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⚡️ 1.7KB full-featured state management inspired by MobX and Solid.js, batteries included ⚡️

Onek version Onek brotli size Onek license

Onek (reads as one-kay or on-ek) is a simple but powerful state management library for React based on a solid foundation of functional reactive data structures from MobX and Solid.js, providing everything needed for managing state in complex React applications, all in a less than 2KB package.


  • 🚀 Reactive observable and computed values - just like MobX, Solid.js and Preact Signals
  • 🎭 Both MobX and Solid.js flavors - choose what you like
  • 🤔 Not opinionated about the structure of your models
  • 👁 Transparency - no data glitches guaranteed
  • 💧 No memory leaks - optimal computed caching without compromises
  • 🧩 Single hook - effortless integration with React components
  • 🤓 Built-in shallow equality - easily optimize re-renders
  • 💾 Compatibility - only ES6 Set and Map are required
  • 💯 100% test coverage with complex cases
  • ⭐️ Written in 100% TypeScript
  • 📦 ...and all in a less than 2KB package

Table of contents


yarn add onek

npm install --save onek


Note: in this section Solid.js flavor will be used. If you want examples for MobX flavor, check out the MobX flavor section.

Observable values

observable is a simple function that accepts an initial value and returns a tuple of getter and setter functions - the same convention as useState from React:

import { observable } from "onek";

// create observable value
const [greeting, setGreeting] = observable("hello!");

// set value directly

// set value with updater function
setGreeting((oldGreeting) => oldGreeting + "!!!");

greeting() === "hola!!!!";
Extra: equality check argument

The second argument to observable might be an equality check function (or true for the built-in shallowEquals implementation):

import { shallowEquals } from "onek";

const [number, setNumber] = observable(1, true);
// or equivalently
const [number, setNumber] = observable(1, shallowEquals);

setNumber(1); // no updates to dependant computeds and reactions
Extra: storing functions in observable

In order to store a function in an observable you need to pass true as the second argument to the setter function. This argument means the setter should store the first argument as-is, without its interpretation as an updater function:

const [callback, setCallback] = observable(() => console.log("hello!"));

setCallback(() => console.log("hola!"), true); // stores callback as is

Computed values

A computed value is like useMemo in React - it's cached and returns the cached value afterwards. All accessed observable or other computed values are automatically tracked, there is no need to specify a dependency list. Changes to these tracked values automatically invalidate the cached value, which is recalculated on next access to the computed:

import { computed } from "onek";

const loudGreeting = computed(() => greeting().toUpperCase());

loudGreeting() === "HOLA!!!!";


loudGreeting() === "HI!";
Extra: equality check argument

The second argument to computed is also an equality check function (or true for the built-in implementation):

const [numbers, setNumbers] = observable([1, 2, 3, 4]);

const sortedNumbers = computed(() => numbers().slice().sort(), true);

const result = sortedNumbers();

console.log(result); // [1,2,3,4]

setNumbers([4, 3, 2, 1]);

sortedNumbers() === result; // result is referrentially the same

Using with React

Using observable and computed in React components is as simple as:

import { computed, observable, useObserver } from "onek";

const [greeting, setGreeting] = observable("hello!");

const loudGreeting = computed(() => greeting().toUpperCase());

const LoudGreeting = () => {
  const observer = useObserver();

  return observer(() => <p>{loudGreeting()}</p>);

const GreetingInput = () => {
  const observer = useObserver();

  return observer(() => (
      onChange={(e) => setGreeting(}

    <GreetingInput />
    <LoudGreeting />

useObserver hook has no arguments and returns an observer function. You can wrap your component code with it or pass it to observable and computed getters in order to get the component update on their changes. Reading observable values outside of the observer fn or without passing it to getters won't subscribe the component to changes:

const [value, setValue] = observable(1);

const Component = () => {
  const observer = useObserver();

  observer(() => value()); // component will rerender on value change
  observer(value); // correct, will rerender as well
  value(observer); // also corrrect
  value(); // no rerender on value change

Actions and transactions

Actions automatically batch updates to observable values, and also make access to observable getters untracked - so if your action is called inside a component's render function or inside a reaction it won't make it re-render on accessed values change.

