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2.1.1 • Public • Published

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To install the stable version:

npm i io-ts fp-ts

Note: fp-ts is a peer dependency for io-ts

The idea

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

A codec can:

  • decode inputs of type I (through decode)
  • encode outputs of type O (through encode)
  • be used as a custom type guard (through is)
class Type<A, O, I> {
    /** a unique name for this codec */
    readonly name: string,

    /** a custom type guard */
    readonly is: (u: unknown) => u 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>

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

The Either type represents a value of one of two possible types (a disjoint union). An instance of Either is either an instance of Left or Right:

type Either<E, A> =
  | {
      readonly _tag: 'Left'
      readonly left: E
  | {
      readonly _tag: 'Right'
      readonly right: A

Convention dictates that Left is used for failure and Right is used for success.


A codec representing string can be defined as:

import * as t from 'io-ts'

const string = new t.Type<string, string, unknown>(
  (input: unknown): input is string => typeof input === 'string',
  // `t.success` and `t.failure` are helpers used to build `Either` instances
  (input, context) => (typeof input === 'string' ? t.success(input) : t.failure(input, context)),
  // `A` and `O` are the same, so `encode` is just the identity function

and we can use it as follows:

import { isRight } from 'fp-ts/lib/Either'

isRight(string.decode('a string')) // true
isRight(string.decode(null)) // false

More generally the result of calling decode can be handled using fold along with pipe (which is similar to the pipeline operator)

import * as t from 'io-ts'
import { pipe } from 'fp-ts/lib/pipeable'
import { fold } from 'fp-ts/lib/Either'

// failure handler
const onLeft = (errors: t.Errors): string => `${errors.length} error(s) found`

// success handler
const onRight = (s: string) => `No errors: ${s}`

pipe(t.string.decode('a string'), fold(onLeft, onRight))
// => "No errors: a string"

pipe(t.string.decode(null), fold(onLeft, onRight))
// => "1 error(s) found"

We can combine these codecs through combinators to build composite types which represent entities like domain models, request payloads etc. in our applications:

import * as t from 'io-ts'

const User = t.type({
  userId: t.number,
  name: t.string

So this is equivalent to defining something like:

type User = {
  userId: number
  name: string

The advantage of using io-ts to define the runtime type is that we can validate the type at runtime, and we can also extract the corresponding static type, so we don’t have to define it twice.

TypeScript integration

Codecs 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 (and comments are preserved).

Static types can be extracted from codecs using the TypeOf operator:

type User = t.TypeOf<typeof User>

// same as
type User = {
  userId: number
  name: string

TypeScript compatibility

The stable version is tested against TypeScript 3.5.2

io-ts version required TypeScript version
2.x+ 3.5.2+
1.6.x+ 3.2.2+
1.5.3 3.0.1+
1.5.2- 2.7.2+

Note. This library is conceived, tested and is supposed to be consumed by TypeScript with the strict flag turned on.

Note. If you are running < typescript@3.0.1 you have to polyfill unknown.

You can use unknown-ts as a polyfill.

Error reporters

A reporter implements the following interface

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

This package exports a default PathReporter reporter


import { PathReporter } from 'io-ts/lib/PathReporter'

const result = User.decode({ name: 'Giulio' })

// => [ 'Invalid value undefined supplied to : { userId: number, name: string }/userId: number' ]

You can define your own reporter. Errors has the following type

interface ContextEntry {
  readonly key: string
  readonly type: Decoder<any, any>

interface Context extends ReadonlyArray<ContextEntry> {}

interface ValidationError {
  readonly value: unknown
  readonly context: Context

interface Errors extends Array<ValidationError> {}


import { pipe } from 'fp-ts/lib/pipeable'
import { fold } from 'fp-ts/lib/Either'

const getPaths = <A>(v: t.Validation<A>): Array<string> => {
  return pipe(
      errors => errors.map(error => error.context.map(({ key }) => key).join('.')),
      () => ['no errors']

console.log(getPaths(User.decode({}))) // => [ '.userId', '.name' ]

Custom error messages

You can set your own error message by providing a message argument to failure


import { either } from 'fp-ts/lib/Either'

const NumberFromString = new t.Type<number, string, unknown>(
  (u, c) =>
    either.chain(t.string.validate(u, c), s => {
      const n = +s
      return isNaN(n) ? t.failure(u, c, 'cannot parse to a number') : t.success(n)

// => ['cannot parse to a number']

