nunjucks: Error: Can't set headers after they are sent. (Express 4)

I don’t use Express, so I probably won’t be able to be much help here. Usually that kind of error is because something is sending output to the client earlier than expected? I don’t have any idea why Nunjucks would be different from Swig in that regard, though.

because you might send no less than 2 times response.

like my code:

  getAllTables().then((data) => res.send(data)).catch((err) => console.log(err));
  next();

I used Promise.then() and my function next() goes before my res.send(), so I had response more than 1 times to front-end. like me, you can write like this.

  getAllTables().then((data) => {
    res.send(data);
    return next();
  }).catch((err) => console.log(err));

@carljm looks like that is the issue if I use

 res.end() 

Instead of

 res.render('template') 

All works fine. looks like asap lib and /Users/carlitux/Projects/project/node_modules/nunjucks/src/environment.js:22:23 are calling the callback before what is expected.

Happened with me while I was sending response twice and not using return with one of them.

router.get('/me', auth, async (req, res) => {
  const user = await User.findById(req.user._id);
  if(!user)  res.status(400).send("User not Found");

  res.send(user)
})

Here when user is not found res.send() is called twice so using return fixed this error.

router.get('/me', auth, async (req, res) => {
  const user = await User.findById(req.user._id);
  if(!user)  return res.status(400).send("User not Found");

  res.send(user)
})

So you can check if res.send() is not called twice as this is one case of getting this error.

I have this problem too. When refresh twice quickly. Then happened Any solution?

nevermind express-flash just tested with simple middleware:

function middleware (req, res, next) {
  return next();
}

and it is failing.

^This is a bug I feel anyone using express would run into. I too have the same issue.

If you have the template render after passing through middleware and the page is refreshed too quickly. The page errors and the app errors.

If you add this code to your project probably works …

const server = http.createServer((req,res)=>{
      res.setHeader('Content-Type', 'text/html');
      res.setHeader('X-Foo', 'bar');
      res.writeHead(200, { 'Content-Type': 'text/plain' });
      res.end('MehdiFilban solved this problem\nYour Header is set NOW\nGOOD LUCK...');
  }).listen(port,()=>{
      console.log('your app is started');
  });