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JavaScript User Authentication With Express and Mongo Sessions and Cookies Authenticating the Username and Password

Kartikey Singh
Kartikey Singh
4,592 Points

Can't understand callback function

UserSchema.statics.authentication = function(email, password, callback){
  User.findOne({ email: email})
    .exec(function(error, user){
      if(error){
        return callback(error);
      }
      else if(!user){
        var err = new Error('User not Found');
        err.status = 401;
        return callback(err);
      }
      bcrypt.compare(password, user.password, function(error, result){
        if(result === true){
          return callback(null, user);
        }
        else{
          return callback();
        }
      });
    });
}

User.authentication(req.body.email, req.body.password, function(err, user){
      if(err || !user){
        //
      }
}

I can't understand how callback function can execute with 2 arguments as well as 1. Sorry since I'm a beginner in javascript I really can't understand the flow of control like when the callback function is executed. Please help me out, Thanks

1 Answer

Christopher Debove
PLUS
Christopher Debove
Courses Plus Student 18,373 Points

Well that's quite an advanced course :o... Maybe you need to see the basics before.

The callback you're call take 2 parameters. Think of it like that :

let callback = function(error, data) {
  // Code of callback;
}

When your calling it with one argument (the error), the parameter error has a value and no data. When your operation is a success you're calling it with null as error and the data.

Nothing too complicated.

But like I said, trying to skip to some advance course like this (with frameworks and third party namespaces) is no good for a beginner. This could discourage you. I really advise you to check the beginner and intermediate courses.