toEqualTypeOf({a: 2})

.toEqualTypeOf fails on excess properties:

// @ts-expect-error
expectTypeOf({a: 1, b: 1}).toEqualTypeOf<{a: number}>()

To allow for extra properties, use .toMatchTypeOf. This is roughly equivalent to an extends constraint in a function type argument.:

expectTypeOf({a: 1, b: 1}).toMatchTypeOf<{a: number}>()

.toEqualTypeOf and .toMatchTypeOf both fail on missing properties:

// @ts-expect-error
expectTypeOf({a: 1}).toEqualTypeOf<{a: number; b: number}>()
// @ts-expect-error
expectTypeOf({a: 1}).toMatchTypeOf<{a: number; b: number}>()

Another example of the difference between .toMatchTypeOf and .toEqualTypeOf, using generics. .toMatchTypeOf can be used for "is-a" relationships:

type Fruit = {type: 'Fruit'; edible: boolean}
type Apple = {type: 'Fruit'; name: 'Apple'; edible: true}

expectTypeOf<Apple>().toMatchTypeOf<Fruit>()

// @ts-expect-error
expectTypeOf<Fruit>().toMatchTypeOf<Apple>()

// @ts-expect-error
expectTypeOf<Apple>().toEqualTypeOf<Fruit>()

Assertions can be inverted with .not:

expectTypeOf({a: 1}).not.toMatchTypeOf({b: 1})

.not can be easier than relying on // @ts-expect-error:

type Fruit = {type: 'Fruit'; edible: boolean}
type Apple = {type: 'Fruit'; name: 'Apple'; edible: true}

expectTypeOf<Apple>().toMatchTypeOf<Fruit>()

expectTypeOf<Fruit>().not.toMatchTypeOf<Apple>()
expectTypeOf<Apple>().not.toEqualTypeOf<Fruit>()

Catch any/unknown/never types:

expectTypeOf<unknown>().toBeUnknown()
expectTypeOf<any>().toBeAny()
expectTypeOf<never>().toBeNever()

// @ts-expect-error
expectTypeOf<never>().toBeNumber()

.toEqualTypeOf distinguishes between deeply-nested any and unknown properties:

expectTypeOf<{deeply: {nested: any}}>().not.toEqualTypeOf<{deeply: {nested: unknown}}>()

You can test for basic JavaScript types:

expectTypeOf(() => 1).toBeFunction()
expectTypeOf({}).toBeObject()
expectTypeOf([]).toBeArray()
expectTypeOf('').toBeString()
expectTypeOf(1).toBeNumber()
expectTypeOf(true).toBeBoolean()
expectTypeOf(() => {}).returns.toBeVoid()
expectTypeOf(Promise.resolve(123)).resolves.toBeNumber()
expectTypeOf(Symbol(1)).toBeSymbol()

.toBe... methods allow for types that extend the expected type:

expectTypeOf<number>().toBeNumber()
expectTypeOf<1>().toBeNumber()

expectTypeOf<any[]>().toBeArray()
expectTypeOf<number[]>().toBeArray()

expectTypeOf<string>().toBeString()
expectTypeOf<'foo'>().toBeString()

expectTypeOf<boolean>().toBeBoolean()
expectTypeOf<true>().toBeBoolean()

.toBe... methods protect against any:

const goodIntParser = (s: string) => Number.parseInt(s, 10)
const badIntParser = (s: string) => JSON.parse(s) // uh-oh - works at runtime if the input is a number, but return 'any'

expectTypeOf(goodIntParser).returns.toBeNumber()
// @ts-expect-error - if you write a test like this, `.toBeNumber()` will let you know your implementation returns `any`.
expectTypeOf(badIntParser).returns.toBeNumber()

Nullable types:

expectTypeOf(undefined).toBeUndefined()
expectTypeOf(undefined).toBeNullable()
expectTypeOf(undefined).not.toBeNull()

expectTypeOf(null).toBeNull()
expectTypeOf(null).toBeNullable()
expectTypeOf(null).not.toBeUndefined()

expectTypeOf<1 | undefined>().toBeNullable()
expectTypeOf<1 | null>().toBeNullable()
expectTypeOf<1 | undefined | null>().toBeNullable()

