Node.js and Express: How to return response after asynchronous operation

Question

I'm new to Node.js, so I'm still wrapping my head around asynchronous functions and callbacks. My struggle now is how to return a response after reading data from a file in an asynchronous operation.

My understanding is that sending a response works like this (and this works for me):

app.get('/search', function (req, res) {
    res.send("request received");
});

However, now I want to read a file, perform some operations on the data, and then return the results in a response. If the operations I wanted to perform on the data were simple, I could do something like this -- perform them inline, and maintain access to the res object because it's still within scope.

app.get('/search', function (req, res) {
    fs.readFile("data.txt", function(err, data) {
        result = process(data.toString());
        res.send(result);
    });
});

However, the file operations I need to perform are long and complicated enough that I've separated them out into their own function in a separate file. As a result, my code looks more like this:

 app.get('/search', function (req, res) {
    searcher.do_search(res.query);
    // ??? Now what ???
});

I need to call res.send in order to send the result. However, I can't call it directly in the function above, because do_search completes asynchronously. And I can't call it in the callback to do_search because the res object isn't in scope there.

Can somebody help me understand the proper way to handle this in Node.js?

Answer 1

To access a variable in a different function, when there isn't a shared scope, pass it as an argument.

You could just pass res and then access both query and send on the one variable within the function.

---

For the purposes of separation of concerns, you might be better off passing a callback instead.

Then do_search only needs to know about performing a query and then running a function. That makes it more generic (and thus reusable).

searcher.do_search(res.query, function (data) {
    res.send(...);
});

function do_search(query, callback) {
    callback(...);
} 

Answer 2

The existing answers are perfectly valid, you can also use async/await keywords since ES2017. Using your own function:

app.get('/search', async(req, res, next) {
  try {
    const answer = await searcher.do_search(req.query);
    res.send(answer);
  }
  catch(error) {
    return next(error);
  }
});