🧶 DeepSignal

Use Preact signals with the interface of a plain JavaScript object.

• DeepSignal works by wrapping the object with a Proxy that intercepts all property accesses and returns the signal value by default.
• This allows you to easily create a deep object that can be observed for changes, while still being able to mutate the object normally.
• Nested objects and arrays are also converted to deep signal objects/arrays, allowing you to create fully reactive data structures.
• The $ prefix returns the signal instance: state.$prop.

---

• Try it on Stackblitz
  • Preact
  • Preact & TypeScript
  • React
  • React & TypeScript
  • Lit
• Or on Codesandbox
  • Preact
  • Preact & TypeScript
  • React
  • React & TypeScript

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Table of contents

• DeepSignal
  • Features
  • Installation
  • Usage
  • API
    • deepSignal
    • get prop() { ... }
    • state.$prop
    • array.$[index]
    • array.$length
    • peek(state, "prop")
    • shallow(obj)
    • state.$prop = signal(value)
    • useDeepSignal
  • Common Patterns
    • Resetting the store
  • When do you need access to signals?
    • Passing the value of a signal directly to JSX
    • Passing a signal to a child component
  • TypeScript
    • DeepSignal
    • RevertDeepSignal
    • Shallow
  • License

Features

• Transparent: deepsignal wraps the object with a proxy that intercepts all property accesses, but does not modify how you interact with the object. This means that you can still use the object as you normally would, and it will behave exactly as you would expect, except that mutating the object also updates the value of the underlying signals.
• Tiny (less than 1kB): deepsignal is designed to be lightweight and has a minimal footprint, making it easy to include in your projects. It's just a small wrapper around @preact/signals-core.
• Full array support: deepsignal fully supports arrays, including nested arrays.
• Deep: deepsignal converts nested objects and arrays to deep signal objects/arrays, allowing you to create fully reactive data structures.
• Lazy initialization: deepsignal uses lazy initialization, which means that signals and proxies are only created when they are accessed for the first time. This reduces the initialization time to almost zero and improves the overall performance in cases where you only need to observe a small subset of the object's properties.
• Stable references: deepsignal uses stable references, which means that the same Proxy instances will be returned for the same objects so they can exist in different places of the data structure, just like regular JavaScript objects.
• Automatic derived state: getters are automatically converted to computeds instead of signals.
• TypeScript support: deepsignal is written in TypeScript and includes type definitions, so you can use it seamlessly with your TypeScript projects, including access to the signal value through the prefix state.$prop.
• State management: deepsignal can be used as a state manager, including state and actions in the same object.

The most important feature is that it just works. You don't need to do anything special. Just create an object, mutate it normally and all your components will know when they need to rerender.

Installation

With Preact

1npm install deepsignal @preact/signals

If you are using deepsignal with Preact (@preact/signals), you should use the deepsignal import. You also need to install @preact/signals.

1import { deepSignal } from "deepsignal";
2
3const state = deepSignal({
4	count: 0,
5});
6
7const Count = () => <div>{state.$count}</div>;

With React

1npm install deepsignal @preact/signals-react

If you are using the library with React (@preact/signals-react), you should use the deepsignal/react import. You also need to install @preact/signals-react.

1import { deepSignal } from "deepsignal/react";
2
3const state = deepSignal({
4	count: 0,
5});
6
7const Count = () => <div>{state.$count}</div>;

• If you want to use deepSignal outside of the components, please follow the React integration guide of @preact/signals-react to choose one of the integration methods.
• For useDeepSignal, no integration is required.

With Lit

Lit now supports Preact Signals, so you can also use deepsignal in Lit.

1npm install deepsignal @lit-labs/preact-signals

If you are using the library just with Lit, you should use the deepsignal/core import. You also need to install @lit-labs/preact-signals and use its SignalWatcher function.

1import { SignalWatcher } from "@lit-labs/preact-signals";
2import { deepSignal } from "deepsignal/core";
3
4const state = deepSignal({
5	count: 0,
6});
7
8class Count extends SignalWatcher(LitElement) {
9	render() {
10		return html`<div>${state.$count}</div>`;
11	}
12}

Without Preact/React/Lit

1npm install deepsignal @preact/signals-core

If you are using the library just with @preact/signals-core, you should use the deepsignal/core import. You also need to install @preact/signals-core.

1import { effect } from "@preact/signals-core";
2import { deepSignal } from "deepsignal/core";
3
4const state = deepSignal({
5	count: 0,
6});
7
8effect(() => {
9	console.log(`Count: ${state.count}`);
10});

This is because the deepsignal import includes a dependency on @preact/signals, while the deepsignal/core import does not. This allows you to use deep signals with either @preact/signals or @preact/signals-core, depending on your needs. Do not use both.

