a-signal

A lightweight, feature-rich signal/event emitter for single events. Perfect for handling state changes, async operations, and event-driven architectures.

Installation

npm install a-signal

Features

Unlike typical event emitters that handle multiple named events, a-signal provides specialized features:

• Single Event Focus: Each signal instance is a dedicated event channel
• Priority-based Listeners: Control execution order with numeric priorities (higher executes first)
• Late Listener Support: Catch events that happened before subscription
• Memory Mode: Remember exact arguments for late subscribers
• Promise-based Waiting: Await next signal emission with optional timeout
• Emission Control: Stop event propagation at any point
• Memory Management: Clear memory and listeners independently
• Extractable Methods: Create clean APIs by extracting methods
• One-time Listeners: Auto-unsubscribe after first execution

Examples

Basic Usage

Simple subscription and emission - the foundation of signal usage.

const signal = new Signal()
signal.on(data => console.log(data))
signal.emit('Hello!')

Priority and Control Flow

Control the order of execution and stop propagation when needed. Useful for middleware-like patterns.

const signal = new Signal({ prioritized: true })

// Higher priority executes first
signal.on(() => console.log('Second'), 1)
signal.on(() => console.log('First'), 100)

// Stop propagation
signal.on(() => {
    console.log('Stop here')
    signal.break()
})

State Management

Perfect for handling initialization states and late-joining components. Combines late listeners with memory to ensure consistent state.

const signal = new Signal({ 
    late: true,      // Get events that happened before subscribing
    memorable: true  // Remember the arguments too
})

signal.emit('state', { value: 42 })

// Later subscriber still gets the event
signal.on((type, data) => {
    console.log(type, data.value) // 'state', 42
})

Async Operations

Convert event-based code into Promise-based code. Great for handling timeouts and async flows.

const signal = new Signal({ timeout: 5000 })

// Wait for next emission
try {
    const value = await signal.wait()
    console.log('Got:', value)
} catch {
    console.log('Timeout')
}

Clean APIs

Simple extraction: Create minimal, focused APIs by extracting just the methods you need.

const signal = new Signal()

// Extract methods to create a clean interface
const { emit, on } = {
    emit: signal.extractEmit(),
    on: signal.extractOn()
}

// Clean usage
on(data => console.log(data))
emit('Hello!')

With full type documentation: For larger applications, create well-documented, type-safe APIs with extracted methods.

class UserAPI {
    /** @type {Signal<[string, {id: number, name: string}]>} */
    #signal = new Signal()

    constructor() {
        /**
         * Emit user events
         * @param {string} event - Event type ('login'|'logout'|'update')
         * @param {{id: number, name: string}} user - User data
         * @returns {void}
         */
        this.emit = this.#signal.extractEmit()

        /**
         * Subscribe to user events
         * @param {(event: string, user: {id: number, name: string}) => void} handler
         * @returns {{ off: () => void }} Subscription handle
         */
        this.on = this.#signal.extractOn()
    }
}

// Usage remains type-safe
const api = new UserAPI()
api.on((event, user) => console.log(`${event}: ${user.name}`))
api.emit('login', { id: 1, name: 'John' })

Memory Management

Control signal's memory and subscription lifecycle. Useful for cleanup and managing long-living signals.

// Clear memory but keep listeners
signal.forget()

// Remove listeners but keep memory
signal.wipe()

// Clear everything
signal.wipe(true)

TypeScript Support

const signal = new Signal<string>()
signal.on((data: string) => console.log(data))

// Multiple arguments
const multiSignal = new Signal<[number, string]>()
multiSignal.on((num, str) => console.log(num, str))

License

MIT License - Volodymyr Ishchenko - feel free to use this project commercially.

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With love ❤️ from Ukraine 🇺🇦