<h1> <img src="./packages/microstates/README/microstates-logo.svg" alt="Microstates Logo" width="200" /><br>Microstates </h1>

Microstates makes working with pure functions over immutable data
feel like working with the classic, mutable models we all know and love.

<details> <summary><strong>Table of Contents</strong></summary>

Features
Why Microstates?
- M in MVC
- Functional Models
What is a Microstate?
Types and Type Composition
Transitions
State Machines
Framework Integrations
microstates npm package
Streaming State
The Vision of Microstates
FAQ
- What if I can't use class syntax?
- What if I can't use Class Properties?
Run Tests

</details> <details> <summary>🎬 <strong>Videos</strong></summary>

Building a shopping cart with Microstates.js - The Frontside YouTube - Taras Mankovski & Alex Regan
State of Enjoyment at Framework Summit 2018 - Charles Lowell
Microstates: The Ember component of state management at EmberATX
Composable State Primitives for JavaScript with Charles Lowell & Taras Mankovski - Devchat.tv
Composable State Management with Microstates.js at Toronto.js - Taras Mankovski

</details> <details> <summary>💬 <strong>Chat</strong></summary>

Join our community on Discord. Everyone is welcome. If you're a new to programming join our #beginner channel where extra care is taken to support those who're just getting started.

</details>

Features

With Microstates added to your project, you get:

🍇 Composable type system
🍱 Reusable state atoms
💎 Pure immutable state transitions without writing reducers
⚡️ Lazy and synchronous out of the box
🦋 Most elegant way to express state machines
🎯 Transpilation free type system
🔭 Optional integration with Observables
⚛ Use in Node.js, browser or React Native
🔬 It's tiny

But, most importantly, Microstates makes working with state fun.

When was the last time you had fun working with state?

For many, the answer is probably never, because state management in JavaScript is an endless game of compromises. You can choose to go fully immutable and write endless reducers. You can go mutable and everything becomes an observable. Or you can setState and lose the benefits of serialization and time travel debugging.

Unlike the view layer, where most frameworks agree on some variation of React's concept of components, none of the current crop of state management tools strike the same balance that React components introduced to the API.

React components have a tiny API. They are functional, simple and extremely reusable. The tiny API gives you high productivity for little necessary knowledge. Functional components are predictable and easy to reason about. They are conceptually simple, but simplicity hides an underlying architecture geared for performance. Their simplicity, predictability and isolation makes them composable and reusable.

These factors combined are what make React style components easy to work with and ultimately fun to write. A Tiny API abstracting a sophisticated architecture that delivers performance and is equally useful on small and big projects is the outcome that we set out to achieve for state management with Microstates.

It's not easy to find the right balance between simplicity and power, but considering the importance of state management in web applications, we believe it's a worthy challenge. Checkout the Vision of Microstates section if you're interested in learning more about where we're going.

What is a Microstate?

A Microstate is just an object that is created from a value and a type. The value is just data, and the type is what defines how you can transition that data from one form into the next. Unlike normal JavaScript objects, microstates are 100% immutable and cannot be changed. They can only derive new immutable microstates through one of their type's transitions.

Types and Type Composition

Microstates comes out of the box with 5 primitive types: Boolean, Number, String, Object and Array.

js
import { create, valueOf } from 'microstates';

let meaningOfLifeAndEverything = create(Number, 42);
console.log(meaningOfLifeAndEverything.state);
//> 42

let greeting = create(String, 'Hello World');
console.log(greeting.state);
//> Hello World

let isOpen = create(Boolean, true);
console.log(isOpen.state);
//> true

// For Object and Array use microstates valueOf method
let foo = create(Object, { foo: 'bar' });
console.log(valueOf(foo));
//> { foo: 'bar' }

let numbers = create(Array, [1, 2, 3, 4]);
console.log(valueOf(numbers));
//> [ 1, 2, 3, 4 ]

Apart from these basic types, every other type in Microstates is built by combining other types. So for example, to create a Person type you could define a JavaScript class with two properties: name which has type String and age which has type Number.

js
class Person {
  name = String;
  age = Number;
}

Once you have a type, you can use that type to create as many people as your application requires:

js
import { create } from 'microstates';

let person = create(Person, { name: 'Homer', age: 39 });

Every microstate created with a type of Person will be an object
extending Person to have a set() method:

txt
+----------------------+
|                      |       +--------------------+
|  Microstate<Person>  +-name--+                    +-concat()->
|                      |       | Microstate<String> +-set()->
|                      |       |                    +-state: 'Homer'
|                      |       +--------------------+
|                      |
|                      |       +--------------------+
|                      +-age---+                    +-increment()->
|                      |       | Microstate<Number> +-decrement()->
|                      |       |                    +-set()->
|                      |       |                    +-state: 39
|                      |       +--------------------+
|                      |
|                      +-set()->
|                      |
+----------------------+

For the five built in types, Microstates automatically gives you transitions that you can use to change their value. You don't have to write any code to handle common operations.