Usage

The usage is similar to Preact's signal, but it works with objects and arrays and the access to the value and signal is reversed. By default, the object returns the value and not the signal:

• Use state.prop to access the value (you don't need to use state.prop.value).
• Use state.prop to mutate the value (you don't need to use state.prop.value).
• Use state.$prop to access the signal instance (only needed for performance optimizations).

This Preact's signals example:

1import { signal, computed } from "@preact/signals";
2
3const count = signal(0);
4const double = computed(() => count.value * 2);
5
6function Counter() {
7	return (
8		<button onClick={() => (count.value += 1)}>
9			{count} x 2 = {double}
10		</button>
11	);
12}

becomes like this with deepsignal:

1import { deepSignal } from "deepsignal";
2
3const state = deepSignal({
4	count: 0,
5	get double() {
6		return state.count * 2;
7	},
8});
9
10function Counter() {
11	return (
12		<button onClick={() => (state.count += 1)}>
13			{state.$count} x 2 = {state.$double}
14		</button>
15	);
16}

You can also add actions inside the deep signal and use it as a state manager.

1import { deepSignal } from "deepsignal";
2
3const store = deepSignal({
4	count: 0,
5	get double() {
6		return store.count * 2;
7	},
8	inc: () => {
9		store.count += 1;
10	},
11});
12
13function Counter() {
14	return (
15		<button onClick={store.inc}>
16			{store.$count} x 2 = {store.$double}
17		</button>
18	);
19}

API

deepSignal

1deepSignal<T extends object>(obj: T): DeepSignal<T>;

The deepSignal function creates a new deep signal. You can read or mutate the underlying signal values by accessing the object's properties just like you would in a regular JavaScript object.

1import { deepSignal } from "deepsignal";
2
3const state = deepSignal({ counter: 0 });
4
5// Reads the value, logs: 0.
6console.log(state.counter);
7
8// Mutates the underlying signal.
9state.counter = 1;
10
11// Reads the value, logs: 1.
12console.log(state.counter);

Writing to a signal is done by mutating the object's properties. Changing a property's value will synchronously update every component and effect that depends on its signal, ensuring your app state is always consistent.

1const state = deepSignal({ counter: 0 });
2
3// Runs the first time, reads the value and outputs: <div>0</div>
4const Counter = () => <div>{state.counter}</div>;
5
6// Runs the first time, reads the value and logs: 0.
7effect(() => {
8	console.log(state.counter);
9});
10
11// Mutates the underlying signal.
12state.counter = 1;
13// The effect runs again, logs: 1.
14// The component renders again, outputs: <div>1</div>.

get prop() { ... }

Using JavaScript getters with deepsignal allows you to define computed properties that are based on the values of other properties, and ensures that the computed properties are automatically updated whenever the values of the other properties change. deepsignal will convert getters to computed values instead of signal instances underneath.

1const state = deepSignal({
2	counter: 1,
3	get double() {
4		return state.counter * 2;
5	},
6});
7
8// Runs the first time, reads value from the getter, logs: 2.
9effect(() => {
10	console.log(state.double);
11});
12
13// Mutates the dependency. The effect runs again, logs: 4.
14state.counter = 2;

state.$prop

You can access the underlying signal of an object's property by using the $ prefix, like so: state.$prop.

1const state = deepSignal({ counter: 0 });
2
3// Runs the first time, read value from signal, logs: 0.
4state.$counter.subscribe(console.log);
5
6// Mutates the underlying signal. The subscription runs again, logs: 1.
7state.counter = 1;

array.$[index]

You can access the underlying signal of an array's item by using the $ prefix, like so: state.$[index].

1const array = deepSignal([0]);
2
3// Runs the first time, read value from signal, logs: 0.
4array.$[0].subscribe(console.log);
5
6// Mutates the underlying signal. The subscription runs again, logs: 1.
7array[0] = 1;

Please note that although the syntax is similar, there's a difference between objects and arrays. In objects, each prop has a signal counterpart (state.prop -> state.$prop) whereas in arrays, the array.$ prop returns a new array of signals, therefore the difference between array[index] and array.$[index].

array.$length

Arrays can access the length signal with a property called array.$length in the same way that objects have access to state.$prop.