Important note: by default all changes to observable values are batched until the end of the current microtask. In order to run reactions synchronously on transaction end, please read the Changing reaction scheduler section.

const [x, setX] = observable(1);
const [y, setY] = observable(2);

const updateValues = action((value) => {
  const xValue = x(); // access to x is not tracked by reaction or component

  setX(0); // these two updates are batched,
  setY(xValue + value); // so components will see updated values at once


A transaction is the same, except it's executed immediately and doesn't make values access untracked:

import { tx } from "onek";

tx(() => {

To get the same behavior as action use utx (Untracked transaction) instead:

const result = utx(() => {

  return x() + y(); // access is untracked

Async operations

Just define an action with async function:

const [data, setData] = observable(null);
const [fetching, setFetching] = observable(false);
const [error, setError] = observable(null);

const fetchData = action(async () => {
  try {
    const responseData = await axios.get("url");
  } catch (err) {
  } finally {

await fetchData();

By default, Onek uses a microtask scheduler for reactions, so updates to observables are batched until the current microtask end. This means both data and fetching values will be consistent when any side effects run.

Extra: async operations for synchronous scheduler

You can configure Onek to use synchronous reaction scheduler that will execute side effects synchronously after each transaction end. In this case you need to use action for promise handlers or utx for code blocks in async function, i.e.:

const fetchData = action(() => {

  return axios
      action((data) => {
      action((err) => {

or with async functions:

const fetchData = action(async () => {

  try {
    const data = await axios.fetch("url");
    utx(() => {
  } catch (err) {
    utx(() => {


A reaction is a way to react to observable or computed changes without involving React. It's the same as the autorun function from MobX:

import { reaction } from "onek";

// will print "Greeting is HOLA!!!!"
const disposer = reaction(() => {
  console.log("Greeting is " + greeting());

setGreeting("Привет!"); // prints "Greeting is Привет!"


setGreeting("Hello!"); // doesn't print anymore; // prints "Greeting is Hello!" again
Extra: reaction destructor

The return value of the reaction body might be a reaction destructor - a function that is called before each reaction run and on disposer call:

const [topic, setTopic] = observable("something");

const disposer = reaction(() => {
  const currentTopic = topic();

  subscribeToTopic(currentTopic, callback);

  return () => {
    unsubscribeFromTopic(currentTopic, callback);

setTopic("different"); // calls destructor function before executing reaction

disposer(); // unsubscribes from topic and won't run anymore

Making models

Solid.js flavor

To compose your observable and computed values into a single model, you can use the following pattern:

function makeModel(initialValue) {
  const [value, setValue] = observable(initialValue);
  const doubleValue = computed(() => value() * 2);

  return {

A downside of this approach is that it's required to explicitly return all model getters/setters/actions, which can be cumbersome for large models. Also it requires defining a convenient TypeScript type for the model:

type Model = ReturnType<typeof makeModel>;

MobX flavor

Another flavor for making Onek models is MobX flavor. It requires importing an additional tiny (~300 bytes) package:

import { makeObservable } from "onek/mobx";

class Model {
  constructor(initialValue) {
    this.value = observable.prop(initialValue);
    this.double = computed.prop(() => this.value * 2);


const model = new Model(10);

// read observable and computed values
model.value === 10;
model.double === 20;

// set observable value
model.value = 100;

It defines observable and computed values on class and then calls makeObservable to create getters/setters on the class instance. The only difference in defining the values on the class is that you need to use .prop modifier on observable/computed factories. Otherwise, the usage of MobX models is equivalent to Solid.js ones - just read the values inside observer function to make a component re-render on changes.

Note: it's safe to call makeObservable more than once on a class instance. This makes it work for inheritance cases where both base and inherited classes have observable values.


