now-deployment/node_modules/now/dist/util/once.ts

import { EventEmitter } from 'events';

export function once<T>(emitter: EventEmitter, name: string): Promise<T> {
  return new Promise((resolve, reject) => {
    function cleanup() {
      emitter.removeListener(name, onEvent);
      emitter.removeListener('error', onError);
    }
    function onEvent(arg: T) {
      cleanup();
      resolve(arg);
    }
    function onError(err: Error) {
      cleanup();
      reject(err);
    }
    emitter.on(name, onEvent);
    emitter.on('error', onError);
  });
}