1const array = deepSignal([0]);
2
3// Runs the first time, logs: 1.
4array.$length.subscribe(console.log);
5
6// Mutates the array. The subscription runs again, logs: 2.
7array.push(1);

peek(state, "prop")

Chances are you will rarely need access to the underlying JavaScript object without subscribing to the current computation except when you have an effect that should write to another signal based on the previous value, but you don't want the effect to be subscribed to that signal. You can use peek(state, "prop") to peek at the value of a signal:

1import { peek } from "deepsignal";
2
3const state = deepSignal({ value: 0, effectCount: 0 });
4
5effect(() => {
6	console.log(state.value);
7
8	// Whenever this effect is triggered, increase `effectCount`, but we don't
9	// want this effect to react to `effectCount`.
10	state.effectCount = peek(state, "effectCount") + 1;
11});

Note that you should only use peek() if you really need it. Reading a signal's value via state.prop is the preferred way in most scenarios.

For primitive values, you can get away with using store.$prop.peek() instead of peek(state, "prop"). But in deepsignal, the underlying signals store the proxies, not the object. That means it's not safe to use state.$prop.peek().nestedProp if prop is an object. You should use peek(state, "prop").nestedProp instead.

shallow(obj)

When using deepsignal, all nested objects and arrays are turned into deep signal objects/arrays. The shallow function is a utility that allows you to declare an object as shallow within the context of deepsignal. Shallow objects do not proxy their properties, meaning changes to their properties are not observed for reactivity. This can be useful for objects that you don't want to be reactive or when you have an object that should not trigger UI updates when changed.

1import { deepSignal, shallow } from "deepsignal";
2
3const shallowObj = { key: "value" };
4const store = deepSignal({
5	someData: shallow(shallowObj),
6});
7
8// Accessing `store.someData` gives you the original object.
9console.log(store.someData === shallowObj); // true
10
11// Mutating `store.someData` does NOT trigger reactivity.
12store.someData.key = "newValue";
13// No reactive update is triggered since `someData` is shallow.

In practice, this means you can have parts of your state that are mutable and changeable without causing rerenders or effects to run. This becomes particularly useful for large datasets or configuration objects that you might want to include in your global state but do not need to be reactive.

Observing reference changes

Although properties of a shallow object are not reactive, the reference to the shallow object itself is observed. If you replace the reference of a shallow object with another reference, it will trigger reactive updates:

1import { deepSignal, shallow } from "deepsignal";
2
3const store = deepSignal({
4	someData: shallow({ key: "value" }),
5});
6
7effect(() => {
8	console.log(store.someData.key);
9});
10
11// Changing the properties of `someData` does not  trigger the `effect`.
12store.someData.key = "changed";
13// No log output.
14
15// But replacing `someData` with a new object triggers the `effect` because store.someData is still tracked.
16store.someData = shallow({ key: "new value" });
17// It will log 'new value'.

With shallow, you have control over the granularity of reactivity in your store, mixing both reactive deep structures with non-reactive shallow portions as needed.

state.$prop = signal(value)

You can modify the underlying signal of an object's property by doing an assignment to the $-prefixed name.

1const state = deepSignal({ counter: 0 });
2
3// Runs the first time, read value from signal, logs: 0.
4state.$counter.subscribe(console.log);
5
6// Replace the signal with a new one.
7state.$counter = signal(10);
8
9// The subscription doesn't run this time; it's a different signal!
10state.counter = 1;

useDeepSignal

Only available on deepsignal and deepsignal/react, not on deepsignal/core.

If you need to create a reference stable version of a deep signal that is hooked to a component instance you can use the useDeepSignal hook:

1import { useDeepSignal } from "deepsignal";
2// or
3import { useDeepSignal } from "deepsignal/react";
4
5function Counter() {
6	const state = useDeepSignal({
7		counter: 0,
8		get double() {
9			return state.counter * 2;
10		},
11	});
12
13	return (
14		<button onClick={() => (state.count += 1)}>
15			Value: {state.$counter}, value x 2 = {state.$double}
16		</button>
17	);
18}

Common Patterns

Resetting the store

If you need to reset your store to some initial values, don't overwrite the reference. Instead, replace each value using something like Object.assign.

1const initialState = { counter: 0 };
2
3const store = deepSignal({
4	...initialState,
5	inc: () => {
6		store.counter++;
7	},
8	reset: () => {
9		Object.assign(store, initialState);
10	},
11});

Take into account that the object that you pass to deepSignal during the creation is also mutated when you mutate the deep signal. Therefore, if you need to keep a set of initial values, you need to store them in a different object or clone it before assigning it to the deepsignal.

When do you need access to signals?

You will only need access to the underlying signals for performance optimizations.

Passing the value of a signal directly to JSX

This works fine but Component will render each time state.counter changes.

