Support multiple events in `on()` #1

@sindresorhus :wave: we've implemented support for multiple events in Probot's "event emitter", it's definitely been useful. We just test for an Array as the eventName then call .on() for each item.

I'm actually a fan of space-separated event names like in jQuery' .on, but I guess Array is fine too.

I'd prefer to be explicit and use arrays containing strings. #12 is a problem since technically, at the moment, event names can be arrays.

It would be useful to change the way to pass the event name so that the listener signature does not change if it starts to listener to multiple events.

it could use the context but it rules out the use of arrow function:

interface Event {
  name: string;
}

class Emittery {
  on(name: string|string[], listener: (this: Event, data: any) => any): () => void {}
  onAny(listener: (this: Event, data: any) => any): () => void {}
}

Or the data and the event could be sent together:

interface Event {
  name: string;
  data: any;
}

class Emittery {
  on(name: string|string[], listener: (ev: Event) => any): () => void {}
  onAny(listener: (ev: Event) => any): () => void {}
  // Would behave like `Promise.race` but would unsubscribe the waiting events
  once(name: string|string[]): Promise<Event>
}

Alternatively, there could be an extra method:

class Emittery {
  on(name: string, listener: (data: any) => any): () => void {}
  onMulti(name: string[], listener: (name: string, data: any) => any): () => void {}
  onAny(listener: (name: string, data: any) => any): () => void {}
}

@novemberborn Any opinion on the above? I'm leaning towards an additional method.

I'm leaning towards an additional method.

👍

An other option, closer to OP, would be to switch the argument order for multiple events:

class Emittery {
  on(name: string|string[], listener: (data: any, name: string) => any): () => void {}
  onAny(listener: (data: any, name: string) => any): () => void {}

  // maybe
  once(name: string|string[]): Promise<[any, string]>
}

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