Observables

🏆 The goal of this library is to provide a lightweight reactivity API for other UI libraries to be built on top of. It follows the "lazy principle" that Svelte adheres to - don't do any unnecessary work and don't place the burden of figuring it out on the developer.

This is a tiny (~850B minzipped) library for creating reactive observables via functions. You can use observables to store state, create computed properties (y = mx b), and subscribe to updates as its value changes.

🪶 Light (~850B minzipped)
💽 Works in both browsers and Node.js
🌎 All types are observable (i.e., string, array, object, etc.)
🕵️‍♀️ Only updates when value has changed
⏱️ Batched updates via microtask scheduler
😴 Lazy by default - efficiently re-computes only what's needed
🔬 Computations via $computed
📞 Effect subscriptions via $effect
♻️ Detects cyclic dependencies
🐛 Debugging identifiers
💪 Strongly typed - built with TypeScript

⏭️ Skip to API

⏭️ Skip to TypeScript

⏭️ Skip to Benchmarks

Here's a simple demo to see how it works:

import { $root, $observable, $computed, $effect, $tick } from '@maverick-js/observables';

$root(async (dispose) => {
  // Create - all types supported (string, array, object, etc.)
  const $m = $observable(1);
  const $x = $observable(1);
  const $b = $observable(0);

  // Compute - only re-computed when `$m`, `$x`, or `$b` changes.
  const $y = $computed(() => $m() * $x()   $b());

  // Effect - this will run whenever `$y` is updated.
  const stop = $effect(() => {
    console.log($y());

    // Called each time `$effect` ends and when finally disposed.
    return () => {};
  });

  $m.set(10); // logs `10` inside effect

  // Wait a tick so update is applied and effect is run.
  await $tick();

  $b.next((prev) => prev   5); // logs `15` inside effect

  // Wait a tick so effect runs last update.
  await $tick();

  // Nothing has changed - no re-compute.
  $y();

  // Stop running effect.
  stop();

  // ...

  // Dispose of all observables inside `$root`.
  dispose();
});

Export Sizes

Average: ~600B. Total: ~850B.

You can also check out the library size on Bundlephobia (less accurate).

Installation

$: npm i @maverick-js/observables

$: pnpm i @maverick-js/observables

$: yarn add @maverick-js/observables

API

$root
$observable
$computed
$effect
$peek
$readonly
$tick
$dispose
onDispose
isObservable
isSubject

`$root`

Computations are generally child computations. When their respective parent is destroyed so are they. You can create orphan computations (i.e., no parent). Orphans will live in memory until their internal object references are garbage collected (GC) (i.e., dropped from memory):

import { $computed } from '@maverick-js/observables';

const obj = {};

// This is an orphan - GC'd when `obj` is.
const $b = $computed(() => obj);

Orphans can make it hard to determine when a computation is disposed so you'll generally want to ensure you only create child computations. The $root function stores all inner computations as a child and provides a function to easily dispose of them all:

import { $root, $observable, $computed, $effect } from '@maverick-js/observables';

$root((dispose) => {
  const $a = $observable(10);
  const $b = $computed(() => $a());

  $effect(() => console.log($b()));

  // Disposes of `$a`, $b`, and `$effect`.
  dispose();
});

// `$root` returns the result of the given function.
const result = $root(() => 10);

console.log(result); // logs `10`

`$observable`

Wraps the given value into an observable function. The observable function will return the current value when invoked fn(), and provide a simple write API via set() and next(). The value can now be observed when used inside other computations created with $computed and $effect.

import { $observable } from '@maverick-js/observables';

const $a = $observable(10);

$a(); // read
$a.set(20); // write (1)
$a.next((prev) => prev   10); // write (2)

Warning Read the $tick section below to understand batched updates.

`$computed`

Creates a new observable whose value is computed and returned by the given function. The given compute function is only re-run when one of it's dependencies are updated. Dependencies are are all observables that are read during execution.

import { $observable, $computed, $tick } from '@maverick-js/observables';

const $a = $observable(10);
const $b = $observable(10);
const $c = $computed(() => $a()   $b());

console.log($c()); // logs 20

$a.set(20);
await $tick();
console.log($c()); // logs 30

$b.set(20);
await $tick();
console.log($c()); // logs 40

// Nothing changed - no re-compute.
console.log($c()); // logs 40

import { $observable, $computed } from '@maverick-js/observables';

const $a = $observable(10);
const $b = $observable(10);
const $c = $computed(() => $a()   $b());

// Computed observables can be deeply nested.
const $d = $computed(() => $a()   $b()   $c());
const $e = $computed(() => $d());

`$effect`

Invokes the given function each time any of the observables that are read inside are updated (i.e., their value changes). The effect is immediately invoked on initialization.

import { $observable, $computed, $effect } from '@maverick-js/observables';

const $a = $observable(10);
const $b = $observable(20);
const $c = $computed(() => $a()   $b());

// This effect will run each time `$a` or `$b` is updated.
const stop = $effect(() => console.log($c()));

// Stop observing.
stop();

You can optionally return a function from inside the $effect that will be run each time the effect re-runs and when it's finally stopped/disposed of:

$effect(() => {
  return () => {
    // Called each time effect re-runs and when disposed of.
  };
});

You can optionally destroy all inner observables when stopping the effect by passing in true to the stop effect function:

// `$c` is from the example above.
const stop = $effect(() => console.log($c()));

// This will dispose of `$a`, `$b`, `$c`, and the effect itself.
stop(true); // <- deep flag

`$peek`

Returns the current value stored inside an observable without triggering a dependency.

import { $observable, $computed, $peek } from '@maverick-js/observables';

const $a = $observable(10);

$computed(() => {
  // `$a` will not be considered a dependency.
  const value = $peek($a);
});

`$readonly`

Takes in the given observable and makes it read only by removing access to write operations (i.e., set() and next()).

import { $observable, $readonly } from '@maverick-js/observables';

const $a = $observable(10);
const $b = $readonly($a);

console.log($b()); // logs 10

// We can still update value through `$a`.
$a.set(20);

console.log($b()); // logs 20

`$tick`

Tasks are batched onto the microtask queue. This means only the last write of multiple write actions performed in the same execution window is applied. You can wait for the microtask queue to be flushed before writing a new value so it takes effect.