Creating your own microstates

Types can be combined with other types freely and Microstates will take care of handling the transitions for you. This makes it possible to build complex data structures that accurately describe your domain.

Let's define another type that uses the person type.

js
class Car {
  designer = Person;
  name = String;
}

let theHomerCar = create(Car, {
  designer: { name: 'Homer', age: 39 },
  name: 'The Homer'
});

theHomerCar object will have the following shape,

txt
+-------------------+           +----------------------+
|                   |           |                      |       +--------------------+
|  Microstate<Car>  |           |  Microstate<Person>  +-name--+                    +-concat()->
|                   |           |                      |       | Microstate<String> +-set()->
|                   +-designer--+                      |       |                    +-state: 'Homer'
|                   |           |                      |       +--------------------+
|                   |           |                      |
|                   |           |                      |       +--------------------+
|                   |           |                      +-age---+                    +-increment()->
|                   |           |                      |       | Microstate<Number> +-decrement()->
|                   |           |                      |       |                    +-set()->
|                   |           |                      |       |                    +-state: 39
|                   |           |                      |       +--------------------+
|                   |           |                      |
|                   |           |                      +-set()->
|                   |           |                      |
|                   |           +----------------------+
|                   |
|                   |           +--------------------+
|                   +-name------+                    +-concat()->
|                   |           | Microstate<String> +-set()->
|                   |           |                    +-state: 'The Homer'
|                   |           +--------------------+
|                   |
|                   +-set()->
|                   |
+-------------------+

You can use the object dot notation to access sub microstates. Using the same example from above:

js
theHomerCar.designer;
//> Microstate<Person>{ name: Microstate<String>'Homer', age: Microstate<Number>39 }

theHomerCar.designer.age.state;
//> 39

theHomerCar.name.state;
//> The Homer

You can use the valueOf() function available from the microstates
module to retrieve the underlying value represented by a microstate.

js
import { valueOf } from 'microstates';

valueOf(theHomerCar);
//> { designer: { name: 'Homer', age: 39 }, name: 'The Homer' }

Array Microstates

Quite often it is helpful to describe your data as a collection of types. For example, a blog might have an array of posts. To do this, you can use the array of type notation [Post]. This signals that Microstates of this type represent an array whose members are each of the Post type.

js
class Blog {
  posts = [Post];
}

class Post {
  id = Number;
  title = String;
}

let blog = create(Blog, {
  posts: [
    { id: 1, title: 'Hello World' },
    { id: 2, title: 'Most fascinating blog in the world' }
  ]
});

for (let post of blog.posts) {
  console.log(post);
}
//> Microstate<Post>{ id: 1, title: 'Hello World' }
//> Microstate<Post>{ id: 2, title: 'Most fascinating blog in the world' }

When you're working with an array microstate, the shape of the Microstate is determined by the value. In this case, posts is created with two items which will, in turn, create a Microstate with two items. Each item will be a Microstate of type Post. If you push another item onto the posts Microstate, it'll be treated as a Post.

js
let blog2 = blog.posts.push({ id: 3, title: 'It is only getter better' });

for (let post of blog2.posts) {
  console.log(post);
}

//> Microstate<Post>{ id: 1, title: 'Hello World' }
//> Microstate<Post>{ id: 2, title: 'Most fascinating blog in the world' }
//> Microstate<Post>{ id: 3, title: 'It is only getter better' }

Notice how we didn't have to do any extra work to define the state
transition of adding another post to the list? That's the power of
composition!

