@arktype/attest

Attest

Attest is a testing library that makes your TypeScript types available at runtime, giving you access to precise type-level assertions and performance benchmarks.

Assertions are framework agnostic and can be seamlessly integrated with your existing Vitest, Jest, or Mocha tests.

Benchmarks can run from anywhere and will deterministically report the number of type instantiations contributed by the contents of the bench call.

If you've ever wondered how ArkType can guarantee identical behavior between its runtime and static parser implementations and highly optimized editor performance, Attest is your answer⚡

Installation

npm install @arktype/attest

Note: This package is still in alpha! Your feedback will help us iterate toward a stable 1.0.

Setup

To use attest's type assertions, you'll need to call our setup/cleanup methods before your first test and after your last test, respectively. This usually involves some kind of globalSetup/globalTeardown config.

For example, in mocha:

import { cleanup, setup } from "@arktype/attest"

export const mochaGlobalSetup = setup

export const mochaGlobalTeardown = cleanup

Bun support is currently pending a bug in the way their source maps translate to stack traces.

Assertions

Here are some simple examples of type assertions and snapshotting:

// @arktype/attest assertions can be made from any unit test framework with a global setup/teardown
describe("attest features", () => {
	it("type and value assertions", () => {
		const even = type("number%2")
		// asserts even.infer is exactly number
		attest<number>(even.infer)
		// make assertions about types and values seamlessly
		attest(even.infer).type.toString.snap("number")
		// including object literals- no more long inline strings!
		attest(even.json).snap({
			intersection: [{ domain: "number" }, { divisor: 2 }]
		})
	})

	it("error assertions", () => {
		// Check type errors, runtime errors, or both at the same time!
		// @ts-expect-error
		attest(() => type("number%0")).throwsAndHasTypeError(
			"% operator must be followed by a non-zero integer literal (was 0)"
		)
		// @ts-expect-error
		attest(() => type({ "[object]": "string" })).type.errors(
			"Indexed key definition 'object' must be a string, number or symbol"
		)
	})

	it("completion snapshotting", () => {
		// snapshot expected completions for any string literal!
		// @ts-expect-error (if your expression would throw, prepend () =>)
		attest(() => type({ a: "a", b: "b" })).completions({
			a: ["any", "alpha", "alphanumeric"],
			b: ["bigint", "boolean"]
		})
		type Legends = { faker?: "🐐"; [others: string]: unknown }
		// works for keys or index access as well (may need prettier-ignore to avoid removing quotes)
		// prettier-ignore
		attest({ "f": "🐐" } as Legends).completions({ "f": ["faker"] })
	})

	it("integrate runtime logic with type assertions", () => {
		const arrayOf = type("<t>", "t[]")
		const numericArray = arrayOf("number | bigint")
		// flexibly combine runtime logic with type assertions to customize your
		// tests beyond what is possible from pure static-analysis based type testing tools
		if (getTsVersionUnderTest().startsWith("5")) {
			// this assertion will only occur when testing TypeScript 5+!
			attest<(number | bigint)[]>(numericArray.infer)
		}
	})
})

Benches

Benches are run separately from tests and don't require any special setup. If the below file was benches.ts, you could run it using something like tsx benches.ts or ts-node benches.ts:

// Combinatorial template literals often result in expensive types- let's benchmark this one!
type makeComplexType<s extends string> = s extends `${infer head}${infer tail}`
	? head | tail | makeComplexType<tail>
	: s

bench("bench type", () => {
	return {} as makeComplexType<"defenestration">
	// This is an inline snapshot that will be populated or compared when you run the file
}).types([169, "instantiations"])

bench(
	"bench runtime and type",
	() => {
		return {} as makeComplexType<"antidisestablishmentarianism">
	},
	fakeCallOptions
)
	// Average time it takes the function execute
	.mean([2, "ms"])
	// Seems like our type is O(n) with respect to the length of the input- not bad!
	.types([337, "instantiations"])

Integration

Setup

If you're a library author wanting to integrate type into your own assertions instead of using the attest API, you'll need to call setup with a list of attestAliases to ensure type data is collected from your own functions:

// attest will only collect type data from functions with names listed in `attestAliases`
setup({ attestAliases: ["yourCustomAssert"] })

// There are many other config options, but some are primarily internal- use others at your own risk!

You'll need to make sure that setup with whatever aliases you need before the first test runs. As part of the setup process, attest will search for the specified assertion calls and cache their types in a temporary file that will be referenced during test execution.

This ensures that type assertions can be made across processes without creating a new TSServer instance for each.

TS Versions

Attest provides utilities for testing your code against multiple versions of TypeScript. This is particularly useful for library authors who want to ensure compatibility across different TypeScript versions.

For example, here's the script ArkType uses to ensure consistency across TS versions in CI:

import { forEachTypeScriptVersion } from "@arktype/attest"
import { shell } from "@arktype/fs"

forEachTypeScriptVersion(() => {
	shell("pnpm test")
})

By default, forEachTypeScriptVersion will look for TypeScript versions in node_modules with the prefix attest-ts. This way, you can maintain a list of versions you'd like to support that will be installed alongside your other dev dependencies and referenced only during these tests:

"devDependencies": {
	"typescript": "5.3.2",
	"attest-ts50": "npm:typescript@5.0",
	"attest-ts51": "npm:typescript@5.1",
	"attest-ts52": "npm:typescript@5.2",
	"attest-ts53": "npm:typescript@5.3",
	"attest-ts54": "npm:typescript@next"
}

Otherwise, you can pass a custom list of directories containing TS installations to forEachTypeScriptVersion's optional second parameter.

APIs

The most flexible attest APIs are getAssertionDataAtPosition and caller.

Here's an example of how you might use them in your own API:

import { getAssertionDataAtPosition, caller } from "@arktype/attest"

const yourCustomAssert = <expectedType>(actualValue: expectedType) => {
	const position = caller()
	const types = getAssertionDataAtPosition(position)
	// assert that the type of actualValue is the same as the type of expectedType
	const relationship = types.args[0].relationships.typeArgs[0]
	if (relationship === undefined) {
		throw new Error(
			`yourCustomAssert requires a type arg representing the expected type, e.g. 'yourCustomAssert<"foo">("foo")'`
		)
	}
	if (relationship !== "equality") {
		throw new Error(
			`Expected ${types.typeArgs[0].type}, got ${types.args[0].type} with relationship ${relationship}`
		)
	}
}

A user might then use yourCustomAssert like this:

import { yourCustomAssert } from "your-package"

test("my code", () => {
	// Ok
	yourCustomAssert<"foo">(`${"f"}oo` as const)
	// Error: `Expected boolean, got true with relationship subtype`
	yourCustomAssert<boolean>(true)
	// Error: `Expected 5, got number with relationship supertype`
	yourCustomAssert<5>(2 + 3)
})

Please don't hesitate to a GitHub issue or discussion or reach out on ArkType's Discord if you have any questions or feedback- we'd love to hear from you! ⛵