Expressions

Intersection

Like its TypeScript counterpart, an intersection combines two existing Types to create a new Type that enforces the constraints of both.

const myObject = type({
	// an email address with the domain arktype.io
	intersected: "string.email & /@arktype\\.io$/"
})

Union

All unions are automatically discriminated to optimize check time and error message clarity.

const unions = type({
	key: "string | number"
})

// operands overlap, but neither transforms data
const okay = type("number > 0").or("number < 10")
// operand transforms data, but there's no overlap between the inputs
const alsoOkay = type("string.numeric.parse").or({ box: "string" })
// operands overlap and transform data, but in the same way
const stillOkay = type("string > 5", "=>", Number.parseFloat).or([
	"0 < string < 10",
	"=>",
	Number.parseFloat
])
// ParseError: An unordered union of a type including a morph and a type with overlapping input is indeterminate
const bad = type({ box: "string.numeric.parse" }).or({ box: "string" })
const sameError = type({ a: "string.numeric.parse" }).or({ b: "string.numeric.parse" })

Brand

Add a type-only symbol to an existing type so that the only values that satisfy is are those that have been directly validated.

const even = type("(number % 2)#even")
type Even = typeof even.infer

const good: Even = even.assert(2)
// TypeScript: Type 'number' is not assignable to type 'Brand<number, "even">'
const bad: Even = 5

Brands can be a great way to represent constraints that fall outside the scope TypeScript, but remember they don't change anything about what is enforced at runtime!

For more information on branding in general, check out this excellent article from Josh Goldberg.

Narrow

Narrow expressions allow you to add custom validation logic and error messages. You can read more about them in their intro section.

const form = type({
	password: "string",
	confirmPassword: "string"
}).narrow((data, ctx) => {
	if (data.password === data.confirmPassword) {
		return true
	}
	return ctx.reject({
		expected: "identical to password",
		// don't display the password in the error message!
		actual: "",
		path: ["confirmPassword"]
	})
})

// ArkErrors: confirmPassword must be identical to password
const out = form({
	password: "arktype",
	confirmPassword: "artkype"
})

If the return type of a narrow is a type predicate, that will be reflected in the inferred Type.

// hover to see how the predicate is propagated to the outer `Type`
const arkString = type("string").narrow(
	(data, ctx): data is `ark${string}` =>
		data.startsWith("ark") ?? ctx.reject("a string starting with 'ark'")
)

Morph

Morphs allow you to transform your data after it is validated. You can read more about them in their intro section.

// hover to see how morphs are represented at a type-level
const trimStringStart = type("string").pipe(str => str.trimStart())

To

If a morph returns an ArkErrors instance, validation will fail with that result instead of it being treated as a value. This is especially useful for using other Types as morphs to validate output or chain transformations.

To make this easier, there's a special to operator that can pipe to a parsed definition without having to wrap it in type to make it a function:

const parseEvenTo = type("string.numeric.parse |> number % 2")

const even = type("number % 2")
// equivalent to parseEvenTo
const parseEvenPipe = type("string.numeric.parse").pipe(even)

Unit

While embedded literal syntax is usually ideal for defining exact primitive values, === and type.unit can be helpful for referencing a non-serialiazable value like a symbol from your type.

const mySymbol = Symbol()

const exactValue = type.unit(mySymbol)

Enumerated

type.enumerated defines a type based on a list of allowed values. It is semantically equivalent to type.unit if provided a single value.

const mySymbol = Symbol()

const exactValueFromSet = type.enumerated(1337, true, mySymbol)

Meta

Metadata allows you to associate arbitrary metadata with your types.

Some metadata is consumed directly by ArkType, for example description is referenced by default when building an error message.

Other properties are introspectable, but aren't used by default internally.

// this validator's error message will now start with "must be a special string"
const specialString = type("string").configure({
	description: "a special string"
})

// sugar for adding description metadata
const specialNumber = type("number").describe("a special number")

Cast

Sometimes, you may want to directly specify how a Type should be inferred without affecting the runtime behavior. In these cases, you can use a cast expression.

// allow any string, but suggest "foo" and "bar"
type AutocompletedString = "foo" | "bar" | (string & {})

const myObject = type({
	autocompletedString: "string" as type.cast<AutocompletedString>
})

Parenthetical

By default, ArkType's operators follow the same precedence as TypeScript's. Also like in TypeScript, this can be overridden by wrapping an expression in parentheses.

// hover to see the distinction!
const groups = type({
	stringOrArrayOfNumbers: "string | number[]",
	arrayOfStringsOrNumbers: "(string | number)[]"
})

this

this is a special keyword that can be used to create a recursive type referencing the root of the current definition.

const disappointingGift = type({
	label: "string",
	"box?": "this"
})

const out = disappointingGift({
	label: "foo",
	box: { label: "bar", box: {} }
})

if (out instanceof type.errors) {
	// ArkErrors: box.box.label must be a string (was missing)
	console.error(out.summary)
} else {
	// narrowed inference to arbitrary depth
	console.log(out.box?.box?.
label: string | undefinedlabel)
}

Unlike its TypeScript counterpart, ArkType's this is not limited to interfaces. It can also be used from within a tuple expression.

// boxes now expects an array of our gift object
const disappointingGifts = type({ label: "string", boxes: "this" }, "[]")

Referencing this from within a scope will result in a ParseError. For similar behavior within a scoped definition, just reference the alias by name:

const types = scope({
	disappointingGift: {
		label: "string",
		// Resolves correctly to the root of the current type
		"box?": "disappointingGift"
	}
}).export()