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: ~850B (brotli)
• Without Computed Maps: ~1KB (brotli)
• Total: ~1.5KB (brotli)

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

Note Maverick Observables is treeshakable and side-effect free so you'll only end up with what you use.

Installation

$: npm i @maverick-js/observables

$: pnpm i @maverick-js/observables

$: yarn add @maverick-js/observables

API

• root
• observable
• computed
• effect
• peek
• readonly
• tick
• computedMap
• computedKeyedMap
• dispose
• onError
• onDispose
• isObservable
• isSubject
• getScope
• getContext
• setContext
• getScheduler

root

Computations are generally child computations. When their respective parent scope 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.
  };
});

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);

computedMap

Note Same implementation as indexArray in Solid JS. Prefer computedKeyedMap when referential checks are required.

Reactive map helper that caches each item by index to reduce unnecessary mapping on updates. It only runs the mapping function once per item and adds/removes as needed. In a non-keyed map like this the index is fixed but value can change (opposite of a keyed map).

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

const source = observable([1, 2, 3]);

const map = computedMap(source, (value, index) => {
  return {
    i: index,
    get id() {
      return value() * 2;
    },
  };
});

console.log(map()); // logs `[{ i: 0, id: $2 }, { i: 1, id: $4 }, { i: 2, id: $6 }]`

source.set([3, 2, 1]);
await tick();

// Notice the index `i` remains fixed but `id` has updated.
console.log(map()); // logs `[{ i: 0, id: $6 }, { i: 1, id: $4 }, { i: 2, id: $2 }]`

computedKeyedMap

Note Same implementation as mapArray in Solid JS. Prefer computedMap when working with primitives to avoid unnecessary re-renders.

Reactive map helper that caches each list item by reference to reduce unnecessary mapping on updates. It only runs the mapping function once per item and then moves or removes it as needed. In a keyed map like this the value is fixed but the index changes (opposite of non-keyed map).

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

const source = observable([{ id: 0 }, { id: 1 }, { id: 2 }]);

const nodes = computedKeyedMap(source, (value, index) => {
  const div = document.createElement('div');

  div.setAttribute('id', String(value.id));
  Object.defineProperty(div, 'i', {
    get() {
      return index();
    },
  });

  return div;
});

console.log(nodes()); // [{ id: 0, i: $0 }, { id: 1, i: $1 }, { id: 2, i: $2 }];

source.next((prev) => {
  // Swap index 0 and 1
  const tmp = prev[1];
  prev[1] = prev[0];
  prev[0] = tmp;
  return [...prev]; // new array
});

await tick();

// No nodes were created/destroyed, simply nodes at index 0 and 1 switched.
console.log(nodes()); // [{ id: 1, i: $0 }, { id: 0, i: $1 }, { id: 2, i: $2 }];

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`

onError

Runs the given function when an error is thrown in a child scope. If the error is thrown again inside the error handler, it will trigger the next available parent scope handler.

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

effect(() => {
  onError((error) => {
    // ...
  });
});

onDispose

Runs the given function when the parent scope computation is being disposed of.

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

const listen = (type, callback) => {
  window.addEventListener(type, callback);
  // Called when the effect is re-run or finally disposed.
  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)));

getScope

Returns the owning scope of the given function. If no function is given it'll return the currently executing parent scope. You can use this to walk up the computation tree.

root(() => {
  effect(() => {
    const $a = observable(0);
    getScope($a); // returns `effect`
    getScope(getScope()); // returns `root`
  });

  getScope(); // returns `root`.
});

getContext

Attempts to get a context value for the given key. It will start from the parent scope and walk up the computation tree trying to find a context record and matching key. If no value can be found undefined will be returned. This is intentionally low-level so you can design a context API in your library as desired.

In your implementation make sure to check if a parent scpoe exists via getScope(). If one does not exist log a warning that this function should not be called outside a computation or render function.

Note See the setContext code example below for a demo of this function.

setContext

Attempts to set a context value on the parent scope with the given key. This will be a no-op if no parent scope is defined. This is intentionally low-level so you can design a context API in your library as desired.

In your implementation make sure to check if a parent scope exists via getScope(). If one does not exist log a warning that this function should not be called outside a computation or render function.

import { root, getContext, setContext } from '@maverick-js/observables';

const key = Symbol();

root(() => {
  setContext(key, 100);
  // ...
  root(() => {
    const value = getContext(key); // 100
  });
});

getScheduler

Returns the global scheduler which can be used to queue additional tasks or synchronously flush the queue.

const scheduler = getScheduler();

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

// Flush queue synchronously
scheduler.syncFlush();

Note See our Scheduler repo for more information.

Debugging

The observable, computed, and effect functions accept a debugging ID (string) as part of their options. 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, { id: 'a' });

// Cyclic dependency chain.
const $b = computed(() => $a() + $c(), { id: 'b' });
const $c = computed(() => $a() + $b(), { id: '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).

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.

❗ Do note that only Maverick and Solid JS are feature complete below which includes nested effects, arbritrary node disposal, context, and error handling.

❗ Nearly all computations in a real world app are going to be less than 10 layers deep, so only the first column really matters. What this benchmark is really showing is how notification propagation scales with computation depth.

Sync

Batched

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 🎉