Methods

on(eventName, listener)

Registers a listener for a given event. Multiple listeners can be registered for the same event.

Parameters:

• eventName (string) – Name of the event to listen to.
• listener (function) – Callback function to execute when the event is emitted.

Returns: The EventEmitter instance (supports chaining).

Example:

          
            1const emitter = new EventEmitter();3emitter.on("data", (msg) => {4  console.log("Received:", msg);5});7emitter.emit("data", "Hello World"); // Logs: Received: Hello World
          
        

Edge Cases:

• Multiple listeners can be attached to the same event; all will be called in order of registration.
• Chaining allows multiple on calls in sequence: emitter.on("a", fn1).on("b", fn2).

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once(eventName, listener)

Registers a one-time listener that will be automatically removed after the first invocation.

Parameters:

• eventName (string) – Event name.
• listener (function) – Callback function.

Returns: The EventEmitter instance (supports chaining).

Example:

          
            1emitter.once("connect", () => console.log("Connected!"));3emitter.emit("connect"); // Logs: Connected!4emitter.emit("connect"); // No output
          
        

Edge Cases:

• The listener is removed immediately after execution.
• Safe to attach multiple once listeners to the same event.

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emit(eventName, ...args)

Triggers an event, calling all registered listeners with the provided arguments.

Parameters:

• eventName (string) – Event to trigger.
• ...args – Any number of arguments to pass to listeners.

Returns: The EventEmitter instance (supports chaining).

Example:

          
            1emitter.on("update", (oldVal, newVal) => {2  console.log(`Changed from ${oldVal} to ${newVal}`);3});5emitter.emit("update", 1, 2); // Logs: Changed from 1 to 2
          
        

Edge Cases:

• If no listeners exist for the event, emit does nothing.
• Errors in one listener do not prevent other listeners from executing; they are logged to console.

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off(eventName, listener)

Removes a specific listener for an event. If no listener is provided, all listeners for that event are removed.

Parameters:

• eventName (string) – Event name.
• listener (function, optional) – Specific listener to remove.

Returns: The EventEmitter instance (supports chaining).

Example:

          
            1const fn = (msg) => console.log(msg);2emitter.on("data", fn);3emitter.off("data", fn); // Removes fn
          
        

Edge Cases:

• Calling off with no listener removes all listeners for the event.
• Safe to call off on an event that has no listeners; does nothing.

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removeAll(eventName)

Utility method to remove all listeners for a specific event.

Parameters:

• eventName (string) – Event name.

Returns: The EventEmitter instance (supports chaining).

Example:

          
            1emitter.on("x", () => {});2emitter.removeAll("x"); // All listeners for "x" removed
          
        

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listeners(eventName)

Returns an array of all registered listeners for an event.

Parameters:

• eventName (string) – Event name.

Returns: Array of functions.

Example:

          
            1emitter.on("msg", () => {});2console.log(emitter.listeners("msg").length); // 1
          
        

Edge Cases:

• Returns an empty array if no listeners are registered.

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