Best way to handle mixed synchronous and asynchronous errors in ExpressJS

Question

Using NodeJS 10.13.0, ExpressJS 4.16.4... Got a controller handling a route that looks like this:

import { Request, Response, NextFunction } from 'express';
import braintree from 'braintree';
import config from '../../server.config';

export function index(req: Request, res: Response, next: NextFunction): void {
  if(!config.braintree) throw new Error('Braintree configuration missing.');
  const gateway: any = braintree.connect(config.braintree); // synchronous
  return gateway.clientToken.generate({})
    .then(response => res.send(response.clientToken))
    .catch(next) // operational error unless config.braintree was changed
}

Reading the ExpressJS docs on error handling, I'm wondering if I'm following best practices - throwing an Error for the synchronous portion and passing the error to next() in the catch for the asynchronous portion.

Any recommendations for improvement?

Answer 1

Considering that promises are in use, both synchronous and asynchronous errors can be consistently handled with async function:

export async function index(req: Request, res: Response, next: NextFunction) {
  try {
    if(!config.braintree)
      throw new Error('Braintree configuration missing.');

    const gateway: any = braintree.connect(config.braintree);
    const response = await gateway.clientToken.generate({})
    res.send(response.clientToken);
  } catch (err) {
    next(err);
  }
}

Since Express doesn't support promises, async function body should be wrapped with try..catch. Considering that try..catch is common for all async middleware functions, it could be moved to a helper:

const asyncMiddleware = (middleware: RequestHandler) => async (req: Request, res: Response, next: NextFunction) => {
  try {
    await middleware(req, res, next);
  } catch (err) {
    next(err)
  }
};

Which is used like:

export const index = asyncMiddleware(async (...) => {...});