Question

I’m using Node.js + Express + nodejs-sqlite3 to make a form that when submited will insert a new row on an slite3 database.
On query sucess I want to write certain response.

So the small big problem is just: Modify a string that will be storing the html to be shown, inside the callback function of sqlite3.run()

I read about closures, and passing an object with methods to modify its own attributes. But it seems it’s not working. It will pass the object attributes and methods, but no change will remain when the callback function ends. I read that objects will be passed as reference, not copies.

This is the code:

app.post("/insert.html", function(req, res){
    function TheBody(){
        this.html = "";
        this.msg = "";
        this.num = "";
    }
    TheBody.prototype.add = function(string){
        this.html = this.html + string;
    }
    var body = new TheBody();

    body.msg = req.body.message;
    body.num = req.body.number;

    var insertCallback = function(data){
        return function(err){
            if( err != null){
                console.log("Can't insert new msg: " + err.message);
                data.add("ERROR-DB");                                               
            } else {
                console.log("Ok. Inserted: " + data.msg);
                console.log(data.html);
                data.add("OK - MSG: "+data.msg+" NUM: "+data.num);
                console.log(data.html);
            }
        };
    };                                                                                   

    var db = new lite.Database('database.db');
    var query = "INSERT INTO outbox (message, number) VALUES (?, ?)";
    db.run(query, [body.msg, body.num], insertCallback(body) );

    res.setHeader('Content-Type', 'text/html');
    res.setHeader('Content-Length', body.html.length);
    res.end(body.html);
}

On server side I’ll see

Ok. Inserted: TestString
[Blank space since data.html still has no information]
OK - MSG: TestString NUM: TestNumber [Showing that indeed was modified inside the function]

But on the client side res.end(body.html); will send an empty string.
The object is not being passed as reference.

What’s missing in the code, and what simpler alternatives I have to change a string variable inside a callback anonymous function?.

I already know I could use response.write() to write directly on the function if it were more simpler. But I discovered it would only work if I use response.end() inside the callback, otherwise (being outside as it is now) it will meet a race condition where the buffer will be closed before sqlite3.run() be able to use response.write().

——– Answered ——–

As hinted by Justin Bicknell and confirmed by George P. Nodejs-sqlite3 functions are run asynchronously. So I was ending the stream to the client before the callback would be called, thus nothing was being printed.
This was a problem more about “This is SPART- nodejs, so write your stuff according to events'” rather than a logic one. I found this kind of programming kind of convoluted but nobody else than me told me to use nodejs. For those wondering about how one could put some order over the order of queries on the database, nodejs-sqlite3 functions returns a database object that is used to chain the next query.

Since I was printing the information to the client just once in every handled event, the resulting object ended like this:

function TheBody(response){
    this.response = response;
}
TheBody.prototype.printAll = function(string){
    this.response.setHeader('Content-Type', 'text/html');
    this.response.setHeader('Content-Length', string.length);
    this.response.end(string);
} 

Preferring that to clutter all the code a lot of res.setHeader() lines.

Answer 1

node-sqlite3 methods are, by default, run in parallel (asynchronously). That means that your code is going through this timeline:

1. Your code calls db.run(...)
2. Your code calls res.end(...)
3. db.run completes and calls your callback.

This is the source of a huge number of questions here on SO, so you can almost certainly find a better answer than anything that I could write here in a reasonable amount of time.

I would start here: How does Asynchronous Javascript Execution happen? and when not to use return statement?