unhandledRejection.js

function unhandledRejection(promise, reason) {
  ...

  const emit = (reason, promise, promiseInfo) => {
    try {
      ...
      return process.emit('unhandledRejection', reason, promise);
    } finally {
      popAsyncContext(asyncId);
    }
  };

  maybeUnhandledPromises.set(promise, {
    reason,
    uid: ++lastPromiseId,
    warned: false,
    domain: process.domain,
    emit
  });
  ...
  ...
}