Note You can read more about microtasks on MDN.

import { $observable } from '@maverick-js/observables';

const $a = $observable(10);

$a.set(10);
$a.set(20);
$a.set(30); // only this write is applied

import { $observable, $tick } from '@maverick-js/observables';

const $a = $observable(10);

// All writes are applied.
$a.set(10);
await $tick();
$a.set(20);
await $tick();
$a.set(30);

`$dispose`

Unsubscribes the given observable and optionally all inner computations. Disposed functions will retain their current value but are no longer reactive.

import { $observable, $dispose } from '@maverick-js/observables';

const $a = $observable(10);
const $b = $computed(() => $a());

// `$b` will no longer update if `$a` is updated.
$dispose($a);

$a.set(100);
console.log($b()); // still logs `10`

The second argument to $dispose is a deep flag which specifies whether all inner computations should also be disposed of:

const $a = $observable();
const $b = $computed(() => $a());
const $c = $effect(() => $b());

$dispose($c, true); // <- deep flag

// `$a`, `$b`, and `$c` are all disposed.

`onDispose`

Runs the given function when the parent computation is disposed of:

import { $effect, onDispose } from '@maverick-js/observables';

const listen = (type, callback) => {
  window.addEventListener(type, callback);
  onDispose(() => window.removeEventListener(type, callback));
};

const stop = $effect(
  listen('click', () => {
    // ...
  }),
);

stop(); // `onDispose` is called

The onDispose callback will return a function to clear the disposal early if it's no longer required:

$effect(() => {
  const dispose = onDispose(() => {});
  // ...
  // Call early if it's no longer required.
  dispose();
});

`isObservable`

Whether the given value is an observable (readonly).

// True
isObservable($observable(10));
isObservable($computed(() => 10));
isObservable($readonly($observable(10)));

// False
isObservable(false);
isObservable(null);
isObservable(undefined);
isObservable(() => {});

`isSubject`

Whether the given value is an observable subject (i.e., can produce new values via write API).

// True
isSubject($observable(10));

// False
isSubject(false);
isSubject(null);
isSubject(undefined);
isSubject(() => {});
isSubject($computed(() => 10));
isSubject($readonly($observable(10)));

Debugging

The $observable, $computed, and $effect functions accept a debugging ID (string) as their second argument. This can be helpful when logging a cyclic dependency chain to understand where it's occurring.

import { $observable, $computed } from '@maverick-js/observables';

const $a = $observable(10, 'a');

// Cyclic dependency chain.
const $b = $computed(() => $a()   $c(), 'b');
const $c = $computed(() => $a()   $b(), 'c');

// This will throw an error in the form:
// $: Error: cyclic dependency detected
// $: a -> b -> c -> b

Note This feature is only available in a development or testing Node environment (i.e., NODE_ENV).

Scheduler

We provide the underlying microtask scheduler incase you'd like to use it:

import { createScheduler } from '@maverick-js/observables';

// Creates a scheduler which batches tasks and runs them in the microtask queue.
const scheduler = createScheduler();

// Queue tasks.
scheduler.enqueue(() => {});
scheduler.enqueue(() => {});

// Schedule a flush - can be invoked more than once.
scheduler.flush();

// Wait for flush to complete.
await scheduler.tick;

Note You can read more about microtasks on MDN.

TypeScript

import {
  isObservable,
  isSubject,
  type Effect,
  type Observable,
  type MaybeObservable,
} from '@maverick-js/observables';

// Types
const observable: Observable<number>;
const computed: Observable<string>;
const effect: Effect;

// Provide generic if TS fails to infer correct type.
const $a = $computed<string>(() => /* ... */);

const $b: MaybeObservable<number>;

if (isObservable($b)) {
  $b(); // Observable<number>
}

if (isSubject($b)) {
  $b.set(10); // ObservableSubject<number>
}

Benchmarks

Layers

This benchmark was taken from cellx. It tests how long it takes for an n deeply layered computation to update. The benchmark can be found here.

Each column represents how deep computations were layered. The average time taken to update the computation out of a 100 runs is used for each library.

Don't take this benchmark too seriously because it hasn't been reviewed properly at all.

Inspiration

@maverick-js/observables was made possible based on my learnings from:

Solid JS
Sinuous
Hyperactiv
Svelte Scheduler

Special thanks to Wesley, Julien, and Solid/Svelte contributors for all their work 🎉

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README.md

README.md

Observables

Export Sizes

Installation

API

`$root`

`$observable`

`$computed`

`$effect`

`$peek`

`$readonly`

`$tick`

`$dispose`

`onDispose`

`isObservable`

`isSubject`

Debugging

Scheduler

TypeScript

Benchmarks

Layers

Inspiration

Files

README.md

Latest commit

History

README.md

File metadata and controls

Observables

Export Sizes

Installation

API

$root

$observable

$computed

$effect

$peek

$readonly

$tick

$dispose

onDispose

isObservable

isSubject

Debugging

Scheduler

TypeScript

Benchmarks

Layers

Inspiration

`$root`

`$observable`

`$computed`

`$effect`

`$peek`

`$readonly`

`$tick`

`$dispose`

`onDispose`

`isObservable`

`isSubject`