sqlite3 callbacks (node.js)

Question

I'm trying to compare an entered email on my website, to ones in the database to see whether it already exists. If it does, then the function returns false and an error is displayed.

var db = new sqlite3.Database('users_db.db');

db.get(
    "SELECT * FROM users WHERE useremail = ?", 
    [email], 
    function (err, rows) {
        if (rows == undefined ){
            global.returnvalue2 = false;
        }
    }
);

What I want is for the function to be run immediately after the selection, so that the returned value is false, and the user record is not created.

At the moment I realise that the callback is being called after everything, so its just making the selection and carrying on throughout the rest of the program until the end.

How can I check if there are any existing records with the same email?

@Soren Because the system runs through 3 checks for the email, password and the "class". If all 3 return true, then the record is created. Because node is skipping this last check of the email, it is returning 'true', and therefore the record is created before the callback is run. — Constantly Confused
– Constantly Confused, Commented Sep 13, 2015 at 16:02
So make a cascading callback -- you cannot do this with sequential programming — Soren
– Soren, Commented Sep 13, 2015 at 16:37

Soren · Accepted Answer · 2015-09-13 16:46:20Z

Make use of the async features in javascript, so your code would look something like this;

var db = new sqlite3.Database('users_db.db');
function checkemail(email, cb) {
    db.get(
         "SELECT * FROM users WHERE useremail = ?", 
         [email],  
         function (err, rows) {
            if (err || rows == undefined ){
                cb("bad email", null)
            } else {
                cb(null,rows)
            } 
         });
}
function checkpassword(pw,cb) {....}
function checkclass(cls,cb) {....}

and then write you code like this;

checkemail(myemail, function(err,rows) {
    if (err) return alert(err);
    checkpassword(pw, function(err, msg) {
       if (err) return alert(err);
       checkclass(cls, function(err, msg) {
           if (err) return alert(err);
           alert("Congratulation you passed all the checks");
       });
    });
});

Thank you so much! I had to edit it to fit my code format, and also at if (err || rows == undefined ) I changed to !=, as being undefined means the email doesn't exist. You're the best <3

dCode · Accepted Answer · 2017-08-15 15:04:08Z

Here's a little one I made.

const sqlite3 = require('sqlite3').verbose();
let db = new sqlite3.Database('iHacks.db');

function get_user_credentials (email, password)
{ return new Promise((rs, rj) => {
  function callback (err, User)
  {
    if (err) rj(err);
    rs(User);
  }

  db.get('select * from users where email=? and password=?', [email, password], callback);
}); }

function login (email, password)
{ return new Promise ((rs, rj) => {
  // Hashing the password.
  password = sha256(password + 'EzSalt');

  // Creating an Error
  const err = new Error('Email or password did not match!');

  // Callback functions
  function check (User)
  {
    rs(User);
  }      

  function fail (err)
  {
    rj(err);
  }

  // Getting the user credentials
  get_user_details(email, password).then(check).catch(fail);    

}); }

login()
  .then(/* Continue code */)
  .catch(err => {throw new Error(err); })
  ;

While a code-only answer is indeed an answer, a bit of explanation (what is the general idea behind your code, important points, warnings, ...) could make it a better answer.

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