Runtime typechecking

Usage no npm install needed!

<script type="module">
  import tsCast from 'https://cdn.skypack.dev/ts-cast';


ts-cast · Coverage Status npm version npm downloads GitHub license

Runtime type checking for Typescript and JavaScript projects. It allows to specify data structures of the "external" resources (requests to the application or responses from other applications).

The idea is based on the Caster that is a function accepts value of unknown type and returns value of target type or throws a TypeError if casting is impossible.

Additionally ts-cast presents tools for:

  • validation casted data
  • transformation of casted data
  • Either and Validation monads support

Almost in all cases ts-cast allows to avoid specification of correspondent type in TypeScript, because the type could be get as ReturnType of the correspondent caster-function.

const Person = struct({ name: string, age: integer });

type Person = ReturnType<typeof Person>;

// the same as

type Person = { name: string, age: number };


npm i ts-cast


Read ts-cast documentation on github:

Getting Started

Let's start with an example:


import { integer, number, string, struct, tuple, array, toBe } from 'ts-cast';
import v from 'validator';

export const Person = struct({
  name: string,
  email: string.restrict(toBe(v.isEmail, "a valid email")),

export const Coords = tuple(number, number);

export const Book = struct({
  title: string,
  annotation: string.optional,
  year: integer,
  authors: array(Person),
  coords: Coords.optional,

export type TPerson = ReturnType<typeof Person>;
export type TCoords = ReturnType<typeof Coords>;
export type TBook = ReturnType<typeof Book>;

Then we can use defined types and their caster-s in this way:


import { Person, TPerson, Book, TBook } from './schema';

const me = Person({ name: "John Smith", email: "john@smith.com" });

const meAgain: TPerson = {
  name: "John Smith",
  email: "john.smith@gmail.com", // but email will not be validated with validator.isEmail

const myBook = Book({
  title: "My First Book",
  year: 2021,
  authors: [
    { name: 'Some Other Guy', email: 'other.guy@gmail.com' },
  coords: [1.2, 23.32]
}, "myBook");

const book: TBook = myBook;

console.dir(book, { depth: null });

TypeScript Analogy

Primitive Types

ts-cast TypeScript
number number
integer number
string string
boolean boolean
nil null
undef undefined


import { integer } from 'ts-cast';

const x = integer(10); // works
const y = integer(Math.PI);  // throws TypeError
const z = integet('10'); // also throws TypeError
import { string } from 'ts-cast';

const a = string('Hello Wordl!'); // works
const b = string(''); // also works
const c = string(null); // throws TypeError
const d = string(20); // throws TypeError again

Literal Types

ts-cast TypeScript Comments
value(V) V as const V extends number \| string \| boolean \| symbol
values(A, B, C, ...) A \| B \| C ... A, B, C ... extends number \| string \| boolean \| symbol
import { values } from 'ts-cast';

const TrafficLight = values('red', 'yellow', 'green');

const green = TrafficLight('green'); // works

type TypeOfGreen = typeof green; // 'red' | 'yellow' | 'green'

const wrong = TrafficLight('blue'); // throws TypeError

Complex Types

ts-cast TypeScript
tuple(caster<T1>, caster<T2>, ...) [T1, T2, ...]
array(caster<T>) T[]
struct({ a: caster<A>, b: caster<B> }) { a: A, b: B }
record(caster<K>, caster<V>) Record<K, V>

Type Modifiers

ts-cast TypeScript
caster<T>.optional T \| undefined
caster<T>.default(v) x: T = v
caster<T>.nullable T \| null
caster<T>.restrict(...) T

Operations over Types

ts-cast TypeScript
union(caster<T1>, caster<T2>, ...) T1 \| T2 \| ...
prod(caster<T1>, caster<T2>, ...) T1 & T2 & ...