# Expressions

### Intersection

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

> **If you don't need to intersect overlapping props, spread instead!**
>
> Computing an intersection is more expensive than [merging props](https://arktype.io/docs/objects#merge), both at runtime and in-editor.
>
> If you can get away with the latter, doing so will make your Types faster and cleaner.

**string**

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

**fluent**

```ts
const FooObject = type({
	foo: "string"
})

// an object requiring both foo and bar
const FoobarObject = FooObject.and({
	bar: "number"
})
```

**tuple**

```ts
// an object requiring both foo and bar
const FoobarObject = type([
	{
		foo: "string"
	},
	"&",
	{
		bar: "number"
	}
])
```

**args**

```ts
// an object requiring both foo and bar
const FoobarObject = type(
	{
		foo: "string"
	},
	"&",
	{
		bar: "number"
	}
)
```

#### n-ary

To create an intersection of many objects directly, `type.and` will avoid the need to chain or compose many binary expressions:

```ts
const FooObject = type({
	foo: "string"
})

// accepts ...definitions
const FoobarObject = type.and(
	FooObject,
	{
		bar: "number"
	},
	{
		baz: "string"
	}
)
```

### Union

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

**string**

```ts
const Unions = type({
	key: "string | number"
})
```

**fluent**

```ts
const Unions = type({
	key: type.string.or(type.number)
})
```

**tuple**

```ts
const Unions = type({
	key: ["string", "|", { name: "string" }]
})
```

**args**

```ts
const Unions = type({
	key: type("string", "|", { name: "string" })
})
```

#### n-ary

To create a union of many objects directly, `type.or` will avoid the need to chain or compose many binary expressions:

```ts
// accepts ...definitions
const Union = type.or(type.string, "number", { key: "unknown" })
```

> **A union that could apply different morphs to the same data throws a ParseError!**
>
> ```ts
> // 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" })
> ```
>
> <details>
> <summary>Learn the set theory behind this restriction</summary>
>
> 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](https://en.wikipedia.org/wiki/Commutative_property) 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 }`
>
> </details>

### Brand

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

**string**

```ts
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
```

**fluent**

```ts
const Even = type.number.divisibleBy(2).brand("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](https://www.learningtypescript.com/articles/branded-types) from [Josh Goldberg](https://github.com/joshuakgoldberg).

### Narrow

Narrow expressions allow you to add custom validation logic and error messages. You can read more about them in [their intro section](https://arktype.io/docs/intro/adding-constraints#narrow).

**fluent**

```ts
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"
})
```

**tuple**

```ts
const Form = type([
	{
		password: "string",
		confirmPassword: "string"
	},
	":",
	(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"
})
```

**args**

```ts
const Form = type(
	{
		password: "string",
		confirmPassword: "string"
	},
	":",
	(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`.

**fluent**

```ts
// 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'")
)
```

**tuple**

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

**args**

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

### Filter

`filter` is similar to `narrow` but applies its predicate to the **input** of a type rather than the output. This distinction matters when your type includes morphs:

- **`narrow`**: validates the **output** (after morphs have been applied)
- **`filter`**: validates the **input** (before morphs run)

```ts
const ShortNumeric = type("string.numeric.parse").filter((s, ctx) => {
	// s is a string here (the input), not a number (the output)
	if (s.length > 10) return ctx.reject("at most 10 characters")
	return true
})

// the filter runs on the input (string) before the morph parses it
ShortNumeric("123") // 123
ShortNumeric("12345678901") // ArkErrors: must be at most 10 characters
```

If the return type of a filter is a type predicate, that will be reflected in the inferred `Type`, just like with `narrow`.

### Pipe

Piping allows you to transform your data after it is validated via one or more sequential **morphs**. You can read more about them in [their intro section](https://arktype.io/docs/intro/morphs-and-more/).

**fluent**

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

**tuple**

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

**args**

```ts
// hover to see how morphs are represented at a type-level
const trimStringStart = type("string", "=>", 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:

**string**

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

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

**fluent**

```ts
const parseEvenTo = type("string.numeric.parse").to("number % 2")

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

**tuple**

```ts
const parseEvenTo = type(["string.numeric.parse", "|>", "number % 2"])

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

**args**

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

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

#### n-ary

Though the fluent `.pipe(...morphsOrTypes)` method already accepts any number of arguments, `type.pipe()` allows you to define such an expression directly without having to explicitly instantiate the first input Type to chain from.

