effective-state

A minimalist reactive programming library that provides fine-grained reactivity primitives for TypeScript applications. It offers a signals-based approach where you create reactive atoms (state containers), derive computed values, and react to changes through effects—all with automatic dependency tracking and optimal re-computation.

Unlike heavier frameworks, effective-state focuses solely on the reactive core: atoms hold state, computed values derive from other reactive sources, and effects run side effects when dependencies change. This makes it perfect for building reactive UIs, state management systems, or any application requiring efficient change propagation.

Features

• ⚡ Minimal, fast, type-safe reactivity
• 🔧 Strict TypeScript support
• 📦 ESM & CJS output
• 🛠️ Zero dependencies

Installation

npm install effective-state

Running Tests (Direct TypeScript)

You can run tests written in TypeScript directly with Node.js 22+:

node --experimental-transform-types --test src/index.test.ts

Quick Start

import { atom, computed, effect } from 'effective-state';

// Create an atom
const count = atom(0);

// Create a computed value
const double = computed(() => count() * 2);

// React to changes
effect(() => {
  console.log(`Count is ${count()}, double is ${double()}`);
});

count(5); // Logs: Count is 5, double is 10

Common Patterns

1. Basic Atom Usage

const value = atom(10);
value(20); // set new value
console.log(value()); // get current value

Tip: In TypeScript, always use the function-call style to get the value of an atom or computed. For example:

const a = atom(2);
const b = atom(3);
console.log(a() + b()); // 5
console.log(`Value is: ${a()}`); // "Value is: 2"

2. Derived/Computed Values

const a = atom(2);
const b = atom(3);
const sum = computed(() => a() + b());
a(5);
console.log(sum()); // 8

3. Effects (Reactions)

const name = atom('Alice');
effect(() => {
  console.log('Hello,', name());
});
name('Bob'); // Logs: Hello, Bob

4. Subscribing to Atom Changes

const n = atom(1);
const unsubscribe = n.subscribe((val) => {
  console.log('n changed to', val);
});
n(2); // Logs: n changed to 2
unsubscribe();
n(3); // No log

5. Nested/Structured State

const user = atom({ name: 'Alice', age: 30 });
effect(() => {
  console.log(user().name);
});
user({ ...user(), name: 'Bob' }); // Logs: Bob

API

atom<T>(initialValue: T): Atom<T>

Creates a reactive atom holding a value. Minimal API:

• Call as function: atom() to read, atom(newValue) to write
• .subscribe(callback) to listen for changes (optional)

computed<T>(fn: () => T): Computed<T>

Creates a computed value that updates when dependencies change.

effect(fn: () => void): () => void

Runs a function whenever its dependencies change. Returns an unsubscribe function.

License

MIT