expect-type

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

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

test('foo types', () => {
  // 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 - or 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
  • Within test frameworks
    • Jest & eslint-plugin-jest
• Similar projects
  • Comparison
• Contributing

Installation and usage

npm install expect-type

import {expectTypeOf} from 'expect-type'

Documentation

The expectTypeOf method takes a single argument, or a generic 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:

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 extra properties:

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

To allow for extra properties, use .toMatchTypeOf. This checks that an object "matches" a type. This is similar to jest's .toMatchObject:

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

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}}>()

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()

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()

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()

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]>()

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<[] | [string | number | Date]>()

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, 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}>()

Within test frameworks

Jest & eslint-plugin-jest

If you're using Jest along with eslint-plugin-jest, you will 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"
      ]
    }
  ],
  // ...
}

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. <200 lines of code - take a look! (tsd, for comparison, is 2.6MB because it ships a patched version of typescript).

Contributing

In most cases, it's worth checking existing issues or creating on 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.