javascript - wrong sequence of compiling

Question

I want to check, if a user is already in my database (sqlite3).

If the user exists: return 500 (ERROR!!)
If the user not exists: return 200 (ok)

Thats my script: node.js+express on serverside.

app.post('/adduser', function(req,res){
    db.serialize(function(){
        var user = req.body.user;
        var password = req.body.password;
        var err = false;

        db.each("SELECT name FROM users", function(err, row) {
            if (user == row.name) {
                console.log(row.name);
                err = true;
                console.log("a: " +err);
                res.send(500);
                return false;
            }
        });

        if (err==false) {
            console.log("b: " +err);
            res.send(200);
        }

    })
});

I get always the success code 200 regardless of whether I submitting an existing username or not. Thats the log from server:

b: false
testuser
a: true

I've entered an exisiting user, but it got code 200. Strange, I get b before a.

Javascript isn't compiled it is ran, it isn't converted into anything, so the word "compile" is incorrect. — Noam Rathaus
– Noam Rathaus, Commented Dec 21, 2013 at 18:59
Why would you return 500 (internal server error) if the user exists. — adeneo
– adeneo, Commented Dec 21, 2013 at 18:59
db.each doesn't (since it takes a callback) seem to run synchronously, so your if(err==false) line will execute long before getting the first result from the database. — Joachim Isaksson
– Joachim Isaksson, Commented Dec 21, 2013 at 19:04
err is the database error, rather than record not found. Amend your first if instead. — punund
– punund, Commented Dec 21, 2013 at 19:13
I want to notify my client, that the user is already in my database and it can't be added. Therefore I use the 500 code — Maddy
– Maddy, Commented Dec 21, 2013 at 19:19

andy137 · Accepted Answer · 2013-12-21 19:30:14Z

Your database queries are called asynchronously. Even if you specify db.serialize, which will cause sqlite to run its sql-queries in the order you specified, the DB query is detached.

This means that the response from the DB (your anonymous callback function in "db.each ...") is called when a result is received.

However, the program is run further, calling your "if (err==false) ..." first.

For example:

    var err = false; // called first

    db.each("SELECT name FROM users", function(err, row) {
       // called 3rd (after result from DB/disk)
    });

    if (err==false) {
       // called second
    }

The db.each() function also accepts a third parameter (see API Documentation), to be called when all queries have finished. According to this, your code should look like:

app.post('/adduser', function(req,res){
  db.serialize(function(){
    var user = req.body.user;
    var password = req.body.password;
    var err = false;

    db.each("SELECT name FROM users", function(err, row) {
        if (user == row.name) {
            console.log(row.name);
            err = true;
            console.log("a: " +err);
            res.send(500);
            return false;
        }
    }, function() {
        if (err==false) {
            console.log("b: " +err);
            res.send(200);
        }
    });
  })
});

When coding node.js, you should always keep in mind that any disk/io/net/db/etc. is called async and the code in the callback will be executed after the following code.

Thanks for your answer. I will vote up your answer, if I have enough reputation

clansaur · Accepted Answer · 2013-12-21 19:18:08Z

0

The 'each' function will call back asynchronously with each of the rows in the result set. You can add a completion handler as a final argument that will be called once all rows have been returned.

db.each("SELECT name FROM users", function(dberr, row) {
    if (user == row.name) {
        console.log(row.name);
        err = true;
        console.log("a: " +err);
        res.send(500);
         return false;
    }
}, function() { // Completion Handler
    if (err==false) {
        console.log("b: " +err);
        res.send(200);
    }
});

NOTE: I've also modified the first argument to your first callback function so that there is no scope confusion.

answered Dec 21, 2013 at 19:18

clansaur

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