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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)


    npm i tcomb

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