events

The events module provides a small EventEmitter implementation for JSH. Many built-in JSH modules use this class as the base for event-driven APIs.

Typical usage looks like this.

const EventEmitter = require('events');

EventEmitter

Creates a new event emitter instance.

Syntax

new EventEmitter()

The implementation stores listeners per event name and returns the emitter instance from most mutating methods so calls can be chained.

on()

Registers a listener for an event.

Syntax

emitter.on(event, listener)

listener must be a function, otherwise TypeError is thrown.

addListener()

Alias of on().

Syntax

emitter.addListener(event, listener)

once()

Registers a listener that runs only once.

Syntax

emitter.once(event, listener)

After the first invocation, the listener is removed automatically.

removeListener()

Removes one matching listener.

Syntax

emitter.removeListener(event, listener)

off()

Alias of removeListener().

Syntax

emitter.off(event, listener)

removeAllListeners()

Removes all listeners for one event, or all events when called without an argument.

Syntax

emitter.removeAllListeners()
emitter.removeAllListeners(event)

emit()

Emits an event and passes all remaining arguments to listeners.

Syntax

emitter.emit(event, ...args)

Return value

• true if at least one listener existed for the event
• false otherwise

If a listener throws while handling a non-error event and the emitter has an error listener, the emitter forwards the error through emit('error', err).

Introspection helpers

listeners()

Returns a shallow copy of the current listeners for an event.

emitter.listeners(event)

listenerCount()

Returns the number of listeners registered for an event.

emitter.listenerCount(event)

eventNames()

Returns the registered event names.

emitter.eventNames()

Listener limits

setMaxListeners()

Sets the warning threshold for listener counts.

emitter.setMaxListeners(n)

getMaxListeners()

Returns the current listener warning threshold.

emitter.getMaxListeners()

The default maximum is 10 listeners per event. When the limit is exceeded, the implementation writes a warning using console.warn(), but it still keeps the listeners.

Usage example

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const EventEmitter = require('events');

const emitter = new EventEmitter();
emitter.on('greet', function(name) {
    console.println('Hello, ' + name + '!');
});

emitter.emit('greet', 'Alice');
emitter.emit('greet', 'Bob');

once() example

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const EventEmitter = require('events');

const emitter = new EventEmitter();
emitter.once('greet', function(name) {
    console.println('Hello, ' + name + '!');
});

emitter.emit('greet', 'Alice');
emitter.emit('greet', 'Bob');

Behavior notes

• The module exports the EventEmitter class directly.
• This is a lightweight implementation, not a full drop-in replacement for Node.js events.
• Listener arrays are copied during emit(), so removing listeners while emitting does not affect the current dispatch pass.
• Only one warning mechanism is provided for too many listeners; it does not prevent registration.