EventEmitter Pattern

Some notes on the EventEmitter pattern and its workings.

A contrived example:

function logWhenSuccessful() {
  someOperation().on('SUCCESS', function () {
    console.log('Operation was successful');
  });
}

function someOperation() {
  let emitter = new EventEmitter();
  emitter.emit('SUCCESS');
  return emitter; // this is to allow chaining with 'on'
}

logWhenSuccessful();

The idea is that when someOperation() is invoked, it emits a SUCCESS event, which should in turn trigger the callback with console.log, but it does not work.

The reason is that the entire script is ran synchronously. When someOperation is called, it emits the SUCCESS event before the listener .on('SUCCESS'...) could be registered. Conceptually, this would work if someOperation were an asynchronous function: when someOperation is called, it would be delegated to background processes and would pass the execution control back to logWhenSuccessful, which would immediately register a SUCCESS event listener. When the asynchronous someOperation completes, it would emit the event and would work as expected.

EventEmitter in Asynchronous Scenario

The asynchronous scenario could be simulated with the following:

function logWhenSuccessful() {
  someOperation().on('SUCCESS', function () {
    console.log('Operation was successful');
  });
}

function someOperation() {
  let emitter = new EventEmitter();
  process.nextTick(function () {
    emitter.emit('SUCCESS');
  });
  return emitter;
}

logWhenSuccessful();

process.nextTick() defers the execution of emitter.emit until the next pass of the event loop, thus allowing the listener to be registered first, before event emitting occurs.

EventEmitter Class Pattern

A perhaps cleaner alternative is to have someOperation extend directly from EventEmitter. This allows an instance of someOperation to register listeners first before calling any functions:

class SomeOperation extends EventEmitter {
  executeOperation() {
    this.emit('SUCCESS');
  }
}

function logWhenSuccessful() {
  let someOperation = new SomeOperation();
  someOperation.on('SUCCESS', function () {
    console.log('Operation was successful');
  });
  someOperation.executeOperation();
}

lowWhenSuccessful();