1. module
The EventEmitter class is defined and exposed by the events module.
2. Synchronous
When the EventEmitter object emits an event, all of the functions attached to that specific event are called synchronously. Any values returned by the called listeners are ignored and will be discarded.
The EventListener calls all listeners synchronously in the order in which they were registered. This is important to ensure the proper sequencing of events and to avoid race conditions or logic errors. When appropriate, listener functions can switch to an asynchronous mode of operation using the setImmediate() or process.nextTick() methods.
3. this
The eventEmitter.emit() method allows an arbitrary set of arguments to be passed to the listener functions. It is important to keep in mind that when an ordinary listener function is called by the EventEmitter, the standard this keyword is intentionally set to reference the EventEmitter to which the listener is attached.
It is possible to use ES6 Arrow Functions as listeners, however, when doing so, the this keyword will no longer reference the EventEmitter instance.
4. 'error' events
When an error occurs within an EventEmitter instance, the typical action is for an 'error' event to be emitted. These are treated as special cases within Node.js.
If an EventEmitter does not have at least one listener registered for the 'error' event, and an 'error' event is emitted, the error is thrown, a stack trace is printed, and the Node.js process exits.
To guard against crashing the Node.js process, a listener can be registered on the process object's 'uncaughtException' event.
As a best practice, listeners should always be added for the 'error' events.
5. 'newListener' Event
All EventEmitters emit the event 'newListener' when new listeners are added and 'removeListener' when existing listeners are removed.
Listeners registered for the 'newListener' event will be passed the event name and a reference to the listener being added.
The EventEmitter instance will emit its own 'newListener' event before a listener is added to its internal array of listeners.
The fact that the event is triggered before adding the listener has a subtle but important side effect: any additional listeners registered to the same name within the 'newListener' callback will be inserted before the listener that is in the process of being added.
6. 'removeListener' events
The 'removeListener' event is emitted after the listener is removed.
7. emitter.setMaxListeners
By default EventEmitters will print a warning if more than 10 listeners are added for a particular event. This is a useful default that helps finding memory leaks.
Obviously, not all events should be limited to just 10 listeners. The emitter.setMaxListeners() method allows the limit to be modified for this specific EventEmitter instance. The value can be set to Infinity (or 0) to indicate an unlimited number of listeners.
To change the default for all EventEmitter instances, the EventEmitter.defaultMaxListeners property can be used.
Take caution when setting the EventEmitter.defaultMaxListeners because the change effects all EventEmitter instances, including those created before the change is made. However, calling emitter.setMaxListeners(n) still has precedence over EventEmitter.defaultMaxListeners.
Note that this is not a hard limit. The EventEmitter instance will allow more listeners to be added but will output a trace warning to stderr indicating that a "possible EventEmitter memory leak" has been detected. For any single EventEmitter, the emitter.getMaxListeners() and emitter.setMaxListeners() methods can be used to temporarily avoid this warning.
The --trace-warnings command line flag can be used to display the stack trace for such warnings.
The emitted warning can be inspected with process.on('warning') and will have the additional emitter, type and count properties, referring to the event emitter instance, the event’s name and the number of attached listeners, respectively. Its name property is set to 'MaxListenersExceededWarning'.
8. emitter.emit(eventName[, ...args])
Synchronously calls each of the listeners registered for the event named eventName, in the order they were registered, passing the supplied arguments to each. Returns true if the event had listeners, false otherwise.
9. emitter.eventNames()
Returns an array listing the events for which the emitter has registered listeners. The values in the array will be strings or Symbols.
10. emitter.on(eventName, listener)
Adds the listener function to the end of the listeners array for the event named eventName. No checks are made to see if the listener has already been added. Multiple calls passing the same combination of eventName and listener will result in the listener being added, and called, multiple times.
By default, event listeners are invoked in the order they are added. The ·emitter.prependListener()· method can be used as an alternative to add the event listener to the beginning of the listeners array.
11. emitter.removeAllListeners([eventName])
Removes all listeners, or those of the specified eventName. Note that it is bad practice to remove listeners added elsewhere in the code, particularly when the EventEmitter instance was created by some other component or module (e.g. sockets or file streams).
12. emitter.removeListener(eventName, listener)
Removes the specified listener from the listener array for the event named eventName.
removeListener will remove, at most, one instance of a listener from the listener array. If any single listener has been added multiple times to the listener array for the specified eventName, then removeListener must be called multiple times to remove each instance.
Note that once an event has been emitted, all listeners attached to it at the time of emitting will be called in order. This implies that any removeListener() or removeAllListeners() calls after emitting and before the last listener finishes execution will not remove them from emit() in progress. Subsequent events will behave as expected.
Because listeners are managed using an internal array, calling this will change the position indices of any listener registered after the listener being removed. This will not impact the order in which listeners are called, but it means that any copies of the listener array as returned by the emitter.listeners() method will need to be recreated.
