Handling unhandled promise rejections in async functions

I am developing a HTTP proxy service and I have observed presence of an odd error message in my logs:

unhandledRejection RequestError: HTTP request error.
    at /dev/rayroute/raygun/src/factories/createRequest.js:107:13
    at processTicksAndRejections (internal/process/task_queues.js:93:5) {
  code: 'RAYGUN_REQUEST_ERROR',
  originalError: Error: test
      at /dev/rayroute/raygun/src/factories/createRequest.js:73:29
      at processTicksAndRejections (internal/process/task_queues.js:93:5)

It is odd because there are plethora of tests to ensure that all errors are handled. It is also odd because I have never never seen unhandled rejection while developing the service (only ever saw it in production logs).

The relevant code looks like this:

const activeRequestHandler = createRequest(requestDefinition);
if (incomingMessage.socket) {
  incomingMessage.socket.on('close', () => {
    if (responseIsReceived) {
      log.trace('client disconnected after response');
    } else {
      log.debug('client disconnected');
      activeRequestHandler.abort(new Error('CLIENT_DISCONNECTED'));
    }
  });
}
try {
  await actions.afterRequestActions(
    context,
    requestDefinition,
    activeRequestHandler
  );
} catch (error) {
  log.error({
    error: serializeError(error),
  }, 'afterRequest threw an error');
}
try {
  responseDefinition = await activeRequestHandler.response;
} catch (error) {
  log.warn({
    error: serializeError(error),
  }, 'an error occurred while waiting for a HTTP response');
  // [..]
}

It is pretty straightforward:

• createRequest initiates a HTTP request and returns a request handler
• the request handler can be used to abort the ongoing request (afterRequestActions aborts request after a hard-timeout); and
• it is used to resolve the response or eventual rejection of the promise

I have written tests to ensure that errors are handled when:

• request rejected
• request aborted
• afterRequestActions throws an error

, but all tests are passing.

It turns out that the problem was that in all my test cases actions.afterRequestActions was resolving/ being rejected before activeRequestHandler.response is resolved. Meanwhile, in production afterRequestActions contains logic that can take substantially longer to execute. I have also learned that even if you declare a try..catch block for your async function, if it resolves before it is await-ted, then you will get an unhandled rejection, i.e.

This code will not warn about unhandled rejection:

const delay = require('delay');
const main = async () => {
  const promise = new Promise((resolve, reject) => {
    setTimeout(() => {
      reject(new Error('Expected rejection.'));
    }, 100);
  });
  await delay(90);
  try {
    await promise;
  } catch (error) {
    console.error(error)
  }
};
main();

But this code will always produce a warning about an unhandled rejection:

const delay = require('delay');
const main = async () => {
  const promise = new Promise((resolve, reject) => {
    setTimeout(() => {
      reject(new Error('Expected rejection.'));
    }, 100);
  });
  await delay(110);
  try {
    await promise;
  } catch (error) {
    console.error(error)
  }
};
main();

The best solution is to add an auxiliary catch block, e.g. This is how I refactored my original code:

const activeRequestHandler = createRequest(requestDefinition);
// Without this we were getting occasional unhandledRejection errors.
// @see https://dev.to/gajus/handling-unhandled-promise-rejections-in-async-functions-5b2b
activeRequestHandler.response.catch((error) => {
  log.warn({
    error: serializeError(error),
  }, 'an error occurred while waiting for a HTTP response (early warning)');
});
if (incomingMessage.socket) {
  incomingMessage.socket.on('close', () => {
    if (responseIsReceived) {
      log.trace('client disconnected after response');
    } else {
      log.debug('client disconnected');
      activeRequestHandler.abort(new Error('CLIENT_DISCONNECTED'));
    }
  });
}
try {
  await actions.afterRequestActions(
    context,
    requestDefinition,
    activeRequestHandler
  );
} catch (error) {
  log.error({
    error: serializeError(error),
  }, 'afterRequest threw an error');
}
try {
  responseDefinition = await activeRequestHandler.response;
} catch (error) {
  log.warn({
    error: serializeError(error),
  }, 'an error occurred while waiting for a HTTP response');
  // [..]
}