Handling errors in express async middleware

Question

I have an async middleware in express, because I want to use await inside it, to clean up my code.

const express = require('express');
const app = express();

app.use(async(req, res, next) => {
    await authenticate(req);
    next();
});

app.get('/route', async(req, res) => {
    const result = await request('http://example.com');
    res.end(result);
});

app.use((err, req, res, next) => {

    console.error(err);

    res
        .status(500)
        .end('error');
})

app.listen(8080);

The problem is that when it rejects, it doesn't go to my error middleware, but if I remove the async keyword and throw inside a middleware it does.

app.get('/route', (req, res, next) => {
    throw new Error('Error');
    res.end(result);
});

So I'm getting UnhandledPromiseRejectionWarning instead of entering my error handling middleware, how can I let the error bubble up, and express handle it?

Answer 1

The problem is that when it rejects, it doesn't go to my error middleware, but if I remove the async keyword and throw inside a middleware it does.

express doesn't support promises currently, support may come in the future release of express@5.x.x

So when you pass a middleware function, express will call it inside a try/catch block.

Layer.prototype.handle_request = function handle(req, res, next) {
  var fn = this.handle;

  if (fn.length > 3) {
    // not a standard request handler
    return next();
  }

  try {
    fn(req, res, next);
  } catch (err) {
    next(err);
  }
};

The problem is that try/catch won't catch a Promise rejection outside of an async function and since express does not add a .catch handler to the Promise returned by your middleware, you get an UnhandledPromiseRejectionWarning.

---

The easy way, is to add try/catch inside your middleware, and call next(err).

app.get('/route', async(req, res, next) => {
    try {
        const result = await request('http://example.com');
        res.end(result);
    } catch(err) {
        next(err);
    }
});

But if you have a lot of async middlewares, it may be a little repetitive.

Since I like my middlewares as clean as possible, and I usually let the errors bubble up, I use a wrapper around async middlewares, that will call next(err) if the promise is rejected, reaching the express error handler and avoiding UnhandledPromiseRejectionWarning

const asyncHandler = fn => (req, res, next) => {
    return Promise
        .resolve(fn(req, res, next))
        .catch(next);
};

module.exports = asyncHandler;

Now you can call it like this:

app.use(asyncHandler(async(req, res, next) => {
    await authenticate(req);
    next();
}));

app.get('/async', asyncHandler(async(req, res) => {
    const result = await request('http://example.com');
    res.end(result);
}));

// Any rejection will go to the error handler

---

There are also some packages that can be used

• async-middleware
• express-async-handler

Answer 2

Answer with asyncHandler is good and usefull, but it is still not comfortable to write this wrapper in every route. I propose to improve it:

const asyncHandler = fn => (req, res, next) => {
    return Promise
        .resolve(fn(req, res, next))
        .catch(next)
}

const methods = [
    'get',
    'post',
    'delete'  // & etc.
]

function toAsyncRouter(router) {
    for (let key in router) {
        if (methods.includes(key)) {
            let method = router[key]
            router[key] = (path, ...callbacks) => method.call(router, path, ...callbacks.map(cb => asyncHandler(cb)))
        }
    }
    return router
}

and now we can do that way:

const router = toAsyncRouter(express().Router())
router.get('/', someAsyncController)

and so one.

Minute ago added a npm module async-express-decorator.