Add combineSignals function

[aleclarson]

This function takes an array of AbortSignal objects, creates a new AbortController which listens to every provided signal, and returns a new AbortSignal.

Motivation: There are times where you have 2+ ways to abort an async operation, and their lifecycles differ, which means multiple AbortControllers (and therefore, multiple AbortSignals) are required.

Proposed implementation

type AbortSignal = {
  addEventListener: (event: 'abort', listener: () => void) => void
  removeEventListener: (event: 'abort', listener: () => void) => void
}

type AbortController<S extends AbortSignal> = {
  signal: S
  abort: () => void
}

declare const AbortController: {
  new <S extends AbortSignal>(): AbortController<S>
}

export function combineSignals<S extends AbortSignal>(signals: S[]): S {
  if (signals.length === 1) {
    return signals[0]
  }
  const controller = new AbortController<S>()
  const abort = () => {
    controller.abort()
    signals.forEach(signal => signal?.removeEventListener('abort', abort))
  }
  signals.forEach(signal => signal?.addEventListener('abort', abort))
  return controller.signal
}