Handling errors in express async middleware

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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 [email protected]

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

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