More .not examples:

expectTypeOf(1).not.toBeUnknown()
expectTypeOf(1).not.toBeAny()
expectTypeOf(1).not.toBeNever()
expectTypeOf(1).not.toBeNull()
expectTypeOf(1).not.toBeUndefined()
expectTypeOf(1).not.toBeNullable()

Detect assignability of unioned types:

expectTypeOf<number>().toMatchTypeOf<string | number>()
expectTypeOf<string | number>().not.toMatchTypeOf<number>()

Use .extract and .exclude to narrow down complex union types:

type ResponsiveProp<T> = T | T[] | {xs?: T; sm?: T; md?: T}
const getResponsiveProp = <T>(_props: T): ResponsiveProp<T> => ({})
type CSSProperties = {margin?: string; padding?: string}

const cssProperties: CSSProperties = {margin: '1px', padding: '2px'}

expectTypeOf(getResponsiveProp(cssProperties))
  .exclude<unknown[]>()
  .exclude<{xs?: unknown}>()
  .toEqualTypeOf<CSSProperties>()

expectTypeOf(getResponsiveProp(cssProperties))
  .extract<unknown[]>()
  .toEqualTypeOf<CSSProperties[]>()

expectTypeOf(getResponsiveProp(cssProperties))
  .extract<{xs?: any}>()
  .toEqualTypeOf<{xs?: CSSProperties; sm?: CSSProperties; md?: CSSProperties}>()

expectTypeOf<ResponsiveProp<number>>().exclude<number | number[]>().toHaveProperty('sm')
expectTypeOf<ResponsiveProp<number>>().exclude<number | number[]>().not.toHaveProperty('xxl')

.extract and .exclude return never if no types remain after exclusion:

type Person = {name: string; age: number}
type Customer = Person & {customerId: string}
type Employee = Person & {employeeId: string}

expectTypeOf<Customer | Employee>().extract<{foo: string}>().toBeNever()
expectTypeOf<Customer | Employee>().exclude<{name: string}>().toBeNever()

Use .pick to pick a set of properties from an object:

type Person = {name: string; age: number}

expectTypeOf<Person>().pick<'name'>().toEqualTypeOf<{name: string}>()

Use .omit to remove a set of properties from an object:

type Person = {name: string; age: number}

expectTypeOf<Person>().omit<'name'>().toEqualTypeOf<{age: number}>()

Make assertions about object properties:

const obj = {a: 1, b: ''}

// check that properties exist (or don't) with `.toHaveProperty`
expectTypeOf(obj).toHaveProperty('a')
expectTypeOf(obj).not.toHaveProperty('c')

// check types of properties
expectTypeOf(obj).toHaveProperty('a').toBeNumber()
expectTypeOf(obj).toHaveProperty('b').toBeString()
expectTypeOf(obj).toHaveProperty('a').not.toBeString()

.toEqualTypeOf can be used to distinguish between functions:

type NoParam = () => void
type HasParam = (s: string) => void

expectTypeOf<NoParam>().not.toEqualTypeOf<HasParam>()

But often it's preferable to use .parameters or .returns for more specific function assertions:

type NoParam = () => void
type HasParam = (s: string) => void

expectTypeOf<NoParam>().parameters.toEqualTypeOf<[]>()
expectTypeOf<NoParam>().returns.toBeVoid()

expectTypeOf<HasParam>().parameters.toEqualTypeOf<[string]>()
expectTypeOf<HasParam>().returns.toBeVoid()

Up to ten overloads will produce union types for .parameters and .returns:

type Factorize = {
  (input: number): number[]
  (input: bigint): bigint[]
}

expectTypeOf<Factorize>().parameters.toEqualTypeOf<[number] | [bigint]>()
expectTypeOf<Factorize>().returns.toEqualTypeOf<number[] | bigint[]>()

expectTypeOf<Factorize>().parameter(0).toEqualTypeOf<number | bigint>()

Note that these aren't exactly like TypeScript's built-in Parameters<...> and ReturnType<...>:

The TypeScript builtins simply choose a single overload (see the Overloaded functions section for more information)

type Factorize = {
  (input: number): number[]
  (input: bigint): bigint[]
}

// overload using `number` is ignored!
expectTypeOf<Parameters<Factorize>>().toEqualTypeOf<[bigint]>()
expectTypeOf<ReturnType<Factorize>>().toEqualTypeOf<bigint[]>()