```ts
// directly accepts ...morphsOrTypes
const trimStartToNonEmpty = type.pipe(
	type.string,
	s => s.trimStart(),
	type.string.atLeastLength(1)
)
```

### Unit

While embedded [literal syntax](https://arktype.io/docs/primitives#number-literals) is usually ideal for defining exact primitive values, `===` and `type.unit` can be helpful for referencing a non-serializable value like a `symbol` from your type.

**fluent**

```ts
const mySymbol = Symbol()

const ExactValue = type.unit(mySymbol)
```

**tuple**

```ts
const mySymbol = Symbol()

const ExactValue = type(["===", mySymbol])
```

**args**

```ts
const mySymbol = Symbol()

const ExactValue = type("===", 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.

**fluent**

```ts
const mySymbol = Symbol()

const ExactValueFromSet = type.enumerated(1337, true, mySymbol)
```

**tuple**

```ts
const mySymbol = Symbol()

const ExactValueFromSet = type(["===", 1337, true, mySymbol])
```

**args**

```ts
const mySymbol = Symbol()

const ExactValueFromSet = type("===", 1337, true, mySymbol)
```

### 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](https://www.typescriptlang.org/tsconfig/#erasableSyntaxOnly) 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`](https://arktype.io/docs/expressions#enumerated).

```ts
enum TsEnum {
	numeric = 1
}

const EnumType = 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:

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

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

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`.

### fn

`type.fn` defines a function with runtime-validated parameters and an optional return type. The result is a `TypedFn`- a callable with `.expression`, `.params`, and `.returns` for introspection.

Parameters are defined using the same syntax as tuple types. A return type can optionally be specified after a `":"` separator.

```ts
const len = type.fn("string | unknown[]", ":", "number")(s => s.length)

len("foo") // 3
len([1, 2]) // 2

len.expression // "(string | Array) => number"

len(true) // TraversalError: value at [0] must be a string or an object (was boolean)
```

Supports defaults, optionals, and variadic parameters:

```ts
const greet = type.fn(
	"string",
	"string = 'world'"
)((greeting, name) => `${greeting}, ${name}!`)

greet("Hello") // "Hello, world!"

const maybeDouble = type.fn(
	"number",
	"boolean?"
)((n, double) => (double ? n * 2 : n))

maybeDouble(5) // 5
maybeDouble(5, true) // 10

const join = type.fn(
	"...",
	"string[]",
	":",
	"string"
)((...parts) => parts.join(","))

join.expression // "(...string[]) => string"
```

If no return type is specified, the return type is inferred from the implementation. If a return type is specified, it will be validated at runtime.

### 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.

**fluent**

```ts
// 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")
```

**tuple**

```ts
// this validator's error message will now start with "must be a special string"
const SpecialString = type([
	"string",
	"@",
	{
		description: "a special string"
	}
])

// sugar for adding description metadata
const SpecialNumber = type(["number", "@", "a special number"])
```

**args**

```ts
// this validator's error message will now start with "must be a special string"
const SpecialString = type("string", "@", {
	description: "a special string"
})

// sugar for adding description metadata
const SpecialNumber = type("number", "@", "a special number")
```

For more control over how to apply the config to the Type's internal structure, a `NodeSelector` can be specified following the config:

**fluent**

```ts
const SelectivelyConfigured = type({
	name: "string",
	age: "number"
}).configure(
	{
		description: "a special string"
	},
	// add the description to all domain keywords
	"domain"
)

SelectivelyConfigured.get("name").description // "a special string"
SelectivelyConfigured.get("age").description // "a special string"
```

**tuple**

```ts
const SelectivelyConfigured = type([
	{
		name: "string",
		age: "number"
	},
	"@",
	{
		description: "a special string"
	},
	// add the description to all domain keywords
	"domain"
])

SelectivelyConfigured.get("name").description // "a special string"
SelectivelyConfigured.get("age").description // "a special string"
```

**args**

```ts
const SelectivelyConfigured = type(
	{
		name: "string",
		age: "number"
	},
	"@",
	{
		description: "a special string"
	},
	// add the description to all domain keywords
	"domain"
)

SelectivelyConfigured.get("name").description // "a special string"
SelectivelyConfigured.get("age").description // "a special string"
```

For more details on the select API, [see the full docs](https://arktype.io/docs/introspection#select).

### 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.

**string**

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

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

**fluent**

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

const MyObj = type({
	autocompletedString: type.string.as<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.

```ts
// 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.

```ts
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)
}
```

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

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