Your First Type

If you already know TypeScript, congratulations- you just learned most of ArkType’s syntax 🎉

Define

import { type } from "arktype"

const user = type({
	name: "string",
	platform: "'android' | 'ios'",
	"versions?": "(number | string)[]"
})

// extract the type if needed
type User = typeof user.infer

If you make a mistake, don’t worry- every definition gets the autocomplete and validation you’re used to from your editor, all within TypeScript’s type system.

Will ArkType crash my TypeScript server?

Thousands of hours of optimization have gone into making validating native type syntax not just feasible, but often much faster than alternatives ⚡

Compose

Suppose we want to move platform and version from our original type to a new device property.

const user = type({
	name: "string",
	// nested definitions don't need to be wrapped
	device: {
		platform: "'android' | 'ios'",
		"versions?": "(number | string)[]"
	}
})

To decouple device from user, just move it to its own type and reference it.

const device = type({
	platform: "'android' | 'ios'",
	"versions?": "(number | string)[]"
})

const user = type({
	name: "string",
	device: device
})

Validate

At runtime, we can pass unknown data to our type and get back either a validated User or an array of clear, customizable errors with a root summary.

const out = user({
	name: "Alan Turing",
	device: {
		platform: "enigma",
		versions: [0, "1", 0n]
	}
})

if (out instanceof type.errors) {
	// hover out.summary to see validation errors
	console.error(out.summary)
} else {
	// hover out to see your validated data
	console.log(`Hello, ${out.name}`)
}

And that’s it! You now know how to to define a Type use it to check your data at runtime.

Next, we’ll take a look at how ArkType extends TypeScript’s type system to handle runtime constraints like maxLength and pattern.