Simple counter

Simple counter - Actions and models
import { action, observable, useObserver } from "onek";

const makeCounter = (initial) => {
  const [count, setCount] = observable(initial);
  const inc = action(() => setCount((count) => count + 1));
  const dec = action(() => setCount((count) => count - 1));
  const reset = action(() => setCount(initial));

  return { count, inc, dec, reset };

const Counter = ({ counter }) => {
  const observer = useObserver();

  const { count, inc, dec, reset } = counter;

  return observer(() => (
      <button onClick={inc}>+</button>
      <button onClick={dec}>-</button>
      <button onClick={reset}>Reset</button>
      Count: {count()}

const counter = makeCounter(0);

root.render(<Counter counter={counter} />);

Counter list

Counter list with stats - Model composition and computed data
import { observable, computed, action, useObserver } from "onek";
import { makeCounter, Counter } from "./Counter";

const makeCountersList = () => {
  const [counters, setCounters] = observable([]);

  const countersCount = computed(() => counters().length);
  const countersSum = computed(() =>
    counters().reduce((sum, counter) => sum + counter.count(), 0)

  const addCounter = action(() => {
    const counter = makeCounter(0);
    setCounters((counters) => [...counters, counter]);

  const removeCounter = action((counter) => {
    setCounters((counters) =>
      counters.filter((_counter) => _counter !== counter)

  const resetAll = action(() => {
    counters().forEach((counter) => counter.reset());

  return {

const CounterStats = ({ count, sum }) => {
  const observer = useObserver();

  return observer(() => (
      <p>Total count: {count()}</p>
      <p>Total sum: {sum()}</p>

const CountersList = ({ model }) => {
  const observer = useObserver();

  return observer(() => (
      <CounterStats count={model.countersCount} sum={model.countersSum} />
      <button onClick={model.addCounter}>Add</button>
      <button onClick={model.resetAll}>Reset all</button>
      {model.counters().map((counter) => (
          <Counter counter={counter} />
          <button onClick={() => model.removeCounter(counter)}>Remove</button>

const countersList = makeCountersList();

root.render(<CountersList model={countersList} />);

Todo List

Todo List - Complex multi-component app
import { action, computed, observable, useObserver } from "onek";

let id = 0;

export const makeTodo = (todoText) => {
  const [text, setText] = observable(todoText);
  const [done, setDone] = observable(false);

  const toggleDone = action(() => {
    setDone((done) => !done);

  return {
    id: id++,

export const makeTodoList = () => {
  const [text, setText] = observable("");
  const [todos, setTodos] = observable([], true);
  const [filter, setFilter] = observable("ALL");

  const doneTodos = computed(() => {
    return todos().filter((todo) => todo.done());

  const undoneTodos = computed(() => {
    return todos().filter((todo) => !todo.done());

  const visibleTodos = computed(() => {
    switch (filter()) {
      case "ALL":
        return todos();
      case "DONE":
        return doneTodos();
      case "UNDONE":
        return undoneTodos();
  }, true);

  const addTodo = action(() => {
    const todo = makeTodo(text());
    setTodos((todos) => [...todos, todo]);

  const removeTodo = action((todo) => {
    setTodos((todos) => todos.filter((_todo) => _todo !== todo));

  const clearDone = action((todo) => {

  return {

  { name: "All", value: "ALL" },
  { name: "Done", value: "DONE" },
  { name: "Undone", value: "UNDONE" },

const NewTodoInput = ({ model }) => {
  const observer = useObserver();

  const { text, setText, addTodo } = model;

  return observer(() => (
      <input onChange={(e) => setText(} value={text()} />
      <button onClick={addTodo} disabled={text().length === 0}>

const TodoListFilter = ({ model }) => {
  const observer = useObserver();

  return observer(() => (
      onChange={(e) => model.setFilter(}
      {{ name, value }) => (
        <option key={value} value={value}>

const Todo = ({ model }) => {
  const observer = useObserver();

  return observer(() => (
    <div className="todo">
          style={{ textDecoration: model.done() ? "line-through" : "none" }}

export const TodoList = ({ model }) => {
  const observer = useObserver();

  return observer(() => (
    <div className="todo-list">
      <button onClick={model.clearDone}>Clear done</button>
      <TodoListFilter model={model} />
      <NewTodoInput model={model} />
      {model.visibleTodos().map((todo) => (
        <Todo key={} model={todo} />


