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.

:::note[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 user = type({
	name: "string",
	device: {
		platform: "'android' | 'ios'",
		"versions?": "(number | string)[]"
	}
})
 
interface RuntimeErrors extends type.errors {
	/**device.platform must be "android" or "ios" (was "enigma")
device.versions[2] must be a number or a string (was bigint)*/
	summary: string
}
 
const narrowMessage = (e: type.errors): e is RuntimeErrors => true
 
// ---cut---
const out = user({
	name: "Alan Turing",
	device: {
		platform: "enigma",
		versions: [0, "1", 0n]
	}
})
 
if (out instanceof type.errors) {
 // ---cut-start---
 // just a trick to display the runtime error
	if (!narrowMessage(out)) throw new Error()
 // ---cut-end---
 // 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.

Your First Type

Define

Compose

Validate

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