Unable to get document using document.findOne() - node.js

Question

I tried to get a document using document.findOne() but it's value is showing undefined .
Here is my code
`app.post("/studentlogin",(req,res)=>
{
let password;
console.log("login page");
bcrypt.hash(req.body.password,saltRounds,(err,hash)=>
{
const user= Student.findOne({srno:req.body.srno});
console.log(user.srno);
if(req.body.srno==user.srno && hash==user.password)
{
session=req.username;
session.userid=req.body.srno;
res.redirect("/");
}
else{
console.log("invalid user");
res.redirect("/studentlogin");
}
});
})`
I'm implementing session authentication using express-session. Here when I log the user it's showing schema and bunch of other stuff which I don't know(The error is too long) . user.srno is also showing as undefined. How can I fix it?
I tried using call-back function which gave me required document correctly. But I want the query to return the correct document and store it in user.
Using callback function
`app.post("/studentlogin",(req,res)=>
{
let password;
console.log("login page");
bcrypt.hash(req.body.password,saltRounds,(err,hash)=>
{
Student.findOne({srno:req.body.srno},(err,result)=>
{
console.log(result);
});
//console.log(user.srno);
if(req.body.srno==user.srno && hash==user.password)
{
session=req.username;
session.userid=req.body.srno;
res.redirect("/");
}
else{
console.log("invalid user");
res.redirect("/studentlogin");
}
});
})`

Answer 1

You need to wait the result from your query on the database before doing the next task like comparing your password, and looks like you just try to log in, you re not going to register a new one, so it's better to use the compare method in Bcrypt like this :
app.post("/studentlogin", async (req , res) => {
const {srno, password} = req.body // destructuring your request is better for visibility
try {
const user = await Student.findOne({srno: srno});//await the result before next step
console.log(user.srno) //you can check
if(!user) {
console.log("invalid user");
// your logic to tell not student found /wrong username or password, and/or redirect
}
const isMatch = await bcrypt.compare(password, user.password) //await the result and this method for comparing the request password and the user's password found
if(!isMatch) {
//your code to tell Wrong username or password
res.redirect("/studentlogin");
} else {
// your code to access to the login.
res.redirect("/");
}
} catch (err) {
console.log(err)
}

Answer 2

The error you are facing is because you are not using async await in your code. use async in your function definition then use await where you are searching the database.
const someFunction = async(req,res)=>{
// your code
const user=await Student.findOne({srno:req.body.srno});
// your code
}
Your code should look like this.
app.post("/studentlogin", async(req,res)=> {
// your code
const user=await Student.findOne({srno:req.body.srno});
// your code
}
console.log(user) to verify.
Hope it helps.