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3.2.29 • Public • Published

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"Si vis pacem, para bellum" - (Vegetius 5th century)

tcomb is a library for Node.js and the browser which allows you to check the types of JavaScript values at runtime with a simple and concise syntax. It's great for Domain Driven Design and for adding safety to your internal code.

TypeScript / Flowtype users

You may want to check out io-ts

IMPORTANT: Running in production

tcomb is supposed to be used in development and is disabled in production. If you want type checks in production you may use


npm install tcomb --save

Code example

A type-checked function:

import t from 'tcomb';
function sum(a, b) {
  return a + b;
sum(1, 's'); // throws '[tcomb] Invalid value "s" supplied to Number'
// using babel-plugin-tcomb
function sum(a: number, b: number) {
  return a + b;

A user defined type:

const Integer = t.refinement(t.Number, (n) => n % 1 === 0, 'Integer');

A type-checked class:

const Person = t.struct({
  name: t.String,              // required string
  surname: t.maybe(t.String),  // optional string
  age: t.Integer,                // required integer
  tags: t.list(t.String)       // a list of strings
}, 'Person');
// methods are defined as usual
Person.prototype.getFullName = function () {
  return `${} ${this.surname}`;
const person = Person({
  surname: 'Canti'
}); // throws '[tcomb] Invalid value undefined supplied to Person/name: String'

Chrome DevTools:





3KB gzipped, no dependencies.

Type safety

All models defined with tcomb are type-checked.

Note. Instances are not boxed, this means that tcomb works great with lodash, Ramda, etc. And you can of course use them as props to React components.

Based on set theory

Domain Driven Design

Write complex domain models in a breeze and with a small code footprint. Supported types / combinators:

  • user defined types
  • structs
  • lists
  • enums
  • refinements
  • unions
  • intersections
  • the option type
  • tuples
  • dictionaries
  • functions
  • recursive and mutually recursive types
  • interfaces

Immutability and immutability helpers

Instances are immutable using Object.freeze. This means you can use standard JavaScript objects and arrays. You don't have to change how you normally code. You can update an immutable instance with the provided update(instance, spec) function:

const person2 = Person.update(person, {
  name: { $set: 'Guido' }

where spec is an object containing commands. The following commands are compatible with the Facebook Immutability Helpers:

  • $push
  • $unshift
  • $splice
  • $set
  • $apply
  • $merge

See Updating immutable instances for details.


Object.freeze calls and asserts are executed only in development and stripped out in production (using process.env.NODE_ENV !== 'production' tests).

Runtime type introspection

All models are inspectable at runtime. You can read and reuse the information stored in your types (in the meta static member). See The meta object in the docs for details.

Libraries exploiting tcomb's RTI:

Easy JSON serialization / deserialization

Encodes / decodes your domain models to / from JSON for free.

Debugging with Chrome DevTools

You can customize the behavior when an assert fails leveraging the power of Chrome DevTools.

// use the default... = function fail(message) {
  throw new TypeError('[tcomb] ' + message); // set "Pause on exceptions" on the "Sources" panel for a great DX
// .. or define your own behavior = function fail(message) {

Pattern matching

const result = t.match(1,
  t.String, () => 'a string',
  t.Number, () => 'a number'
console.log(result); // => 'a number'

Babel plugin

Using babel-plugin-tcomb you can also write (Flow compatible) type annotations:

function sum(a: number, b: number): number {
  return a + b;

TypeScript definition file



How to Build a standalone bundle

git clone
cd tcomb
npm install
npm run dist

Will output 2 files:

  • dist/tcomb.js (development)
  • dist/tcomb.min.js (production) Object.freeze calls and asserts stripped out

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The MIT License (MIT)

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