Isntnt is a collection of composable JavaScript runtime type predicates with TypeScript type guard declarations. Supports generics including union and intersection types.

Generics

above

(floor: number) => (value: unknown) => value is number

const isAboveZero = above(0)

and

<T extends Array<Predicate<any>>>(...predicates: T) => (value: unknown) => value is Predicate<Intersect<Static<T[number]>>>

const isBetween0And21 = and(above(0), below(21))

const isUser = shape({ name: isString })
const hasEmailAddress = at('email', isString)

const isUserWithEmail = and(isUser, hasEmailAddress) // (value: unknown) => { name: string } & { email: string }

array

<T>(predicate: Predicate<T>) => (value: unknown) => value is Array<T>

const isAnyArray = array(isAny) // (value: unknown) => value is Array<any>

at

<T extends PropertyKey, U>(key: T, predicate: Predicate<U>) => (value: unknown) => value is { [P in T]: U }

const isAnyAtFoo = at('foo', isAny) // (value: unknown) => value is { foo: any }

below

(max: number) => (value: unknown) => value is number

const isBelow21 = below(21)

either

<T extends Array<Primitive>>(...literalValues: T) => (value: unknown) => value is T[number]

const isFooOrBar = either('foo', 'bar') // (value: unknown) => value is 'foo' | 'bar'

has

<T extends PropertyKey>(key: T) => (value: unknown) => value is { [P in T]: unknown }

const hasFoo = has('foo') // (value: unknown) => value is { 'foo': unknown }

instance

<T extends Constructor<any, any>>(constructor: T) => (value: unknown) => value is InstanceType<T>

const isInstanceofString = instance(String) // (value: unknown) => value is String

literal

<T extends Primitive>(literalValue: T) => (value: unknown) => value is T

const is42 = literal(42) // (value: unknown) => value is 42

max

<T extends number>(max: number) => (value: unknown) => value is number

const isMax255 = max(255)

maybe

<T>(predicate: Predicate<T>) => (value: unknown) => value is T | null | undefined

const isMaybeString = maybe(isString) // (value: unknown) => value is string | null | undefined

min

(min: number) => (value: unknown) => value is number

const isMin18 = min(18)

noneable

Aliases maybe

nullable

<T>(predicate: Predicate<T>) => (value: unknown) => value is T | null

const isNullableString = nullable(isString) // (value: unknown) => value is string | null

object

<T>(predicate: Predicate<T>) => (value: unknown) => value is Record<any, T>

const isEnum = object(isUint) // (value: unknown) => value is Record<any, number>

optional

<T>(predicate: Predicate<T>) => (value: unknown) => value is T | undefined

const isOptionalString = optional(isString) // (value: unknown) => value is string | undefined

or

<T extends Array<Predicate<any>>>(...predicates: T) => (value: unknown) => value is Static<T[number]>

const isStringOrNumber = or(isString, isNumber) // (value: unknown) => value is string | number

record

<T extends PropertyKey, U>(keyPredicate: Predicate<T>, valuePredicate: Predicate<U>) => (value: unknown) => value is Record<T, U>

const isDictionary = record(isString, isInt) // (value: unknown) => value is Record<string, number>

shape

<T extends Record<PropertyKey, Predicate<any>>>(predicates: T) => (value: unknown) => value is { [P in keyof T]: Static<T[P]> }

Note: Actual signature also considers optional members ({ name?: T }) in its Predicate type

const isCoordinate = shape({ x: isNumber, y: isNumber }) // (value: unknown) => value is { x: number, y: number }

test

(expression: RegExp) => (value: unknown) => value is string

const isSlug = test(/^[\w-]+$/)

tuple

<T extends Array<any>>(...predicates: { [K in keyof T]: Predicate<T[K]> }) => (value: unknown) => value is T

const isPoint = tuple(isNumber, isNumber) // (value: unknown) => value is [number, number]

Predicates

isAny

(value: unknown) => value is any

Always returns true.

