ERR_HTTP_HEADERS_SENT: Cannot set headers after they are sent to the client

That particular error occurs whenever you try to send more than one response to the same request and is usually caused by improper asynchronous code.

The code you show in your question does not appear like it would cause that error, but I do see code in a different route here that would cause that error.

Where you have this:

if (!user) {
  errors.email = "User not found";
  res.status(404).json({ errors });
}

You need to change it to:

if (!user) {
  errors.email = "User not found";
  res.status(404).json({ errors });
  // stop further execution in this callback
  return;
}

You don't want the code to continue after you've done res.status(404).json({ errors }); because it will then try to send another response.

In addition, everywhere you have this:

if (err) throw err;

inside an async callback, you need to replace that with something that actually sends an error response such as:

if (err) {
    console.log(err);
    res.sendStatus(500);
    return;
}

throwing inside an async callback just goes back into the node.js event system and isn't thrown to anywhere that you can actually catch it. Further, it doesn't send a response to the http request. In otherwords, it doesn't really do what the server is supposed to do. So, do yourself a favor and never write that code in your server. When you get an error, send an error response.

Since it looks like you may be new here, I wanted to compliment you on including a link to your full source code at https://github.com/lourdesr/devlog because it's only by looking at that that I was able to see this place where the error is occuring.

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I was receiving this error because of a foolish mistake on my part. I need to be more careful when referencing my other working code. The truly embarrassing part is how long I spent trying to figure out the cause of the error. Ouf!

Bad:

return res
  .send(C.Status.OK)
  .json({ item });

Good:

return res
  .status(C.Status.OK)
  .json({ item });

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Use ctrl + F hotkey and find all 'res.' keywords then replace them with a prefix of 'return ',

change  all 'res.' to  'return res.'

something like this:

res.send() change to --> return res.send()

maybe you have 'res.' in some block, like if() statement

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Sorry for the Late response, As per the mongoose documentation "Mongoose queries are not promises. They have a .then() function for co and async/await as a convenience. However, unlike promises, calling a query's .then() can execute the query multiple time"

so to use promises

mongoose.Promise = global.Promise //To use the native js promises

Then

var promise = Profile.findOne({ user: req.user.id }).exec()
promise.then(function (profile){
    if (!profile) {
      throw new Error("User profile not found") //reject promise with error
    }
    return res.status(200).json(profile) //return user profile      
}).catch(function (err){
    console.log(err); //User profile not found
    return res.status(404).json({ err.message }) //return your error msg
})

here is an nice article about switching out callbacks with promises in Mongoose

and this answer on mongooses promise rejection handling Mongoose right promise rejection handling

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There is a simple fix for the node error [ERR_HTTP_HEADERS_SET]. You need to add a return statement in front of your responses to make sure your router exits correctly on error:

router.post("/", async (req, res) => {
    
    let user = await User.findOne({email: req.body.email});   
    if (!user) **return** res.status(400).send("Wrong user");
    
});

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Because of multiple response sending in your request. if you use return key word in your else condition your code will run properly

if (!profile) {
    return res.status(404).json({ error: "No Profile Found" });
}
else {
    **return** res.json(profile);
}

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I got the same error using express and mongoose with HBS template engine. I went to Expressjs and read the docs for res.render, and it says // if a callback is specified, the rendered HTML string has to be sent explicitly. So I wasnt originally sending my html explicitly in the callback,. This is only for a contact form btw, not login info, albeit GET

//Original
let { username, email } = req.query;  //My get query data easier to read

res.status(200).render('index', { username, email });

//Solution without error. Second param sending data to views, Third param callback

res.status(200).render('index', { username, email }, (err, html)=>{
      res.send(html);
 });

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you have to enable Promises in your programm, in my case i enabled it in my mongoose schema by using mongoose.Promise = global.Promise . This enables using native js promises.

other alternatives to this soloution is :

var mongoose = require('mongoose');
// set Promise provider to bluebird
mongoose.Promise = require('bluebird');

and

// q
mongoose.Promise = require('q').Promise;

but you need to install these packages first.

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My problem besides not returning, i was forgetting to await an asynchronous function in the handler. So handler was returning and after a bit the async function did its thing. 🤦🏻‍♀️

Before:

req.session.set('x', {...});
req.session.save();
return req.status(200).end();

When i needed to await:

req.session.set('x', {...});
await req.session.save();
return req.status(200).end();

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In react, if your are calling the function in useEffect hook, make sure to add a dependency to the dependency Array.

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I had this error from an if statement not having an else block.

if(someCondition) {
await () => { } 

}

await () => { } 

I changed the above to this below and solved my issue

if(someCondition) {
await () => { } 

} else { 
await () => { } 
}

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This also happen when you tries to send the multiple response for a same request !! So make sure you always use return keyword to send response to client inorder to stop the further processing !!

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Where you have this:

if (!user) { errors.email = "User not found"; res.status(404).json({ errors }); }

You need to change it to:

if (!user) { errors.email = "User not found"; return res.status(404).json({ errors }); }

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