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 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

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()

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]>()

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]>()

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()

Check that functions never return:

const thrower = () => {
  throw 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>()

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
• 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).