ExpressJS中间件还捕获错误？

Question

I'm writing middleware that I'm applying at the route level, like so:

router.get('/foo', myMiddleware, (req, res) => { ... });

so I can do some stuff with the request. But I also need to catch errors to do some special handling. I know I can add a handler afterwards like this:

... (req, res) => { ... }, myErrorHandler);

and it'll get called just fine.

But my question is, is there any way to have a single piece of middleware that can do all of this so I don't need two points of integration? I tried calling req.on('error', (err) => { ... }) within my middleware but it never seems to be called.

Answer 1

Express comes with a built-in error handler that takes care of any errors that might be encountered in the app. This default error-handling middleware function is added at the end of the middleware function stack.

// Router
router.get('/foo', myMiddleware, (req, res) => { ... });

// Router Error Handler
router.use(function (err, req, res, next) {

});

Answer 2

I ended up solving this by writing a helper function that wraps the actual handler. It looks like this:

function checkPage(handler: express.RequestHandler) {
    return async (req: express.Request, res: express.Response, next: express.NextFunction) => {
        let _write = res.write;
        res.write = chunk => {
            if (req.query.verbose) {
                return _write.call(res, `<p>${chunk}</p>`);
            } else {
                return true;
            }
        }

        try {
            await handler(req, res, next);
            res.write('<hr/><p style="color:green;"><b>happy</b></p>');
        } catch (err) {
            res.write(`<p style="color:red;">${err}</p>`);
            res.write('<hr/><p style="color:red;"><b>SAD!</b></p>')
        }

        res.end();
    }
}

Then in my route handler, I just use it like so:

router.get('/foo', checkPage(async (req, res, next) => {
    ...
    res.write('stuff');
    ...
}));