cito

Check types at runtime

• Small: 1/4 of superstruct, 1/15 of zod
• Support recursive type declarations
• Descriptive error messages
• Full TypeScript support
• JIT compilation for fast validation checks
• Only loose assertions: does not warn on extra keys

Usage

import {assert, is, object, number, string, array} from 'cito'

type Post = typeof Post.infer
const Post = object({
  id: number,
  title: string,
  link: string.optional,
  author: object({id: number}),
  tags: array(string)
})

const data = {
  id: 42,
  title: 'Hello world',
  author: {id: 42},
  tags: ['hello']
}

// Throws if data is invalid, data is type as Post
assert(data, Post)

// data is typed as Post in the block, does not throw
if (is(data, Post)) {
  // use data
}

// Assert and name the input data
const post = Post(data)

Defining objects using a class

Object types can be declared using a class, which has the following advantages over plain objects:

• Optional properties

  It is possible to mark properties as optional which will be reflected in the inferred type:

  import {object, string} from 'cito'
  const WithOptional = object(
    class {
      required = string
      present = string.optional
      optional? = string.optional
    }
  )
  type WithOptional = typeof WithOptional.infer
  //   ^? {required: string, present: string | undefined, optional?: string | undefined}

• Recursive types

  Declare recursive types with full type inference without having to resort to manual type definition.

  Note: recursive types cannot be JIT compiled

  import {any, object} from 'cito'
  type Node = typeof Node.infer
  const Node = object(
    class Node {
      next = object(Node)
      prev = object(Node)
      data = any
    }
  )
  type List = typeof List.infer
  const List = object({
    head: Node.optional
  })

Api

Cito exports the following public members.

const string: Type<string>
const number: Type<number>
const bigint: Type<bigint>
const boolean: Type<boolean>
const symbol: Type<symbol>
const date: Type<Date>
const any: Type<any>
const func: Type<Function>
function literal<T>(value: T): Type<T>
function nullable<T>(inner: Type<T>): Type<T | null>
function optional<T>(inner: Type<T>): Type<T | undefined>
function instance<T>(constructor: new (...args: any[]) => T): Type<T>
function tuple<T>(...types: T): Type<Tuple<T>>
function record<T>(inner: Type<T>): Type<Record<string, T>>
function object<T>(definition: T): Type<Object<T>>
function union<T>(...types: T): Type<Union<T>>
function array<T>(inner: Type<T>): Type<Array<T>>
function enums<T>(types: T): Type<keyof T>
function lazy<T>(fn: () => Type<T>): Type<T>
function assert<T>(value: unknown, type: Type<T>): asserts value is T
function is<T>(value: unknown, type: Type<T>): value is T
function compile<T>(type: T): {check: (value) => value is T}
type Infer<T> = T extends Type<infer U> ? U : ...

A Type has the following api:

interface Validator<T> {
  (value: any): value is T
}

interface Type<T> {
  // This special property allows you to infer the type `T`
  infer: T

  // Call the instance to type check a value and return it if valid
  (value: any): T

  // A type that includes `T` and `null`
  nullable: Type<T | null>

  // A type that includes `T` and `undefined`
  optional: Type<T | undefined>

  // Returns a new type that narrows `T` to a subtype `E`
  narrow<E extends T>(): Type<E>

  // Create a new instance of type `T`, does not type check at runtime
  new (value: any): T

  // Returns a boolean indicating whether input is of type `T`
  is(input: any): input is T

  // Returns a new type which validates both `T` and `E`
  and<E>(validate: Validator<E>): Type<T & E>

  // Returns a new type which validates either `T` or `E`
  or<E>(validate: Validator<E>): Type<T | E>
}

Custom types can be created using the type function:

import {type} from 'cito'

const regex = type((value): value is RegExp => value instanceof RegExp)

regex(/(.*?)/g)
regex('this will throw')

Benchmarks

Making the comparison with superstruct and zod:

The benchmark code is adapted from typed, which is MIT License Copyright (c) 2022 CodBot

benchmark        time (avg)             (min … max)       p75       p99      p995
--------------------------------------------------- -----------------------------
zod           68.25 µs/iter     (59.4 µs … 1.19 ms)   66.5 µs  173.3 µs  203.6 µs
superstruct  252.25 µs/iter   (217.9 µs … 779.4 µs)  239.4 µs  513.5 µs  548.3 µs
cito           21.1 µs/iter    (19.4 µs … 283.9 µs)   20.2 µs   33.8 µs   44.3 µs

summary for validate
  cito
   3.23x faster than zod
   11.96x faster than superstruct

cito jit     381.57 ns/iter (349.56 ns … 723.29 ns) 378.48 ns 720.73 ns 723.29 ns
typebox jit  767.31 ns/iter   (726.39 ns … 1.11 µs) 768.16 ns   1.11 µs   1.11 µs

summary for validate jit
  cito jit
   2.01x faster than typebox jit

The data used in the benchmarks is from SpaceX's GraphQL API.