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

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

Interact with the demo live on StackBlitz.

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

// 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()));

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

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

$b.update((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();

Export Sizes

Total: if you import everything it'll be ~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

$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 update(). 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.update((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 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 update()).

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.

isComputed

Whether the given function is a computed observable.

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

isComputed(() => {}); // false

const $a = $observable(10);
isComputed($a); // false

const $b = $computed(() => $a() + 10);
isComputed($b); // true

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.

Types

import { $computed, type Observable, type Computation } from '@maverick-js/observables';

const observable: Observable<number>;
const computed: Computation<number>;

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

Inspiration

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

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