More examples of ways to work with functions - parameters using .parameter(n) or .parameters, and return values using .returns:

const f = (a: number) => [a, a]

expectTypeOf(f).toBeFunction()

expectTypeOf(f).toBeCallableWith(1)
expectTypeOf(f).not.toBeAny()
expectTypeOf(f).returns.not.toBeAny()
expectTypeOf(f).returns.toEqualTypeOf([1, 2])
expectTypeOf(f).returns.toEqualTypeOf([1, 2, 3])
expectTypeOf(f).parameter(0).not.toEqualTypeOf('1')
expectTypeOf(f).parameter(0).toEqualTypeOf(1)
expectTypeOf(1).parameter(0).toBeNever()

const twoArgFunc = (a: number, b: string) => ({a, b})

expectTypeOf(twoArgFunc).parameters.toEqualTypeOf<[number, string]>()

.toBeCallableWith allows for overloads. You can also use it to narrow down the return type for given input parameters.:

type Factorize = {
  (input: number): number[]
  (input: bigint): bigint[]
}

expectTypeOf<Factorize>().toBeCallableWith(6)
expectTypeOf<Factorize>().toBeCallableWith(6n)

.toBeCallableWith returns a type that can be used to narrow down the return type for given input parameters.:

type Factorize = {
  (input: number): number[]
  (input: bigint): bigint[]
}
expectTypeOf<Factorize>().toBeCallableWith(6).returns.toEqualTypeOf<number[]>()
expectTypeOf<Factorize>().toBeCallableWith(6n).returns.toEqualTypeOf<bigint[]>()

.toBeCallableWith can be used to narrow down the parameters of a function:

type Delete = {
  (path: string): void
  (paths: string[], options?: {force: boolean}): void
}

expectTypeOf<Delete>().toBeCallableWith('abc').parameters.toEqualTypeOf<[string]>()
expectTypeOf<Delete>()
  .toBeCallableWith(['abc', 'def'], {force: true})
  .parameters.toEqualTypeOf<[string[], {force: boolean}?]>()

expectTypeOf<Delete>().toBeCallableWith('abc').parameter(0).toBeString()
expectTypeOf<Delete>().toBeCallableWith('abc').parameter(1).toBeUndefined()

expectTypeOf<Delete>()
  .toBeCallableWith(['abc', 'def', 'ghi'])
  .parameter(0)
  .toEqualTypeOf<string[]>()

expectTypeOf<Delete>()
  .toBeCallableWith(['abc', 'def', 'ghi'])
  .parameter(1)
  .toEqualTypeOf<{force: boolean} | undefined>()

You can't use .toBeCallableWith with .not - you need to use ts-expect-error::

const f = (a: number) => [a, a]

// @ts-expect-error
expectTypeOf(f).toBeCallableWith('foo')

You can also check type guards & type assertions:

const assertNumber = (v: any): asserts v is number => {
  if (typeof v !== 'number') {
    throw new TypeError('Nope !')
  }
}

expectTypeOf(assertNumber).asserts.toBeNumber()

const isString = (v: any): v is string => typeof v === 'string'
expectTypeOf(isString).guards.toBeString()

Assert on constructor parameters:

expectTypeOf(Date).toBeConstructibleWith('1970')
expectTypeOf(Date).toBeConstructibleWith(0)
expectTypeOf(Date).toBeConstructibleWith(new Date())
expectTypeOf(Date).toBeConstructibleWith()

expectTypeOf(Date).constructorParameters.toEqualTypeOf<
  | []
  | [value: string | number]
  | [value: string | number | Date]
  | [
      year: number,
      monthIndex: number,
      date?: number | undefined,
      hours?: number | undefined,
      minutes?: number | undefined,
      seconds?: number | undefined,
      ms?: number | undefined,
    ]
>()

Constructor overloads:

class DBConnection {
  constructor()
  constructor(connectionString: string)
  constructor(options: {host: string; port: number})
  constructor(..._: unknown[]) {}
}

expectTypeOf(DBConnection).toBeConstructibleWith()
expectTypeOf(DBConnection).toBeConstructibleWith('localhost')
expectTypeOf(DBConnection).toBeConstructibleWith({host: 'localhost', port: 1234})
// @ts-expect-error - as when calling `new DBConnection(...)` you can't actually use the `(...args: unknown[])` overlaod, it's purely for the implementation.
expectTypeOf(DBConnection).toBeConstructibleWith(1, 2)