Object Microstates

You can also create an object microstate with {Post}. The difference is that the collection is treated as an object. This can be helpful when creating normalized data stores.

js
class Blog {
  posts = { Post };
}

class Post {
  id = Number;
  title = String;
}

let blog = create(Blog, {
  posts: {
    '1': { id: 1, title: 'Hello World' },
    '2': { id: 2, title: 'Most fascinating blog in the world' }
  }
});

blog.posts.entries['1'];
//> Microstate<Post>{ id: 1, title: 'Hello World' }

blog.posts.entries['2'];
//> Microstate<Post>{ id: 2, title: 'Most fascinating blog in the world' }

Object type microstates have Object transitions, such as assign, put and delete.

js
let blog2 = blog.posts.put('3', { id: 3, title: 'It is only getter better' });

blog2.posts.entries['3'];
//> Microstate<Post>{ id: 3, title: 'It is only getter better' }

Transitions

Transitions are the operations that let you derive a new state from an existing state. All transitions return another Microstate. You can use state charts to visualize microstates. For example, the Boolean type can be described with the following statechart.

The Boolean type has a toggle transition which takes no arguments and creates a new microstate with the state that is opposite of the current state.

Here is what this looks like with Microstates.

js
import { create } from 'microstates';

let bool = create(Boolean, false);

bool.state;
//> false

let inverse = bool.toggle();

inverse.state;
//> true

Pro tip Remember, Microstate transitions always return a Microstate. This is true both inside and outside the transition function. Using this convention can allow composition to reach crazy levels of complexity.

Let's use a Boolean in another type and see what happens.

js
class App {
  name = String;
  notification = Modal;
}

class Modal {
  text = String;
  isOpen = Boolean;
}

let app = create(App, {
  name: 'Welcome to your app',
  notification: {
    text: 'Hello there',
    isOpen: false
  }
});

let opened = app.notification.isOpen.toggle();
//> Microstate<App>

valueOf(opened);
//> {
// name: 'Welcome to your app',
// notification: {
//   text: 'Hello there',
//   isOpen: true
// }}

Microstate transitions always return the whole object. Notice how we invoked the boolean transition app.notification.isOpen, but we didn't get a new Boolean microstate? Instead, we got a completely new App where everything was the same except for that single toggled value.

Transitions for built-in types

The primitive types have predefined transitions:

Boolean
- toggle(): Microstate - return a Microstate with opposite boolean value
String
- concat(str: String): Microstate - return a Microstate with str added to the end of the current value
Number
- increment(step = 1: Number): Microstate - return a Microstate with number increased by step, default is 1.
- decrement(step = 1: Number): Microstate - return a Microstate with number decreased by step, default is 1.
Object
- assign(object): Microstate - return a Microstate after merging object into current object.
- put(key: String, value: Any): Microstate - return a Microstate after adding value at given key.
- delete(key: String): Microstate - return a Microstate after removing property at given key.
Array
- map(fn: (Microstate) => Microstate): Microstate - return a Microstate with mapping function applied to each element in the array. For each element, the mapping function will receive the microstate for that element. Any transitions performed in the mapping function will be included in the final result.
- push(value: any): Microstate - return a Microstate with value added to the end of the array.
- pop(): Microstate - return a Microstate with last element removed from the array.
- shift(): Microstate - return a Microstate with element removed from the array.
- unshift(value: any): Microstate - return a Microstate with value added to the beginning of the array.
- filter(fn: state => boolean): Microstate - return a Microstate with filtered array. The predicate function will receive state of each element in the array. If you return a falsy value from the predicate, the item will be excluded from the returned microstate.
- clear(): Microstate - return a microstate with an empty array.

Many transitions on primitive types are similar to methods on original classes. The biggest difference is that transitions always return Microstates.

Type transitions

Define the transitions for your types using methods. Inside of a transition, you can invoke any transitions you like on sub microstates.

js
import { create } from 'microstates';

class Person {
  name = String;
  age = Number;

  changeName(name) {
    return this.name.set(name);
  }
}

let homer = create(Person, { name: 'Homer', age: 39 });

let lisa = homer.changeName('Lisa');

Chaining transitions

Transitions can be composed out of any number of subtransitions. This is often referred to as "batch transitions" or "transactions". Let's say that when we authenticate a session, we need to both store the token and indicate that the user is now authenticated. To do this, we can chain transitions. The result of the last operation will become a new microstate.

js
class Session {
  token = String;
}

class Authentication {
  session = Session;
  isAuthenticated = Boolean;

  authenticate(token) {
    return this.session.token.set(token).isAuthenticated.set(true);
  }
}

class App {
  authentication = Authentication;
}

let app = create(App, { authentication: {} });

let authenticated = app.authentication.authenticate('SECRET');

valueOf(authenticated);
//> { authentication: { session: { token: 'SECRET' }, isAuthenticated: true } }

The `initialize` transition

Just as every state machine must begin life in its "start state", so too must every microstate begin life in the right state. For those cases where the start state depends on some logic, there is the initialize transition. The initialize transition is just like any other transition, except that it will be automatically called within create on every Microstate that declares one.

