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1.12.1 • Public • Published

Tiny Types

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TinyTypes is an npm module that makes it easy for TypeScript and JavaScript projects to give domain meaning to primitive types. It also helps to avoid all sorts of bugs and makes your code easier to refactor. Learn more.

API Docs

API documentation is available at


To install the module from npm:

npm install --save tiny-types

Defining Tiny Types

An int on its own is just a scalar with no meaning. With an object, even a small one, you are giving both the compiler and the programmer additional information about what the value is and why it is being used.

Jeff Bay, Object Calisthenics

Single-value types

To define a single-value TinyType - extend from TinyTypeOf<T>():

import { TinyTypeOf } from 'tiny-types';
class FirstName extends TinyTypeOf<string>() {}
class LastName  extends TinyTypeOf<string>() {}
class Age       extends TinyTypeOf<number>() {}

Every tiny type defined this way has a readonly property value of type T, which you can use to access the wrapped primitive value. For example:

const firstName = new FirstName('Jan');
firstName.value === 'Jan';


Each tiny type object has an equals method, which you can use to compare it by value:

    name1 = new FirstName('Jan'),
    name2 = new FirstName('Jan');
name1.equals(name2) === true; 


An additional feature of tiny types is a built-in toString() method:

const name = new FirstName('Jan');
name.toString() === 'FirstName(value=Jan)';

Which you can override if you want to:

class Timestamp extends TinyTypeOf<Date>() {
    toString() {
        return `Timestamp(value=${this.value.toISOString()})`;
const timestamp = new Timestamp(new Date());
timestampt.toString() === 'Timestamp(value=2018-03-12T00:30:00.000Z))'

Multi-value and complex types

If the tiny type you want to model has more than one value, or you want to perform additional operations in the constructor, extend from TinyType directly:

import { TinyType } from 'tiny-types';
class Person extends TinyType {
    constructor(public readonly firstName: FirstName,
                public readonly lastName: LastName,
    ) {

You can also mix and match both of the above definition styles:

import { TinyType, TinyTypeOf } from 'tiny-types';
class UserName extends TinyTypeOf<string>() {}
class Timestamp extends TinyTypeOf<Date>() {
    toString() {
        return `Timestamp(value=${this.value.toISOString()})`;
abstract class DomainEvent extends TinyTypeOf<Timestamp>() {}
class AccountCreated extends DomainEvent {
    constructor(public readonly username: UserName, timestamp: Timestamp) {
const event = new AccountCreated(new UserName('jan-molak'), new Timestamp(new Date()));

Even such complex types still have both the equals and toString methods:

    now = new Date(2018, 2, 12, 0, 30),
    event1 = new AccountCreated(new UserName('jan-molak'), new Timestamp(now)),
    event2 = new AccountCreated(new UserName('jan-molak'), new Timestamp(now));
event1.equals(event2) === true;
event1.toString() === 'AccountCreated(username=UserName(value=jan-molak), value=Timestamp(value=2018-03-12T00:30:00.000Z))'

Serialisation to JSON

Every TinyType defines a toJSON() method, which returns a JSON representation of the object. This means that you can use TinyTypes as Data Transfer Objects.

Single-value TinyTypes are serialised to the value itself:

import { TinyTypeOf } from 'tiny-types';
class FirstName extends TinyTypeOf<string>() {}
const firstName = new FirstName('Jan');
firstName.toJSON() === 'Jan'

Complex TinyTypes are serialised recursively:

import { TinyType, TinyTypeOf } from 'tiny-types';
class FirstName extends TinyTypeOf<string>() {}
class LastName extends TinyTypeOf<string>() {}
class Age extends TinyTypeOf<number>() {}
class Person extends TinyType {
        public readonly firstName: FirstName,
        public readonly lastName: LastName,
        public readonly age: Age,
    ) {
const person = new Person(new FirstName('Bruce'), new LastName('Smith'), new Age(55));
person.toJSON() === { firstName: 'Bruce', lastName: 'Smith', age: 55 }

De-serialisation from JSON

Although you could define standalone de-serialisers, I like to define them as static factory methods on the TinyTypes themselves:

import { TinyTypeOf } from 'tiny-types';
class FirstName extends TinyTypeOf<string>() {
    static fromJSON = (v: string) => new FirstName(v);
const firstName = new FirstName('Jan'),
FirstName.fromJSON(firstName.toJSON()).equals(firstName) === true

When working with complex TinyTypes, you can use the (experimental) Serialised interface to reduce the likelihood of your custom fromJSON method being incompatible with toJSON:

import { TinyTypeOf, TinyType, Serialised } from 'tiny-types';
class EmployeeId extends TinyTypeOf<number>() {
    static fromJSON = (id: number) => new EmployeeId(id);
class DepartmentId extends TinyTypeOf<string>() {
    static fromJSON = (id: string) => new DepartmentId(id);
class Allocation extends TinyType {
    static fromJSON = (o: Serialised<Allocation>) => new Allocation(
        EmployeeId.fromJSON(o.employeeId as number),
        DepartmentId.fromJSON(o.departmentId as string),
    constructor(public readonly employeeId: EmployeeId, public readonly departmentId: DepartmentId) {

This way de-serialising a complex type becomes trivial:

const allocation = new Allocation(new EmployeeId(1), new DepartmentId('engineering'));
const deserialised = Allocation.fromJSON({ departmentId: 'engineering', employeeId: 1 });
allocation.equals(deserialised) === true

Although Serialised is by no means 100% foolproof as it's only limited to checking whether your input JSON has the same fields as the object you're trying to de-serialise, it can at least help you to avoid errors caused by typos.

Your feedback matters!

Do you find TinyTypes useful? Give it a star!

Found a bug? Need a feature? Raise an issue or submit a pull request.

Have feedback? Let me know on twitter: @JanMolak


TinyTypes library is licensed under the Apache-2.0 license.

- Copyright © 2018- Jan Molak


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