1const state = deepSignal({ counter: 0 });
2
3// Reads value from signal, outputs: <div>0</div>
4const Component = () => <div>{state.counter}</div>;
5
6// Mutates the underlying signal. The component is rerender again and outputs: <div>1</div>
7state.counter = 1;

We can pass the signal directly to JSX and Preact will mutate the DOM instead of rerendering Component:

1const state = deepSignal({ counter: 0 });
2
3// Reads value from signal, outputs: <div>0</div>
4const Component = () => <div>{state.$counter}</div>;
5
6// Mutates the underlying signal. The DOM is mutated to: <div>1</div>
7state.counter = 1;

Passing a signal to a child component

This also works fine, but Parent will render each time state.counter changes.

1const state = deepSignal({ counter: 0 });
2
3const Child = ({ counter }) => <span>{counter}</div>;
4const Parent = () => <Child counter={state.counter} />;
5
6// Mutates the underlying signal. Parent is rerender again.
7state.counter = 1;

You can pass the signal directly to the child:

1const state = deepSignal({ counter: 0 });
2
3const Child = ({ counter }) => <span>{counter}</div>;
4const Parent = () => <Child counter={state.$counter} />;
5
6// Mutates the underlying signal. Parent is not rerender.
7state.counter = 1;

Be aware that if you do so, counter will become a regular Preact signal. If you need to access or mutate its value inside Child, you'd need to use counter.value.

TypeScript

deepsignal has TypeScript support, which means that you can use it seamlessly with your TypeScript projects. deepsignal includes type definitions that provide type safety when working with deep signal objects, which can help you avoid common type-related mistakes and improve the overall quality of your code.

There is one caveat to consider when using deepsignal with TypeScript: you need to use the non-null assertion operator when accessing signals. This is because mutations don't work unless the signals are optional, so TypeScript does not know that the signal instance will always be defined and it will treat it as a possibly-undefined value. To fix this, you can use the non-null assertion operator (!) to tell TypeScript that the signal instance is definitely defined.

For example, consider the following code:

1const state = deepSignal({
2  counter: 1
3});
4
5console.log(state.$counter.value); // error: Object is possibly 'undefined'.
6console.log(state.$counter!.value); // 1

If we try to access the value property of the $counter signal, TypeScript will error because it does not know that the $counter signal is defined.

The same happens with arrays:

1const array = deepSignal([1]);
2
3console.log(array.$[0].value); // error: Object is possibly 'undefined'.
4console.log(array.$![0].value); // 1

Note that here the position of the non-null assertion operator changes because array.$ is an object in itself.

DeepSignal exports two types, one to convert from a plain object/array to a deepSignal instance, and other to revert from a deepSignal instance back to the plain object/array.

DeepSignal

You can use the DeepSignal type if you want to manually convert a type to a deep signal outside of the deepSignal function, but this is usually not required.

One scenario where this could be useful is when doing external assignments. Imagine this case:

1import { deepSignal } from "deepsignal";
2
3type State = { map: Record<string, boolean> };
4
5const state = deepSignal<State>({ map: {} });

If you want to assign a new object to state.map, TypeScript will complain because it expects a deep signal type, not a plain object one:

1const newMap: State["map"] = { someKey: true };
2
3state.map = newMap; // This will fail because state.map expects a deep signal type.

You can use the DeepSignal type to convert a regular type into a deep signal one:

1import type { DeepSignal } from "deepsignal";
2
3const newMap: DeepSignal<State["map"]> = { someKey: true };
4
5state.map = newMap; // This is fine now.

You can also manually cast the type on the fly if you prefer:

1state.map = newMap as DeepSignal<typeof newMap>;

1state.map = { someKey: true } as DeepSignal<State["map"]>;

RevertDeepSignal

You can use the RevertDeepSignal type if you want to recover the type of the plain object/array using the type of the deepSignal instance. For example, when you need to use Object.values().

1import type { RevertDeepSignal } from "deepsignal";
2
3const values = Object.values(store as RevertDeepSignal<typeof store>);

Shallow

You can use the Shallow type if you want to type a store that contains a shallow object.

1import type { Shallow } from "deepsignal";
2
3type Store = {
4	obj: { a: number };
5	shallowObj: { b: number };
6};
7
8const store = deepSignal<Store>({
9	obj: { a: 1 },
10	shallowObj: shallow({ b: 2 }),
11});

License

MIT, see the LICENSE file.

---

This library is hugely inspired by the work of @solkimicreb with proxies and lazy initialization on @nx-js/observer-util.