You can even use this mechanism to transition the microstate to one with a completely different type and value.

For example:

js
class Person {
  firstName = String;
  lastName = String;

  initialize({ firstname, lastname } = {}) {
    let initialized = this;

    if (firstname) {
      initialized = initialized.firstName.set(firstname);
    }

    if (lastname) {
      initialized = initialized.lastName.set(lastname);
    }

    return initialized;
  }
}

The `set` transition

The set transition is the only transition that is available on all types. It can be used to replace the value of the current Microstate with another value.

js
import { create } from 'microstates';

let number = create(Number, 42).set(43);

number.state;
//> 43

Pro tip: Microstates will never require you to understand Monads in order to use transitions, but if you're interested in learning about the primitives of functional programming that power Microstates, you may want to checkout funcadelic.js.

Transition scope

Microstates are composable, and they work exactly the same no matter what other microstate they're a part of. For this reason, Microstate transitions only have access to their own transitions and the transitions of the microstates they contain. What they do not have is access to their context. This is similar to how components work. The parent component can render children and pass data to them, but the child components do not have direct access to the parent component. The same principle applies in Microstates, so as a result, it benefits from the same advantages of isolation and composability that make components awesome.

State Machines

A state machine is a system that has a predefined set of states. At any given point, the state machine can only be in one of these states. Each state has a predefined set of transitions that can be derived from that state. These constraints are beneficial to application architecture because they provide a way to identify application state and suggest how the application state can change.

From its conception, Microstates was created to be the most convenient way to express state machines. The goal was to design an API that would eliminate the barrier of using state machines and allow for them to be composable. After almost two years of refinement, the result is an API that has evolved significantly beyond what we typically associate with code that expresses state machines.

xstate for example, is a great specimen of the classic state machine API. It's a fantastic library and it addresses the very real need for state machines and statecharts in modern applications. For purposes of contrast, we'll use it to illustrate the API choices that went into Microstates.

Explicit Transitions

Most state machine libraries focus on finding the next state given a configuration. For example, this xstate declaration describes what state id to match when in a specific state.

js
import { Machine } from 'xstate';

const lightMachine = Machine({
  key: 'light',
  initial: 'green',
  states: {
    green: {
      on: {
        TIMER: 'yellow'
      }
    },
    yellow: {
      on: {
        TIMER: 'red'
      }
    },
    red: {
      on: {
        TIMER: 'green'
      }
    }
  }
});

Microstates does not do any special state resolution. You explicitly declare what happens on a state transition. Here is what a similar state machine looks like in Microstates.

js
class LightMachine {
  color = String;

  initialize({ color = 'green' } = {}) {
    return this.color.set(color);
  }

  timer() {
    switch (this.color.state) {
      case 'green': return this.color.set('yellow');
      case 'yellow': return this.color.set('red');
      case 'red':
      default:
        return this.color.set('green');
    }
  }
}

With Microstates, you explicitly describe what happens on transition and define the matching mechanism.

Transition methods

transitionTo is often used by state machine libraries to trigger state transition. Here is an example with xstate library,

js
const nextState = lightMachine.transition('green', 'TIMER').value;

//> 'yellow'

Microstates does not have such a method. Instead, it relies on vanilla JavaScript property lookup. The method invocation is equivalent to calling transitionTo with name of the transition.

js
import { create } from 'microstates';

let lightMachine = create(LightMachine);

const nextState = lightMachine.timer();

nextState.color.state;
//> 'yellow'

Immutable Object vs Immutable Data Structure

When you create a state machine with xstate, you create an immutable object. When you invoke a transition on an xstate state machine, the value of the object is the ID of the next state. All of the concerns of immutable value change as a result of state change are left for you to handle manually.

Microstates treats value as part of the state machine. It allows you to colocate your state transitions with reducers that change the value of the state.