isAny(value)

isArray

(value: unknown) => value is Array<unknown>

isArray(value)

isArrayLike

(value: unknown) => value is Record<number, unknown>

isArrayLike(value)

isBigInt

(value: unknown) => value is bigint

isBigInt(value)

isBoolean

(value: unknown) => value is boolean

isBoolean(value)

isDate

(value: unknown) => value is Date

isDate(value)

isDictionary

(value: unknown) => value is Record<any, string>

isDictionary(value)

isFalse

(value: unknown) => value is false

isFalse(value)

isFunction

(value: unknown) => value is Function

isFunction(value)

isInt

(value: unknown) => value is number

isInt(value)

isInt8

(value: unknown) => value is number

isInt8(value)

isInt16

(value: unknown) => value is number

isInt16(value)

isInt32

(value: unknown) => value is number

isInt32(value)

isLength

(value: unknown) => value is number

isLength(value)

isMap

(value: unknown) => value is Map<any, unknown>

isMap(value)

isNegative

(value: unknown) => value is number

isNegative(value)

isNever

(value: unknown) => value is never

Always returns false;

isNever(value)

isNone

(value: unknown) => value is null | undefined

isNone(value)

isNull

(value: unknown) => value is null

isNull(value)

isNumber

(value: unknown) => value is number

isNumber(value)

isObject

(value: unknown) => value is object

isObject(value)

isObjectLike

(value: unknown) => value is ObjectLike

isObjectLike(value)

isPlainObject

(value: unknown) => value is {}

isPlainObject(value)

isPositive

(value: unknown) => value is number

isPositive(value)

isPrimitive

(value: unknown) => value is Primitive

isPrimitive(value)

isRegExp

(value: unknown) => value is RegExp

isRegExp(value)

isSerializable

(value: unknown) => value is Serializable

isSerializable(value)

isSerializableArray

(value: unknown) => value is Array<Serializable>

isSerializableArray(value)

isSerializableNumber

(value: unknown) => value is number

isSerializableNumber(value)

isSerializableObject

(value: unknown) => value is Record<string, Serializable>

isSerializableObject(value)

isSerializablePrimitive

(value: unknown) => value is SerializablePrimitive

isSerializablePrimitive(value)

isSet

(value: unknown) => value is Set<unknown>

isSet(value)

isSome

(value: unknown) => value is Some

isSome(value)

isString

(value: unknown) => value is string

isString(value)

isSymbol

(value: unknown) => value is symbol

isSymbol(value)

isTrue

(value: unknown) => value is true

isTrue(value)

isUint

(value: unknown) => value is number

isUint(value)

isUint8

(value: unknown) => value is number

isUint8(value)

isUint16

(value: unknown) => value is number

isUint16(value)

isUint32

(value: unknown) => value is number

isUint32(value)

isUndefined

(value: unknown) => value is undefined

isUndefined(value)

isWeakMap

(value: unknown) => value is WeakMap<any, unknown>

isWeakMap(value)

isWithLength

(value: unknown) => value is { length: number }

isWithLength(value)

Types

Intersect

Intersect<A | B> // A & B

Maybe

Maybe<T> // T | null | undefined

type MaybeString = Maybe<string> // string | null | undefined

None

None // null | undefined

Nullable

Nullable<T> // T | null

type NullableString = Nullable<string> // string | null

Optional

Optional<T> // T | undefined

type OptionalString = Optional<string> // string | undefined

Predicate

Predicate<T> // (value: unknown, ...rest: Array<unknown>) => value is T

Primitive

Primitive // null | undefined | boolean | number | string | symbol | bigint

Serializable

Serializable // SerializableArray | SerializableObject | SerializablePrimitive

SerializableArray

SerializableArray // Array<Serializable>

SerializableObject

SerializableObject // Partial<{ [key: string]: Serializable }>

SerializablePrimitive

SerializablePrimitive // null | boolean | number | string

Some

Some // Function | boolean | bigint | number | string | symbol | object
Some<T> // Exclude<T, undefined | null>

// Make sure `T` is not `null` or `undefined`
type Option<T extends Some, E extends Error> = T | E

// Remove `null` or `undefined` from a type
type MaybeString = Optional<string> // string | null | undefined
type SomeString = Some<MaybeString> // string

Static

Static<Predicate<T>> // T

type True = Static<typeof isTrue> // true