Check function this parameters:

function greet(this: {name: string}, message: string) {
  return `Hello ${this.name}, here's your message: ${message}`
}

expectTypeOf(greet).thisParameter.toEqualTypeOf<{name: string}>()

Distinguish between functions with different this parameters:

function greetFormal(this: {title: string; name: string}, message: string) {
  return `Dear ${this.title} ${this.name}, here's your message: ${message}`
}

function greetCasual(this: {name: string}, message: string) {
  return `Hi ${this.name}, here's your message: ${message}`
}

expectTypeOf(greetFormal).not.toEqualTypeOf(greetCasual)

Class instance types:

expectTypeOf(Date).instance.toHaveProperty('toISOString')

Promise resolution types can be checked with .resolves:

const asyncFunc = async () => 123

expectTypeOf(asyncFunc).returns.resolves.toBeNumber()

Array items can be checked with .items:

expectTypeOf([1, 2, 3]).items.toBeNumber()
expectTypeOf([1, 2, 3]).items.not.toBeString()

You can also compare arrays directly:

expectTypeOf<any[]>().not.toEqualTypeOf<number[]>()

Check that functions never return:

const thrower = () => {
  throw new Error('oh no')
}

expectTypeOf(thrower).returns.toBeNever()

Generics can be used rather than references:

expectTypeOf<{a: string}>().not.toEqualTypeOf<{a: number}>()

Distinguish between missing/null/optional properties:

expectTypeOf<{a?: number}>().not.toEqualTypeOf<{}>()
expectTypeOf<{a?: number}>().not.toEqualTypeOf<{a: number}>()
expectTypeOf<{a?: number}>().not.toEqualTypeOf<{a: number | undefined}>()
expectTypeOf<{a?: number | null}>().not.toEqualTypeOf<{a: number | null}>()
expectTypeOf<{a: {b?: number}}>().not.toEqualTypeOf<{a: {}}>()

Detect the difference between regular and readonly properties:

type A1 = {readonly a: string; b: string}
type E1 = {a: string; b: string}

expectTypeOf<A1>().toMatchTypeOf<E1>()
expectTypeOf<A1>().not.toEqualTypeOf<E1>()

type A2 = {a: string; b: {readonly c: string}}
type E2 = {a: string; b: {c: string}}

expectTypeOf<A2>().toMatchTypeOf<E2>()
expectTypeOf<A2>().not.toEqualTypeOf<E2>()

Distinguish between classes with different constructors:

class A {
  value: number
  constructor(a: 1) {
    this.value = a
  }
}
class B {
  value: number
  constructor(b: 2) {
    this.value = b
  }
}

expectTypeOf<typeof A>().not.toEqualTypeOf<typeof B>()

class C {
  value: number
  constructor(c: 1) {
    this.value = c
  }
}

expectTypeOf<typeof A>().toEqualTypeOf<typeof C>()

Known limitation: Intersection types can cause issues with toEqualTypeOf:

// @ts-expect-error the following line doesn't compile, even though the types are arguably the same.
// See https://github.com/mmkal/expect-type/pull/21
expectTypeOf<{a: 1} & {b: 2}>().toEqualTypeOf<{a: 1; b: 2}>()

To workaround for simple cases, you can use a mapped type:

type Simplify<T> = {[K in keyof T]: T[K]}

expectTypeOf<Simplify<{a: 1} & {b: 2}>>().toEqualTypeOf<{a: 1; b: 2}>()

But this won't work if the nesting is deeper in the type. For these situations, you can use the .branded helper. Note that this comes at a performance cost, and can cause the compiler to 'give up' if used with excessively deep types, so use sparingly. This helper is under .branded because it deeply transforms the Actual and Expected types into a pseudo-AST:

// @ts-expect-error
expectTypeOf<{a: {b: 1} & {c: 1}}>().toEqualTypeOf<{a: {b: 1; c: 1}}>()

expectTypeOf<{a: {b: 1} & {c: 1}}>().branded.toEqualTypeOf<{a: {b: 1; c: 1}}>()

Be careful with .branded for very deep or complex types, though. If possible you should find a way to simplify your test to avoid needing to use it:

