Question

In my index.js, I have an exports function that is supposed to send data back to the client via ajax on pressing a submit button. However, when the user presses submit, the data seems to get sent over before it the data gets modified. When pressing submit one more time, it sends the data that was previously modified as if clicking the submit button only sends the ‘previously’ set data. This is my code:

        var tabledata = getRecordFromDatabase(key);
        if(tabledata.length === 0)
            tabledata = 'There is no matched record in the database';
        res.contentType('text/html');
        res.send({'matched':tabledata});

So to illustrate the error: I click submit after filling out a form and receive back the message “There is no matched record in the database”. I hit submit a second time without changing anything in the form I just filled. This time record data is actually sent to me. Why could this be?

Answer 1

If whatever you’re doing in getRecordFromDatabase is asynchronous and non-blocking, then node.js is behaving as it should. Node.js is non-blocking – it doesn’t stop and wait for processes to complete (unless those processes are intentionally written to block, which is usually avoided in node.js). This is beneficial, because it keeps the server free to accept new requests and process many requests at once.

If your database call is asynchronous, you’re not waiting for it to return before you res.send(). That’s why your first submit returns back empty. Most likely, by the time you hit submit a second time, your DB call has finally returned, and that’s why you get a result.

It’s hard to give you a code-based answer to your problem, because you abstracted away what is happening in your DB call method. But typically, an asynchronous call would go something like:

getRecordFromDatabase(key, function(err, data){
    if(data.length === 0)
        data = 'There is no matched record in the database';
    res.contentType('text/html');
    res.send({'matched':data});
});

This way, you are passing a function to execute as a callback to your asynchronous method – when the async call completes, it executes the callback, which then executes the res.send() with the appropriate data.