You can also use the withMessage helper from io-ts-types

Implemented types / combinators

Type TypeScript codec / combinator
null null t.null or t.nullType
undefined undefined t.undefined
void void t.void or t.voidType
string string t.string
number number t.number
boolean boolean t.boolean
unknown unknown t.unknown
array of unknown Array<unknown> t.UnknownArray
array of type Array<A> t.array(A)
record of unknown Record<string, unknown> t.UnknownRecord
record of type Record<K, A> t.record(K, A)
function Function t.Function
literal 's' t.literal('s')
partial Partial<{ name: string }> t.partial({ name: t.string })
readonly Readonly<A> t.readonly(A)
readonly array ReadonlyArray<A> t.readonlyArray(A)
type alias type T = { name: A } t.type({ name: A })
tuple [ A, B ] t.tuple([ A, B ])
union A | B t.union([ A, B ])
intersection A & B t.intersection([ A, B ])
keyof keyof M t.keyof(M) (only supports string keys)
recursive types t.recursion(name, definition)
branded types / refinements t.brand(A, predicate, brand)
integer t.Int (built-in branded codec)
exact types t.exact(type)
strict t.strict({ name: A }) (an alias of t.exact(t.type({ name: A })))

Recursive types

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

interface Category {
  name: string
  categories: Array<Category>

const Category: t.Type<Category> = t.recursion('Category', () =>
    name: t.string,
    categories: t.array(Category)

Mutually recursive types

interface Foo {
  type: 'Foo'
  b: Bar | undefined

interface Bar {
  type: 'Bar'
  a: Foo | undefined

const Foo: t.Type<Foo> = t.recursion('Foo', () =>
    type: t.literal('Foo'),
    b: t.union([Bar, t.undefined])

const Bar: t.Type<Bar> = t.recursion('Bar', () =>
    type: t.literal('Bar'),
    a: t.union([Foo, t.undefined])

Branded types / Refinements

You can brand / refine a codec (any codec) using the brand combinator

// a unique brand for positive numbers
interface PositiveBrand {
  readonly Positive: unique symbol // use `unique symbol` here to ensure uniqueness across modules / packages

const Positive = t.brand(
  t.number, // a codec representing the type to be refined
  (n): n is t.Branded<number, PositiveBrand> => n >= 0, // a custom type guard using the build-in helper `Branded`
  'Positive' // the name must match the readonly field in the brand

type Positive = t.TypeOf<typeof Positive>
same as
type Positive = number & t.Brand<PositiveBrand>

Branded codecs can be merged with t.intersection

// t.Int is a built-in branded codec
const PositiveInt = t.intersection([t.Int, Positive])

type PositiveInt = t.TypeOf<typeof PositiveInt>
same as
type PositiveInt = number & t.Brand<t.IntBrand> & t.Brand<PositiveBrand>

Exact types

You can make a codec exact (which means that additional properties are stripped) using the exact combinator

const ExactUser = t.exact(User)

User.decode({ userId: 1, name: 'Giulio', age: 45 }) // ok, result is right({ userId: 1, name: 'Giulio', age: 45 })
ExactUser.decode({ userId: 1, name: 'Giulio', age: 43 }) // ok but result is right({ userId: 1, name: 'Giulio' })

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 C = t.TypeOf<typeof C>

// same as
type C = {
  foo: string
} & {
  bar?: number | undefined

You can apply partial to an already defined codec via its props field

const PartialUser = t.partial(User.props)

type PartialUser = t.TypeOf<typeof PartialUser>

// same as
type PartialUser = {
  name?: string
  age?: number

Custom types

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

import { either } from 'fp-ts/lib/Either'

// represents a Date from an ISO string
const DateFromString = new t.Type<Date, string, unknown>(
  (u): u is Date => u instanceof Date,
  (u, c) =>
    either.chain(t.string.validate(u, c), s => {
      const d = new Date(s)
      return isNaN(d.getTime()) ? t.failure(u, 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.

Generic Types

Polymorphic codecs are represented using functions. For example, the following typescript:

interface ResponseBody<T> {
  result: T
  _links: Links

interface Links {
  previous: string
  next: string

Would be:

// where `t.Mixed = t.Type<any, any, unknown>`
const responseBody = <C extends t.Mixed>(codec: C) =>
    result: codec,
    _links: Links

const Links = t.type({
  previous: t.string,
  next: t.string

And used like:

const UserModel = t.type({
  name: t.string

functionThatRequiresRuntimeType(responseBody(t.array(UserModel)), ...params)


You can pipe two codecs if their type parameters do align

const NumberCodec = new t.Type<number, string, string>(
  (s, c) => {
    const n = parseFloat(s)
    return isNaN(n) ? t.failure(s, c) : t.success(n)

const NumberFromString = t.string.pipe(NumberCodec, 'NumberFromString')


  • io-ts@2.x
  • io-ts@1.x
    • geojson-iots - codecs for GeoJSON as defined in rfc7946 made with io-ts
    • graphql-to-io-ts - Generate typescript and corresponding io-ts types from a graphql schema

Tips and Tricks

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, O(log(n)) vs O(n)

Beware that keyof is designed to work with objects containing string keys. If you intend to define a numbers enumeration, you have to use an union of number literals :

const HttpCode = t.union([
  // etc...

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  • vmarchaud
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