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1.1.5 • Public • Published

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The idea

Blog post: "Typescript and validations at runtime boundaries" by @lorefnon

A value of type Type<A, O, I> (called "runtime type") is the runtime representation of the static type A.

Also a runtime type can

  • decode inputs of type I (through decode)
  • encode outputs of type O (through encode)
  • be used as a custom type guard (through is)
export type mixed = object | number | string | boolean | symbol | undefined | null
class Type<A, O = A, I = mixed> {
  readonly _A: A
  readonly _O: O
  readonly _I: I
    /** a unique name for this runtime type */
    readonly name: string,
    /** a custom type guard */
    readonly is: (v: mixed) => v is A,
    /** succeeds if a value of type I can be decoded to a value of type A */
    readonly validate: (input: I, context: Context) => Either<Errors, A>,
    /** converts a value of type A to a value of type O */
    readonly encode: (a: A) => O
  ) {}
  /** a version of `validate` with a default context */
  decode(i: I): Either<Errors, A>

Note. The Either type is defined in fp-ts, a library containing implementations of common algebraic types in TypeScript.


A runtime type representing string can be defined as

import * as t from 'io-ts'
export class StringType extends t.Type<string> {
  // equivalent to Type<string, string, mixed> as per type parameter defaults
  readonly _tag: 'StringType' = 'StringType'
  constructor() {
      (m): m is string => typeof m === 'string',
      (m, c) => ( ? t.success(m) : t.failure(m, c)),

A runtime type can be used to validate an object in memory (for example an API payload)

const Person = t.type({
  name: t.string,
  age: t.number
// validation succeeded
Person.decode(JSON.parse('{"name":"Giulio","age":43}')) // => Right({name: "Giulio", age: 43})
// validation failed
Person.decode(JSON.parse('{"name":"Giulio"}')) // => Left([...])

TypeScript compatibility

The stable version is tested against TypeScript 2.7.x+

Error reporters

A reporter implements the following interface

interface Reporter<A> {
  report: (validation: Validation<any>) => A

This package exports two default reporters

  • PathReporter: Reporter<Array<string>>
  • ThrowReporter: Reporter<void>


import { PathReporter } from 'io-ts/lib/PathReporter'
import { ThrowReporter } from 'io-ts/lib/ThrowReporter'
const result = Person.decode({ name: 'Giulio' })
// => ['Invalid value undefined supplied to : { name: string, age: number }/age: number']
// => throws 'Invalid value undefined supplied to : { name: string, age: number }/age: number'


  • io-ts-types - A collection of runtime types and combinators for use with io-ts
  • io-ts-reporters - Error reporters for io-ts
  • geojson-iots - Runtime types for GeoJSON as defined in rfc7946 made with io-ts
  • graphql-to-io-ts - Generate typescript and cooresponding io-ts types from a graphql schema

TypeScript integration

Runtime types can be inspected


This library uses TypeScript extensively. Its API is defined in a way which automatically infers types for produced values


Note that the type annotation isn't needed, TypeScript infers the type automatically based on a schema.

Static types can be extracted from runtime types using the TypeOf operator

type IPerson = t.TypeOf<typeof Person>
// same as
type IPerson = {
  name: string
  age: number

Implemented types / combinators

import * as t from 'io-ts'
Type TypeScript Runtime type / combinator
null null t.null or t.nullType
undefined undefined t.undefined
string string t.string
number number t.number
boolean boolean t.boolean
any any t.any
never never t.never
object object t.object
integer t.Integer
array of any Array<mixed> t.Array
array of type Array<A> t.array(A)
dictionary of any { [key: string]: mixed } t.Dictionary
dictionary of type { [K in A]: B } t.dictionary(A, B)
function Function t.Function
literal 's' t.literal('s')
partial Partial<{ name: string }> t.partial({ name: t.string })
readonly Readonly<T> t.readonly(T)
readonly array ReadonlyArray<number> t.readonlyArray(t.number)
type alias type A = { name: string } t.type({ name: t.string })
tuple [ A, B ] t.tuple([ A, B ])
union A \| B t.union([ A, B ]) or t.taggedUnion(tag, [ A, B ])
intersection A & B t.intersection([ A, B ])
keyof keyof M t.keyof(M)
recursive types see Recursive types t.recursion(name, definition)
refinement t.refinement(A, predicate)
exact types t.exact(type)
strict types (deprecated) t.strict({ name: t.string })

Recursive types

Recursive types can't be inferred by TypeScript so you must provide the static type as a hint

// helper type
type ICategory = {
  name: string
  categories: Array<ICategory>
const Category = t.recursion<ICategory>('Category', self =>
    name: t.string,
    categories: t.array(self)

Tagged unions

If you are encoding tagged unions, instead of the general purpose union combinator, you may want to use the taggedUnion combinator in order to get better performances

const A = t.type({
  tag: t.literal('A'),
  foo: t.string
const B = t.type({
  tag: t.literal('B'),
  bar: t.number
// the actual presence of the tag is statically checked
const U = t.taggedUnion('tag', [A, B])


You can refine a type (any type) using the refinement combinator

const Positive = t.refinement(t.number, n => n >= 0, 'Positive')
const Adult = t.refinement(Person, person => person.age >= 18, 'Adult')

Exact types

You can make a runtime type alias exact (which means that only the given properties are allowed) using the exact combinator

const Person = t.type({
  name: t.string,
  age: t.number
const ExactPerson = t.exact(Person)
Person.decode({ name: 'Giulio', age: 43, surname: 'Canti' }) // ok
ExactPerson.decode({ name: 'Giulio', age: 43, surname: 'Canti' }) // fails

Strict types (deprecated)

Note. This combinator is deprecated, use exact instead.

