Add async emit methods to EventEmitter #18646

• Version:
  v8.9.4

• Platform:
  Darwin pumba-5.local 17.4.0 Darwin Kernel Version 17.4.0: Sun Dec 17 09:19:54 PST 2017; root:xnu-4570.41.2~1/RELEASE_X86_64 x86_64

• Subsystem:
  events

https://nodejs.org/api/events.html#events_asynchronous_vs_synchronous instructs users to cause asynchronous event execution in the event handlers by using setImmediate or process.nextTick, which means that, when using EventEmitter#emit, asynchronous event handling is left to clients, and the EventEmitter has no control over whether an event is handled asynchronously or synchronously. In cases where the EventEmitter is sensitive to performance, the EventEmitter itself can dictate the event emission is asynchronous via code similar to the following:

class MyClass extends EventEmitter {
  // ...
  doSomething() {
    process.nextTick(() => this.emit.bind(this).call('myEvent', {foo:'bar'}))
    // ...
  }
}

The emit line above could be replaced by something convenient like

this.emitNextTick('myEvent', {foo:'bar'})
// or
this.emitImmediate('myEvent', {foo:'bar'})

or, even more conveniently,

this.emitAsync('myEvent', {foo:'bar'})

which uses either setImmediate or process.nextTick according to the current environment.