📈 Announcing ArkType 2.1 📈

Expressions

Intersection

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

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

A union that could apply different morphs to the same data throws a ParseError!

// operands overlap, but neither transforms data
const  = ("number > 0").or("number < 10")
// operand transforms data, but there's no overlap between the inputs
const  = ("string.numeric.parse").or({ box: "string" })
// operands overlap and transform data, but in the same way
const  = ("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  = ({ box: "string.numeric.parse" }).or({ box: "string" })
const  = ({ a: "string.numeric.parse" }).or({ b: "string.numeric.parse" })
Learn the set theory behind this restriction

If you're relatively new to set-based types, that error might be daunting, but if you take a second to think through the example, it becomes clear why this isn't allowed. The logic of bad is essentially:

  • If the input is an object where box is a string, parse and return it as a number
  • If the input is an object where box is a string, return it as a string

There is no way to deterministically return an output for this type without sacrificing the commutativity of the union operator.

sameError may look more innocuous, but has the same problem for an input like { a: "1", b: "2" }.

  • Left branch would only parse a, resulting in { a: 1, b: "2" }
  • Right branch would only parse b, resulting in { a: "1", b: 2 }

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

const :  = .assert(2)
// TypeScript: Type 'number' is not assignable to type 'Brand<number, "even">'
const :  = 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  = ({
	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  = 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  = ("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  = ("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  = ("string.numeric.parse |> number % 2")

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

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  = Symbol()

const  = type.unit()

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  = Symbol()

const  = type.enumerated(1337, true, )

valueOf

type.valueOf defines a Type from a TypeScript enum or enum-like object.

`enum` should be avoided in modern TypeScript

Over time, TS has shifted away from features that affect the .js it ultimately outputs, including enum.

With the introduction of the --erasableSyntaxOnly option to facilitate type-stripping, enum is no longer considered a best practice.

type.valueOf exists primarily to facilitate integration with legacy code that relies on enum, but if you have the option, prefer transparently defining value sets via ["tupleLiterals"] as const, { objectLiterals: true } as const, or directly via type.enumerated.

enum TsEnum {
	numeric = 1
}

const  = type.valueOf(TsEnum) // Type<1>

It is almost semantically identical to type.enumerated(...Object.values(o)). The only exception occurs when an object has an entry with a numeric value and entry with that value as a key mapping back to the original:

// this is the structure TsEnum compiles to in JS
const  = {
	numeric: 1,
	"1": "numeric"
} as 

// only allows the number 1 even though it is inferred
// to also allow the string "numeric"
const  = type.valueOf()

Notice EquivalentObject doesn't include "numeric" because it inverts a numeric value entry.

We recommend type.enumerated as the more transparent option for converting value references to a Type. However, if the described inverted entry pairs can't exist on your object, you can safely use type.valueOf.

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  = ("string").configure({
	description: "a special string"
})

// sugar for adding description metadata
const  = ("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  = "foo" | "bar" | (string & {})

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

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

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

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

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

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