Variable value remains unchanged after running multiple queries in Node JS-CodePudding

I am trying to check if a user with a given email id is registered previously. If it is, then a variable named uniqueUser, is set to false. However, this variable remains unchanged even when the condition is true. It's value changes locally, but once the promise is resolved, the value gets back to previous one.

Here are the codes :-

router.post('/register-user', multer_profile_pic.any(), (req, res, next) => {
   
    var uniqueUser = true; //----> This value is being focused
    const user = req.body;
   
    
    var query = `SELECT * FROM users WHERE email=$1`;
  
    var values = [user.email];

   db.query(query,values)
            .then(result => {
                if(result.rowCount > 0) {
                    uniqueUser = false;
                    console.log("uniqueUser=" uniqueUser); //----> Here value becomes false
                   
                }
                
            })
            .catch(err => {
                
                uniqueUser=false;
            });
    
   
    if(uniqueUser) {
        console.log(uniqueUser); //----> Here value becomes true
      //INSERT QUERY HERE
    }
    
    const josn_resp = {
        response: response,
        message:message
    }
    res.json(josn_resp);
});

CodePudding user response：

This problem happened because you don't understand the promise event loop how work.

In your code db.query(query,values) and if(uniqueUser) { execute in same time in event loop and actually your variable uniqueUser never changed.

You should change your code like bellow:

router.post('/register-user', multer_profile_pic.any(), (req, res, next) => {

  var uniqueUser = true; //----> This value is being focused
  const user = req.body;


  var query = `SELECT * FROM users WHERE email=$1`;

  var values = [user.email];

  db.query(query, values)
    .then(result => {
      if (result.rowCount > 0) {
        uniqueUser = false;
        console.log('uniqueUser='   uniqueUser); //----> Here value becomes false
      }

      if (uniqueUser) {
        console.log(uniqueUser); //----> Here value becomes true
        //INSERT QUERY HERE

        return db.query('insert-query');
      }

      return Promise.resolve();
    })
    .then(() => {
      const josn_resp = {
        response: response,
        message: message,
      };

      res.json(josn_resp);
    })
    .catch(err => {
      res.status(400);
      res.json({message: err.message})
    });
});

If you use nodeJs 12 and above I suggest use async/await:

router.post('/register-user', multer_profile_pic.any(), async (req, res, next) => {
  const user = req.body;

  try {
    const result = await db.query(`SELECT * FROM users WHERE email=$1`, [user.email]);
    if (result.rowCount === 0) {
      await db.query('insert-query');
    }

    const josn_resp = {
      response: response,
      message: message,
    };

    res.json(josn_resp);
  } catch (err) {
    res.status(400);
    res.json({message: err.message})
  }
});