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tcomb-builder is an immutable interface for tcomb-form

Tcomb builder is deprecated. We will review and accept reasonable PRs, but we are not actively developing this project.


npm install tcomb-builder tcomb-validation


  1. Immutability

  2. Replaces deeply nested options objects in your codebase

  3. Yields a standard tcomb type and options object


If you use tcomb-form, you probably have a lot of code that looks like this:

import tcomb from 'tcomb-form';

function validation(value) {
  return value.length > 0
      ? null
      : 'Please provide a value';

const FirstName = tcomb.refinement(

FirstName.getValidationErrorMessage = validation;

const options = {
  factory: function TextField(props)...,
  label: 'First Name',

// <tcomb.Form type={FirstName} options={options} />

This code has a few problems:

  1. Mutability makes the type hard to reuse.

    It is hard to reuse the FirstName type in another place that requires slightly modified validation, e.g. a max length restriction, because getValidationErrorMessage is set as a static function on the type.

  2. Mutability makes the options object hard to reuse.

    Creating another field, e.g. 'Last Name', would require copy-pasting or Object.assigning the options object.

  3. Nested options objects are hard to maintain.

    If the above field is placed inside a tcomb.struct, the options object is deeply nested. The more structs, the more nesting.

Thanks to its immutable design, tcomb-builder offers a cleaner syntax that encourages code-reuse.

import tcomb from 'tcomb-form';

import { BaseBuilder } from 'tcomb-builder';

function validation(value) {
  return value.length > 0
      ? null
      : 'Please provide a value';

const TextBuilder = new BaseBuilder()
  .setTypeAndValidate('Text', tcomb.String)
  .setValidation('LengthValidation', validation);

const firstName = TextBuilder.setLabel('First Name');

// <tcomb.Form type={firstName.getType()} options={firstName.getOptions()} />

This code is equivalent to the first example, with the following advantages:

  1. Immutability makes the type and options objects easy to reuse.

    Making a lastName field is as easy as TextBuilder.setLabel('Last Name'). Every .set method returns a new builder, so TextBuilder itself is never modified.

  2. Type and options objects are built together.

    When using structs, you don't have to worry about maintaining parallel type and options structures. tcomb-builder is one interface for all of tcomb, tcomb-validation, and tcomb-form. The parallel structures are maintained internally and accessed with the .getType and .getOptions methods.


There is thorough documentation within the docs folder.

Start with the documentation on what this library exports ( and the builder documentation (

Background Reading


Here is an example of a simple form. All primitives and widgets builders extend the BaseBuilder. Note that widgets are primitives that have a template set on them.

import { primitives, validators, widgets } from 'tcomb-builder';

const name = widgets.TextBuilder.setLabel('Name');

const dateOfBirth = widgets.TextBuilder
  .setLabel('Date of Birth');

const occupation = widgets.TextBuilder

const bananaStand = widgets.CheckboxBuilder
  .setLabel('Worked at banana stand');
const chickenDance = widgets.CheckboxBuilder
  .setLabel('Does chicken dance');
const hugeMistake = widgets.CheckboxBuilder
  .setLabel('Has made a huge mistake');
const none = widgets.CheckboxBuilder
  .setLabel('None of the above');

const noneKey = 'isNone';
const crossValidation = validators.combine([
  validators.checkbox.minMax({ min: 1, max: 2 }),

const foodGroup = widgets.CheckboxGroupBuilder
  .setValidation('SelectOneOrTwo', crossValidation)
  .setField('bananaStand', bananaStand)
  .setField('chickenDance', chickenDance)
  .setField('hugeMistake', hugeMistake)
  .setLabel('Character Traits')
  .setField(noneKey, none);

export default widgets.StructBuilder
  .setField('name', name)
  .setField('dateOfBirth', dateOfBirth)
  .setField('occupation', occupation)
  .setField('foodGroup', foodGroup)

Once exported, the page builder provides us with its tcomb type and options blob, which can be passed directly to tcomb-form without modification.

const type = InvestorInformationBuilder.getType();
const options = InvestorInformationBuilder.getOptions();
<tcomb.Form type={type} options={options} />

The end result, once tcomb template factories have been created for the underlying primitive types—text field, drop down, checkbox, and struct template primitives in this example—is that the form will render exactly as we expect it to.


Builders are composable, and this library uses three terms to describe them:

  1. Primitives: Builders that define a single input (e.g. a text field) or a structure (e.g. struct); primitive builders are found in the primitives folder.

  2. Widgets: Primitives that have a template provider already set on them. They are found in the widgets folder.

  3. Forms: Builders that define an entire page (e.g. Credit Card Information); examples are found in the examples folder

Setting Templates

When you first set up tcomb-builder, it is recommended to create an instance of LazyTemplateInterface, which defines a mapping of component types to templates. With this interface and the setLazyTemplateFactory function, your templates are defined in one place (the LazyTemplateInterface), which makes refactoring easier.

If you have a Node backend, you can use the same builders on both the client and server. The builders on the server will never realize the lazy templates, so you don't have to require React as a dependency on the server. See the API docs for more details.