Optimizing React re-renders with check flag

Reaction scheduler

The reaction scheduler is a function that's called at the end of the first transaction executed after the previous scheduler run. It has one argument - a runner function that should somehow be " scheduled" to run. The default implementation of the scheduler is a microtask Promise-based scheduler:

const reaction = (runner) => Promise.resolve().then(runner);

configure({ reaction });

This is a good compromise between speed and developer experience, but sometimes you might want to run all reactions synchronously at transaction end (for example, this is done in the Onek test suite):

const reaction = (runner) => runner();

configure({ reaction });

Another alternative to the default microtask scheduler is a macrotask scheduler:

const reaction = (runner) => setTimeout(runner, 0);

configure({ reaction });

Reaction exception handler

The default exception handler for auto-run reactions is just console.error. It can be configured by the reactionExceptionHandler option:

  reactionExceptionHandler: (exception) => {
    // some exception handling logic

Memory leaks: why not?

Onek does not have memory leaks while maintaining optimal caching for computed values. There is no keepAlive option like in MobX, and here's why. When a computed value has lost its last subscriber or is being read in untracked context without existing subscribers, it enters a **passive ** state. This state means the computed is no longer referenced by any observable or other computed, but still holds references to its dependencies, so it can check later if some of them changed.

How is the change detection possible without guarantees that values stored in observables and computeds are immutable? The answer is simple: along with the value, observable and computed store a revision - an immutable plain object that is new each time an observable or computed is updated. This allows the implementation of reselect-like logic of checking computed dependencies with very small overhead and preserve cached values without any memory leaks.

API Documentation


Here are some general interfaces used in the following documentation:

import { ComputedImpl, ObservableImpl } from "./types";

type ISubscriber = ComputedImpl | ReactionImpl;

interface Getter<T> {
  (subscriber?: ISubscriber): T;

interface ObservableGetter<T> extends Getter<T> {
  instance: IObservable<T>;

interface ComputedGetter<T> extends Getter<T> {
  instance: IComputed<T>;

  destroy(): void;

interface Setter<T> {
  (value?: T | UpdaterFn<T>, asIs?: boolean): void;

type CheckFn<T> = (prev: T, next: T) => boolean;
type UpdaterFn<T> = (prevValue: T) => T;


function observable<T>(
  value: T,
  checkFn?: boolean | CheckFn<T>
): readonly [ObservableGetter<T>, Setter<T>];

Creates a getter and setter for reactive value. The value argument is the value stored in the observable instance, and the checkFn is a function that's used for checking if the new value from the setter is the same as the old one.

The getter is a function that can accept a ISubscriber - return value of useObserver hook or the value of instanceattribute of a computed getter.

The setter function can accept a value argument that can be of a generic type or an updater function that returns a value of the generic type.

The second argument to the setter function is an asIs boolean that indicates if the value should be stored as is without interpreting it as an updater function.

The setter also can be called without arguments - this will mark the observable as changed without changing its value. This can be useful when you mutate the observable value directly without changing the reference to it.


function computed<T>(
  fn: () => T,
  checkFn?: boolean | CheckFn<T>
): ComputedGetter<T>;


type Destructor = (() => void) | null | undefined | void;

type Disposer = (() => void) & { run: () => void };

function reaction(fn: () => Destructor, manager?: () => void): Disposer;


function action<Args extends any[], T>(
  fn: (...args: Args) => T
): (...args: Args) => T;


function tx(fn: () => void): void;


function utx<T>(fn: () => T, subscriber = null): T;


function untracked<Args extends any[], T>(
  fn: (...args: Args) => T
): (...args: Args) => T;


function useObserver(): ISubscriber | undefined;


function shallowEquals<T>(prev: T, next: T): boolean;




Eugene Daragan

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