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.

It can be used in your existing test files - or anywhere other type-checked file you'd like.

Example

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

test('foo types', () => {
  // make sure `foo` has type {a: number}
  expectTypeOf(foo).toMatchTypeOf({a: 1})
  expectTypeOf(foo).toHaveProperty('a').toBeNumber()

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

See the documentation for lots more examples.

Contents

• Contents
• Installation and usage
• Documentation
  • Features
• 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 that two objects have equivalent types to .toEqualTypeOf:

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

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

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

.toMatchTypeOf checks that an object "matches" a type - that is, it has all the expected properties with correct types. This is similar to jest's .toMatchObject:

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

When there's no instance/runtime variable for the expected type, you can use generics:

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

Assertions can be inverted:

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

Catch any/unknown/never types:

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

Test for basic javascript types:

expectTypeOf(() => 1).toBeFunction()
expectTypeOf({}).toBeObject()
expectTypeOf([]).toBeArray()
expectTypeOf('').toBeString()
expectTypeOf(1).toBeNumber()
expectTypeOf(true).toBeBoolean()
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()

Most assertions can be inverted with .not:

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

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()

Assert on function 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: number; b?: number}>().not.toEqualTypeOf<{a: number}>()
expectTypeOf<{a: number; b?: number | null}>().not.toEqualTypeOf<{a: number; b?: number}>()
expectTypeOf<{a: number; b?: number | null}>().toEqualTypeOf<{a: number; b?: number | null}>()

Similar projects

Other projects with similar goals:

• ts-expect exports several generic helper types to perform type assertions
• dtslint does type checks via comment directives and tslint
• tsd-check is a CLI that runs the TypeScript type checker over assertions
• 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 expectType(...).not
• first-class support for:
  • any (as well as unknown and never).
    • 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
  • array item values
  • nullable
• assertions on types "matching" rather than exact type equality, for "is-a" relationships e.g. expectTypeOf(square).toMatchTypeOf<Shape>()
• built into existing tooling with no dependencies. No extra build step, cli tool, or lint plugin is needed. Just import the function and start writing tests.