Framework Integrations

React.js
Ember.js
Create a PR if you created an integration that you'd like to add to this list.
Create an issue if you'd like help integrating Microstates with a framework

`microstates` npm package

The microstates package provides the Microstate class and functions that operate on Microstate objects.

You can import the microstates package using:

bash
npm install microstates

# or

yarn add microstates

Then import the libraries using:

js
import Microstate, { create, from, map } from 'microstates';

create(Type, value): Microstate

The create function is conceptually similar to Object.create. It creates a Microstate object from a type class and a value. This function is lazy, so it should be safe in most high performant operations even with complex and deeply nested data structures.

js
import { create } from 'microstates';

create(Number, 42);
//> Microstate

from(any): Microstate

from allows the conversion of any POJO (plain JavaScript object) into a Microstate. Once you've created a Microstate, you can perform operations on all properties of the value.

js
import { from } from 'microstates';

from('hello world');
//Microstate<String>

from(42).increment();
//> Microstate<Number>

from(true).toggle();
//> Microstate<Boolean>

from([1, 2, 3]);
//> Microstate<Array<Number>>

from({ hello: 'world' });
//> Microstate<Object>

from is lazy, so you can consume any deeply nested POJO and
Microstates will allow you to perform transitions with it. The cost of
building the objects inside of Microstates is paid whenever you reach
for a Microstate inside. For example, let o = from({ a: { b: { c: 42 }}}) doesn't do anything until you start to read the properties with
dot notiation like o.entries or with iteration / destructuring.

js
let [[[[[[c]]]]]] = from({a: { b: {c: 42}}});
valueOf(c.increment());

// { a: { b: { c: 43 }}}

let greeting = from({ hello: ['world']});
let [ world ] = greeting.entries.hello;
valueOf(world.concat('!!!'));
// { hello: [ 'world!!!' ]}

map(microstate, fn): Microstate

The map function invokes the function for each microstate in an array microstate. It is usually used to map over an array of microstates and return an array of components. The mapping function will receive each microstate in the array. You can invoke transitions on each microstate as you would usually.

js
let numbers = create([Number], [1, 2, 3, 4]);

<ul>
  {map(numbers, number => (
    <li onClick={() => number.increment()}>{number.state}</li>
  ))}
</ul>;

Streaming State

A microstate represents a single immutable value with transitions to derive the next value. Microstates provides a mechanism called Identity that allows to emit stream of Microstates. When you create an Identity from a Microstate, you get an object that has the same shape the original microstate. Every composed microstate becomes an identity and every transition gets wrapped in side effect emitting behaviour specific to the identity's constructor. Identity wrapped Microstate offer the following benefits over raw Microstates.

A common performance optimization used by all reactive engines is to prevent re-renders for components who’s props have not changed. The most efficient way to determine if a value has not changed it to perform an exact equality check, for example: prevValue === currentValue. If the reference is the same, then consider the value unchanged. The Identity makes this possible with Microstates by internally managing how the Identity is constructed as a result of a transition. It will automatically determine which branches of microstates are unchanged and reuse previous identities for those branches.

Memoized Getters

Microstates are immutable which makes it safe for us to memoize computations that are derived off their state. Identies will automatically memoize getters and return previously computed value when the microstate backing the identity has not changed. When a transition is invoked on the identity, the part of the identity tree that are changed will be re-created effectively invalidating the cache for the changed parts of the identity. The getters will be recomputed for state that is changed.

Debounce no-op transitions

Identities automatically prevent unnecessary re-renders by debouncing transitions that do not change the value. This eliminates the need for shouldComponentUpdate hooks for pure components because it is safe to assume that a component that is re-rendering is re-rendering as a result of a transition that changed the value. In other words, if the current state and the state being transitioned to are the same, then a "new state" that would be the same is not emitted.

js
let id = Store(from({ name: 'Charles' }), next => {
  console.count('changed');
  id = next;
});

id.name.set('Charles');
id.name.set('Charles');

The above transitions would be debounced because they do not change the value. The update callback would not be called, even though set operation is called twice.

Identity Constructors

Microstates comes with two Identity constructors: Store and Observable. Store will send next identity to a callback. Observable will create a stream of identities and send next identity through the stream.