You can make a runtime type strict (which means that only the given properties are allowed) using the strict combinator

const Person = t.type({
  name: t.string,
  age: t.number
const StrictPerson = t.strict(Person.props)
Person.decode({ name: 'Giulio', age: 43, surname: 'Canti' }) // ok
StrictPerson.decode({ name: 'Giulio', age: 43, surname: 'Canti' }) // fails

Mixing required and optional props

You can mix required and optional props using an intersection

const A = t.type({
  foo: t.string
const B = t.partial({
  bar: t.number
const C = t.intersection([A, B])
type CT = t.TypeOf<typeof C>
// same as
type CT = {
  foo: string
  bar?: number

Custom types

You can define your own types. Let's see an example

import * as t from 'io-ts'
// represents a Date from an ISO string
const DateFromString = new t.Type<Date, string>(
  (m): m is Date => m instanceof Date,
  (m, c) =>
    t.string.validate(m, c).chain(s => {
      const d = new Date(s)
      return isNaN(d.getTime()) ? t.failure(s, c) : t.success(d)
  a => a.toISOString()
const s = new Date(1973, 10, 30).toISOString()
// right(new Date('1973-11-29T23:00:00.000Z'))
// left(errors...)

Note that you can deserialize while validating.

Tips and Tricks

Is there a way to turn the checks off in production code?

No, however you can define your own logic for that (if you really trust the input)

import * as t from 'io-ts'
import { Either, right } from 'fp-ts/lib/Either'
const { NODE_ENV } = process.env
export function unsafeDecode<A, O>(value: t.mixed, type: t.Type<A, O>): Either<t.Errors, A> {
  if (NODE_ENV !== 'production' || type.encode !== t.identity) {
    return type.decode(value)
  } else {
    // unsafe cast
    return right(value as A)
// or...
import { failure } from 'io-ts/lib/PathReporter'
export function unsafeGet<A, O>(value: t.mixed, type: t.Type<A, O>): A {
  if (NODE_ENV !== 'production' || type.encode !== t.identity) {
    return type.decode(value).getOrElseL(errors => {
      throw new Error(failure(errors).join('\n'))
  } else {
    // unsafe cast
    return value as A

Union of string literals

Use keyof instead of union when defining a union of string literals

const Bad = t.union([
  // etc...
const Good = t.keyof({
  foo: null,
  bar: null,
  baz: null
  // etc...


  • unique check for free
  • better performance
  • quick info stays responsive

Known issues

VS Code might display weird types for nested types

const NestedInterface = t.type({
  foo: t.string,
  bar: t.type({
    baz: t.string
type NestedInterfaceType = t.TypeOf<typeof NestedInterface>
Hover on NestedInterfaceType will display
type NestedInterfaceType = {
    foo: string;
    bar: t.TypeOfProps<{
        baz: t.StringType;
instead of
type NestedInterfaceType = {
  foo: string;
  bar: {
    baz: string

Solution: the clean and alias functions

The pattern

// private runtime type
const _NestedInterface = t.type({
  foo: t.string,
  bar: t.type({
    baz: t.string
// a type alias using interface
export interface NestedInterface extends t.TypeOf<typeof _NestedInterface> {}
// Two possible options for the exported runtime type
// a clean NestedInterface which drops the kind...
export const NestedInterface = t.clean<NestedInterface, NestedInterface>(_NestedInterface)
NestedInterface: t.Type<NestedInterface, NestedInterface, t.mixed>
// ... or an alias of _NestedInterface which keeps the kind
export const NestedInterface = t.alias(_NestedInterface)<NestedInterface, NestedInterface>()
    foo: t.StringType;
    bar: t.InterfaceType<{
        baz: t.StringType;
    }, t.TypeOfProps<{
        baz: t.StringType;
    }>, t.OutputOfProps<{
        baz: t.StringType;
    }>, t.mixed>;
}, NestedInterface, NestedInterface, t.mixed>
// you can also alias the props
interface NestedInterfaceProps extends t.PropsOf<typeof _NestedInterface> {}
export const NestedInterface = t.alias(_NestedInterface)<NestedInterface, NestedInterface, NestedInterfaceProps>()
const NestedInterface: t.InterfaceType<NestedInterfaceProps, NestedInterface, NestedInterface, t.mixed>


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