get all passing student and count - node.js

Question

i want to getting document above mark 35, but i didn't success.
my api
router.get('/pass_student', async function(req, res, next){
await mark.find()
.exec()
.then(result => {
for(var j = 0; j < result.length; j++){
for(var i = 0; i < result[0].mark.length; i++){
console.log(result[j].mark[i].mark)
if(result[j].mark[i].mark >= 35){
const response = {
count: result.length,
}
res.send(response)
}
}
}
})
.catch(err => {
res.send(err);
})
})
my model
const mongoose = require('mongoose');
const markSchema = new mongoose.Schema({
mark:[{
mark: String
}],
subject:{
type: mongoose.Schema.Types.ObjectId,
ref: 'subject'
},
student:{
type: mongoose.Schema.Types.ObjectId,
ref: 'student'
}
})
module.exports = mongoose.model("mark", markSchema);
node:6044) UnhandledPromiseRejectionWarning: Error [ERR_HTTP_HEADERS_SENT]: Cannot set headers after they are sent to the client

Answer 1

Try this,
router.get('/pass_student', function(req, res, next){
mark.find()
.exec()
.then(result => {
let response;
for(var j = 0; j < result.length; j++){
for(var i = 0; i < result[0].mark.length; i++){
console.log(result[j].mark[i].mark)
if(result[j].mark[i].mark >= 35){
response = {
count: result.length,
}
}
}
res.send(response);
}
})
.catch(err => {
res.send(err);
})
});

Answer 2

Few observations I'd like to share based on your code are:
1. You are mixing async/await and promises, if you're calling an asynchronous method with await, then there is no need to use then/catch blocks. Also two things to be remember always while using await
the immediate parent function in which you are using await should be declared with async and
Always use try catch block to handle errors with async/await
2. The Error you are getting is because the res.send(response); statement is written inside For loop. In express you can send the response only once for a single request. In your case the server sends response after the first iteration in loop and on the next iteration it throws the error you got.
3. The response you are sending is not the actual result you want. It will always give you the length of the records returned from db i.e your count calculation logic is wrong.
Now lets come to the solution:
The easiest and best solution would be to change the type of marks in your schema to array of numbers instead of string, which actually makes sense. And by changing it to number you can easily get the count using mongoose query only. In that case your code and query will be:
router.get('/pass_student', async (req, res) => {
try {
const count = mark.find({mark: {$gt: 35}}).count();
return res.status(200).json({count});
/*
OR you can do;
const result = mark.find({mark: {$gt: 35}}).lean();
return res.status(200).json({count: result.length});
*/
} catch(err) {
return res.status(500).json({err});
}
});
Or if you don't wanna change the type of your field, change your code like:
router.get('/pass_student', async (req, res) => {
try {
const result = mark.find({}).lean();
const count = 0;
result.forEach(elem => {
const temp = elem.mark.find(i => Number(i) >= 35);
if(temp) count = count + 1;
});
return res.status(200).json({count});
} catch(err) {
return res.status(500).json({err});
}
});
Do let me know if you have any issues.
Hope this helps :)