In my current project using the MERN stack, I have noticed that certain functions in the server-side related to fetching or saving data in MongoDB involve a lot of callbacks. To prevent getting stuck in callback hell, I am exploring a promises-based approach. However, I have run into an issue with this promised-based solution. There is a specific condition where I need to either continue executing the code or return a response and halt.
const mId = req.body.mId;
const cId = req.body.cId;
const lId = req.body.lId;
LP.findOne({
'userId': lId,
'courseId': cId
})
.then( lP => {
return lP.materials;
})
.then( materials => {
if( materials.includes(mId) ) {
console.log('A')
return res.json({'status': 'success'})
}
else {
materials.push(materialId)
return LP.findOneAndUpdate({
'userId': learnerId,
'courseId': courseId
}, {
$set: { materials: materials}
}, {
new: true
})
}
})
.then( update => {
console.log('B')
return res.json({ 'status': 'success' })
})
.catch( err => {
return res.json({ 'status': 'fail' })
})
In the above code snippet, after A is printed, B is also displayed, triggering further execution which results in the error "Cannot set headers after they are sent to the client." While I understand this behavior is typical of promises, I am looking for possible solutions to avoid this problem. How can I return a response early on without continuing the code execution?
Thank you.