Store(microstate, callback)

Store identity constructor takes two arguments: microstate and a callback. It returns an identity. When a transition is invoked on the identity, the callback will receive the next identity.

js
import { Store, from, valueOf } from 'microstates';

let initial = create(Number, 42);

let last;

last = Store(initial, next => (last = next));

last.increment();
//> undefined
// callback will be invoked syncronously on transition

// last here will reference the last
last.increment();
//> undefined

valueOf(last);
//> 44

The same mechanism can be used with React or any other reactive environment.

js
import React from 'react';
import { Store, create } from 'microstates';

class Counter extends React.Component {
  state = {
    last: Store(create(Number, 42), next => this.setState({ last: next }))
  };
  render() {
    let { last } = this.state;
    return (
      <button onClick={() => last.increment()}>Increment {last.state}</button>
    );
  }
}

Observable.from(microstate)

Microstates provides an easy way to convert a Microstate which represents a single value into a Observable stream of values. This is done by passing a Microstate to Observable.from function. This function will return a Observable object with a subscribe method. You can subscribe to the stream by passing an observer to the subscribe function. Once you subscribe, you will synchronously receive a microstate with middleware installed that will cause the result of transitions to be pushed through the stream.

You should be able to use to any implementation of Observables that supports Observer.from using symbol-observable. We'll use RxJS for our example.

js
import { from as observableFrom } from 'rxjs';
import { create } from 'microstates';

let homer = create(Person, { firstName: 'Homer', lastName: 'Simpson' });

let observable = observableFrom(homer);

let last;
let subscription = observable.subscribe(next => {
  // capture the next microstate coming through the stream
  last = next;
});

last.firstName.set('Homer J');

valueOf(last);
//> { firstName: 'Homer J', lastName: 'Simpson' }

The Vision of Microstates

What if switching frameworks were easy? What if a company could build domain specific code that worked across frameworks? Imagine what it would be like if your tools stayed with you as you progressed in your career as an engineer. This is the world that we hope to create with Microstates.

Shared Solutions

Imagine never having to write another normalized data store again because someone made a normalized data store Microstate that you can compose into your app's Microstate.

In the future (not currently implemented), you will be able to write a normalized data store like this,

js
import Normalized from 'future-normalized-microstate';

class MyApp {
  store = Normalized.of(Product, User, Category);
}

The knowledge about building normalized data stores is available in libraries like Ember Data, Orbit.js, Apollo and urql, yet many companies end up rolling their own because these tools are coupled to other stacks.

As time and resources permit, we hope to create a solution that will be flexible enough for use in most applications. If you're interested in helping us with this, please reach out.

Added Flexibility

Imagine if your favourite Calendar component came with a Microstate that allowed you to customize the logic of the calendar without touching the rendered output. It might looks something like this,

js
import Calendar from 'awesome-calendar';
import { filter } from 'microstates';

class MyCalendar extends Calendar.Model {
  // make days as events
  days = Day.of([Event]);

  // component renders days from this property
  get visibleDays() {
    return filter(this.days, day => day.state.status !== 'finished');
  }
}

<Calendar.Component model={MyCalendar} />;

Currently, this is pseudocode, but Microstates was architected to allow for these kinds of solutions.

Framework Agnostic Solutions

Competition moves our industry forward but consensus builds ecosystems.

Unfortunately, when it comes to the M(odel) of the MVC pattern, we are seeing neither competition nor consensus. Every framework has its own model layer that is not compatible with others. This makes it difficult to create truly portable solutions that can be used on all frameworks.

It creates lock-in that is detrimental to the businesses that use these frameworks and to the developers who are forced to make career altering decisions before they fully understand their choices.

We don't expect everyone to agree that Microstates is the right solution, but we would like to start the conversation about what a shared primitive for state management in JavaScript might look like. Microstates is our proposal.

In many ways, Microstates is a beginning. We hope you'll join us for the ride and help us create a future where building stateful applications in JavaScript is much easier than it is today.

FAQ

What if I can't use class syntax?

Classes are functions in JavaScript, so you should be able to use a function to do most of the same things as you would with classes.

js
class Person {
  name = String;
  age = Number;
}

☝️ is equivalent to 👇

js
function Person() {
  this.name = String;
  this.age = Number;
}

What if I can't use Class Properties?

Babel compiles Class Properties into class constructors. If you can't use Class Properties, then you
can try the following.

js
class Person {
  constructor() {
    this.name = String;
    this.age = Number;
  }
}

class Employee extends Person {
  constructor() {
    super();
    this.boss = Person;
  }
}