Events

Typically, event names are represented by a camel-cased string, however,
there aren’t any strict restrictions on that, as any string will be accepted.

Functions can then be attached to objects, to be executed when an event
is emitted. These functions are called listeners. Inside a listener
function, this refers to the EventEmitter that the listener was
attached to.

To access the EventEmitter class, require('events').EventEmitter.

When an EventEmitter instance experiences an error, the typical action is
to emit an 'error' event. Error events are treated as a special case in node.
If there is no listener for it, then the default action is to print a stack
trace and exit the program.

All EventEmitters emit the event 'newListener' when new listeners are
added and when a listener is removed.

emitter.on(event, listener)

Adds a listener to the end of the listeners array for the specified event.
No checks are made to see if the listener has already been added. Multiple
calls passing the same combination of event and listener will result in the
listener being added multiple times.

Returns emitter, so calls can be chained.

emitter.once(event, listener)

Returns emitter, so calls can be chained.

emitter.removeListener(event, listener)

Remove a listener from the listener array for the specified event.
Caution: changes array indices in the listener array behind the listener.

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 event, then removeListener must be called
multiple times to remove each instance.

Returns emitter, so calls can be chained.

Removes all listeners, or those of the specified event. It’s not a good idea to
remove listeners that were added elsewhere in the code, especially when it’s on
an emitter that you didn’t create (e.g. sockets or file streams).

Returns emitter, so calls can be chained.

emitter.setMaxListeners(n)

By default EventEmitters will print a warning if more than 10 listeners are
added for a particular event. This is a useful default which helps finding
memory leaks. Obviously not all Emitters should be limited to 10. This function
allows that to be increased. Set to zero for unlimited.

EventEmitter.defaultMaxListeners

emitter.setMaxListeners(n) sets the maximum on a per-instance basis.
This class property lets you set it for all instances,
current and future, effective immediately. Use with care.

Note that emitter.setMaxListeners(n) still has precedence over
EventEmitter.defaultMaxListeners.

emitter.listeners(event)

Returns an array of listeners for the specified event.

Execute each of the listeners in order with the supplied arguments.

Returns true if event had listeners, false otherwise.

Class Method: EventEmitter.listenerCount(emitter, event)

Return the number of listeners for a given event.

Event: ‘newListener’

• event {String} The event name
• listener {Function} The event handler function

This event is emitted any time a listener is added. When this event is triggered,
the listener may not yet have been added to the array of listeners for the event.

Event: ‘removeListener’

• event {String} The event name
• listener {Function} The event handler function