// This *should* result in an error, but the "branding" mechanism produces too large a type and TypeScript just gives up! https://github.com/microsoft/TypeScript/issues/50670
expectTypeOf<() => () => () => () => 1>().branded.toEqualTypeOf<() => () => () => () => 2>()

// @ts-expect-error the non-branded implementation catches the error as expected.
expectTypeOf<() => () => () => () => 1>().toEqualTypeOf<() => () => () => () => 2>()

So, if you have an extremely deep type that ALSO has an intersection in it, you're out of luck and this library won't be able to test your type properly:

// @ts-expect-error this fails, but it should succeed.
expectTypeOf<() => () => () => () => {a: 1} & {b: 2}>().toEqualTypeOf<
  () => () => () => () => {a: 1; b: 2}
>()

// this succeeds, but it should fail.
expectTypeOf<() => () => () => () => {a: 1} & {b: 2}>().branded.toEqualTypeOf<
  () => () => () => () => {a: 1; c: 2}
>()

Another limitation: passing this references to expectTypeOf results in errors.:

class B {
  b = 'b'

  foo() {
    // @ts-expect-error
    expectTypeOf(this).toEqualTypeOf(this)
    // @ts-expect-error
    expectTypeOf(this).toMatchTypeOf(this)
  }
}

// Instead of the above, try something like this:
expectTypeOf(B).instance.toEqualTypeOf<{b: string; foo: () => void}>()

Overloads limitation for TypeScript <5.3: Due to a TypeScript bug fixed in 5.3, overloaded functions which include an overload resembling (...args: unknown[]) => unknown will exclude unknown[] from .parameters and exclude unknown from .returns:

type Factorize = {
  (...args: unknown[]): unknown
  (input: number): number[]
  (input: bigint): bigint[]
}

expectTypeOf<Factorize>().parameters.toEqualTypeOf<[number] | [bigint]>()
expectTypeOf<Factorize>().returns.toEqualTypeOf<number[] | bigint[]>()

This overload, however, allows any input and returns an unknown output anyway, so it's not very useful. If you are worried about this for some reason, you'll have to update TypeScript to 5.3+.

Why is my assertion failing?

For complex types, an assertion might fail when it should if the Actual type contains a deeply-nested intersection type but the Expected doesn't. In these cases you can use .branded as described above:

// @ts-expect-error this unfortunately fails - a TypeScript limitation prevents making this pass without a big perf hit
expectTypeOf<{a: {b: 1} & {c: 1}}>().toEqualTypeOf<{a: {b: 1; c: 1}}>()

expectTypeOf<{a: {b: 1} & {c: 1}}>().branded.toEqualTypeOf<{a: {b: 1; c: 1}}>()

Where is .toExtend?

A few people have asked for a method like toExtend - this is essentially what toMatchTypeOf is. There are some cases where it doesn't precisely match the extends operator in TypeScript, but for most practical use cases, you can think of this as the same thing.

Internal type helpers

🚧 This library also exports some helper types for performing boolean operations on types, checking extension/equality in various ways, branding types, and checking for various special types like never, any, unknown. Use at your own risk! Nothing is stopping you from using these beyond this warning:

All internal types that are not documented here are not part of the supported API surface, and may be renamed, modified, or removed, without warning or documentation in release notes.

For a dedicated internal type library, feel free to look at the source code for inspiration - or better, use a library like type-fest.

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 a 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 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. Until then they will take a certain amount of squinting.

Concrete "expected" objects vs type arguments

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 type argument 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 type argument 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>()

Overloaded functions

Due to a TypeScript design limitation, the native TypeScript Parameters<...> and ReturnType<...> helpers only return types from one variant of an overloaded function. This limitation doesn't apply to expect-type, since it is not used to author TypeScript code, only to assert on existing types. So, we use a workaround for this TypeScript behaviour to assert on all overloads as a union (actually, not necessarily all - we cap out at 10 overloads).

