ExpressJS doesn't wait for my promise

Question

I'm making a search-page on my server. When the endpoint is reached and the user waits for the search function to return the results and render the page Express falls through to the 404 handler instead, and I get the following error when I suppose the render function is called:

Error: Can't set headers after they are sent.

What am I doing wrong?

router.get("/", async (req, res) => {
    try {
        const queryString = req.query.q;

        const user = helper.checkAndGetUser(req, res);

        let s = String(queryString), searchedTags = [""];
        if(s.indexOf(",") > -1){
            searchedTags = s.replace(" ", "").split(",");
        }

        const options = {
            "query": {tags: {$all: searchedTags}, _forSale: true}
        };

        const results = await Search.search(options).then(result => result).catch(err => {
            throw err;
        });

        //This res.render -call is called after the 404 splat-route.
        return res.render("partial/search.pug", {user: user, search: {
            query: queryString,
            results: results
        }});

        //If I'd use res.send for debugging, it is instead called before the splat-route, like the following:
        return res.send(results);
    } catch(err) {
        next(err);
    }
});

module.exports = router;

I register the router:

const search = require("./search.js");
app.use("/search", search);

Followed by the 404 splat-route:

app.get("*", async (req, res, next) => {

    const user = helper.checkAndGetUser(req, res);

    res.status(404);
    res.render("partial/404.pug", {user: user});
});

To clarify: My question is how can I make the res.render function get called just as the res.send function?

UPDATE [2017-10-05]: I continued with another part of the site, a similar endpoint, and discovered that sending the result provided by the promise worked as expected if using res.send but not res.render. Using res.render the 404-handler kicked in again. Can this be a bug in Express?

Answer 1

This happens if you attempt to write to res after it is sent, so you must be calling additional code after res.render() or you already responded before calling that.

change it to return res.render(...) so it exits the functions, otherwise it will continue through the function and hit other res.render()s etc.

Something is up with that error handler also. I will update my post in a few mins with tips (on phone). It should probably have (req, res, next) and call return next(err) and pass it to your error handling middleware.

Here is the pattern I like to use in async/await Express:

// these routes occur in the order I show them

app.get('/route', async (req, res, next) => {
    try {
        const data = 'asdf'
        const payload = await something(data)
            .then((result) => createPayload(result))

        // remember, if you throw anywhere in try block, it will send to catch block
        // const something = willFail().catch((error) => {
        //     throw 'Custom error message:' + error.message
        // })

        // return from the route so nothing else is fired
        return res.render('route', { payload })
    } catch (e) {
        // fire down to error middleware
        return next(e)
    }
})

// SPLAT
app.get('*', async (req, res, next) => {
    // if no matching routes, return 404
    return res.status(404).render('error/404')
})

// ERRORS
app.use(async (err, req, res, next) => {
    // if err !== null, this middleware fires
    // it has a 4th input param "err"
    res.status(500).render('error/500')
    // and do whatever else after...
    throw err
})

Note: next() callback called without param is treated as no error, and proceeds to the next middleware. If anything is passed in, it will fire the error middleware with the param as the value of err in the error handling middleware. You can use this technique in routes and other middlewares, as long as the error middleware comes last. Mind your use of return with res.send/render() to prevent double setting headers.

NEW:

Something looks a little bit off with that .then() having a callback in it. I don't see logically where err would come from since the value of the resolved promise goes into the .then() function as result. At this point, it is suspect and should be removed or refactored if possible. This part here:

try {
    let results = [];
    await Search.search(options).then(result => {
        results = result;
    }, err => {
        throw err;
    });

    console.log("res.render");
    return res.render("partial/search.pug", {user: user, search: {
        query: string,
        results: results
    }});
} catch(err) {
    next(err);
}

First, here is about what I would expect to see with async/await syntax:

router.get("/", async (req, res, next) => {

    try {
        const queryString = req.query.q;
        const user = helper.checkAndGetUser(req, res);

        let s = String(queryString), searchedTags = [""];
        if (s.indexOf(",") > -1) {
            searchedTags = s.replace(" ", "").split(",");
        }
        const options = {
            "query": { tags: { $all: searchedTags }, _forSale: true }
        };

        // If a promise is ever rejected inside a try block,
        // it passes the error to the catch block.
        // If you handle it properly there, you avoid unhandled promise rejections.

        // Since, we have async in the route function, we can use await
        // we assign the value of Search.search(options) to results.
        // It will not proceed to the render statement
        // until the entire promise chain is resolved.
        // hence, then(data => { return data }) energizes `results`
        const results = await Search.search(options)
            .then(data => data)
            // If any promise in this chain is rejected, this will fire
            // and it will throw the error to the catch block
            // and your catch block should pass it through to your
            // error handling middleware
            .catch(err => { throw 'Problem occurred in index route:' + err });

        return res.render("partial/search.pug", {
            user: user, search: {
                query: string,
                results: results
            }
        });
    } catch (err) {
        // look at the top how we added next as the 3rd, callback parameter
        return next(err);
    }
});

module.exports = router;

Error handler:

// notice how we add `err` as first parameter
app.use((err, req, res, next) => {

    const user = helper.checkAndGetUser(req, res);

    res.status(404);
    res.render("partial/404.pug", {user: user});
});

From the Express docs:

Define error-handling middleware functions in the same way as other middleware functions, except error-handling functions have four arguments instead of three: (err, req, res, next). For example:

app.use(function (err, req, res, next) {
  console.error(err.stack)
  res.status(500).send('Something broke!')
})

http://expressjs.com/en/guide/error-handling.html

That might be your true issue because the error handler should only fire if next() is called with any input, but yours appears to be firing every time like a normal middleware, so I suspect it's because there is no err parameter on that middleware function, so it is treated as a normal one.

The Default Error Handler

Express comes with a built-in error handler, which 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.

If you pass an error to next() and you do not handle it in an error handler, it will be handled by the built-in error handler; the error will be written to the client with the stack trace. The stack trace is not included in the production environment.

If you call next() with an error after you have started writing the response (for example, if you encounter an error while streaming the response to the client) the Express default error handler closes the connection and fails the request.

So when you add a custom error handler, you will want to delegate to the default error handling mechanisms in Express, when the headers have already been sent to the client:

// code example in docs

Note that the default error handler can get triggered if you call next() with an error in your code more than once, even if custom error handling middleware is in place.

I also recommend using that splat route app.get('*', async (req, res, next) => {}) right above the error handler middlware (aka as the last loaded route in your list). This will catch all unmatched routes, such as /sih8df7h6so8d7f and forward the client to your 404. I think the error handler middlware is more suited for error 500 and clean formatted type errors because it gives you a function that can parse the value of next(err) anytime it is called from a route.

I usually do this for authentication failures with JSON web token (as the first line of code inside every auth required route):

if (!req.person) return res.status(403).render('error/403')

I realize some of this may fry your wig wholesale, so try all this stuff out and see each piece working before you determine if you would like to utilize it or not.