Within test frameworks

Vitest

expectTypeOf is built in to vitest, so you can import expectTypeOf from the vitest library directly if you prefer. Note that there is no set release cadence, at time of writing, so vitest may not always be using the very latest version.

import {expectTypeOf} from 'vitest'
import {mount} from './mount.js'

test('my types work properly', () => {
  expectTypeOf(mount).toBeFunction()
  expectTypeOf(mount).parameter(0).toMatchTypeOf<{name: string}>()

  expectTypeOf(mount({name: 42})).toBeString()
})

Jest & eslint-plugin-jest

If you're using Jest along with eslint-plugin-jest, and you put assertions inside test(...) definitions, you may get warnings from the jest/expect-expect rule, complaining that "Test has no assertions" for tests that only use expectTypeOf().

To remove this warning, configure the ESLint rule to consider expectTypeOf as an assertion:

"rules": {
  // ...
  "jest/expect-expect": [
    "warn",
    {
      "assertFunctionNames": [
        "expect", "expectTypeOf"
      ]
    }
  ],
  // ...
}

Limitations

A summary of some of the limitations of this library. Some of these are documented more fully elsewhere.

1. Intersection types can result in failures when the expected and actual types are not identically defined, even when they are effectively identical. See Why is my assertion failing for details. TL;DR: use .brand in these cases - and accept the performance hit that it comes with.
2. toBeCallableWith will likely fail if you try to use it with a generic function or an overload. See this issue for an example and how to work around it.
3. (For now) overloaded functions might trip up the .parameter and .parameters helpers. This matches how the built-in TypeScript helper Parameters<...> works. This may be improved in the future though (see related issue).
4. expectTypeOf(this).toEqualTypeOf(this) inside class methods does not work.

Similar projects

Other projects with similar goals:

• tsd is a CLI that runs the TypeScript type checker over assertions
• ts-expect exports several generic helper types to perform type assertions
• dtslint does type checks via comment directives and tslint
• type-plus comes with various type and runtime TypeScript assertions
• static-type-assert type assertion functions

Comparison

The key differences in this project are:

• a fluent, jest-inspired API, making the difference between actual and expected clear. This is helpful with complex types and assertions.
• inverting assertions intuitively and easily via expectTypeOf(...).not
• checks generics properly and strictly (tsd doesn't)
• first-class support for:
  • any (as well as unknown and never) (see issues outstanding at time of writing in tsd for never and any).
    • This can be especially useful in combination with not, to protect against functions returning too-permissive types. For example, const parseFile = (filename: string) => JSON.parse(readFileSync(filename).toString()) returns any, which could lead to errors. After giving it a proper return-type, you can add a test for this with expect(parseFile).returns.not.toBeAny()
  • object properties
  • function parameters
  • function return values
  • constructor parameters
  • class instances
  • array item values
  • nullable types
• assertions on types "matching" rather than exact type equality, for "is-a" relationships e.g. expectTypeOf(square).toMatchTypeOf<Shape>()
• built into existing tooling. No extra build step, cli tool, IDE extension, or lint plugin is needed. Just import the function and start writing tests. Failures will be at compile time - they'll appear in your IDE and when you run tsc.
• small implementation with no dependencies. Take a look! (tsd, for comparison, is 2.6MB because it ships a patched version of TypeScript).

TypeScript backwards-compatibility

There is a CI job called test-types that checks whether the tests still pass with certain older TypeScript versions. To check the supported TypeScript versions, refer to the job definition.

Contributing

In most cases, it's worth checking existing issues or creating one to discuss a new feature or a bug fix before opening a pull request.

Once you're ready to make a pull request: clone the repo, and install pnpm if you don't have it already with npm install --global pnpm. Lockfiles for npm and yarn are gitignored.

If you're adding a feature, you should write a self-contained usage example in the form of a test, in test/usage.test.ts. This file is used to populate the bulk of this readme using eslint-plugin-codegen, and to generate an "errors" test file, which captures the error messages that are emitted for failing assertions by the TypeScript compiler. So, the test name should be written as a human-readable sentence explaining the usage example. Have a look at the existing tests for an idea of the style.

After adding the tests, run npm run lint -- --fix to update the readme, and npm test -- --updateSnapshot to update the errors test. The generated documentation and tests should be pushed to the same branch as the source code, and submitted as a pull request. CI will test that the docs and tests are up to date if you forget to run these commands.

Documentation of limitations through tests

Limitations of the library are documented through tests in usage.test.ts. This means that if a future TypeScript version (or library version) fixes the limitation, the test will start failing, and it will be automatically removed from the documentation once it no longer applies.