无法弄清楚为什么我不断收到 [ERR_HTTP_HEADERS_SENT] 错误

Question

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

I have seen other posts about this same issue, but I wasn't able to figure out how to fix my problem. I have code handling a post request like so:

router.post('/', function(req, res) {
  var movies = JSON.parse(fs.readFileSync('movies.json'));
  var title = req.body.title;
  movies.forEach(function(movie) {
    if (title == movie.title) {
      return res.render('movie', { watched: false });
    } else {
      return res.render('movie', { watched: true });
    }
  });
});

I understand the issue is due to res.render() being there twice, but even with a return statement it still gives me the same error so I am having a hard time understanding how it is getting sent twice. Shouldn't it only be sending once due to the return keyword?

Also, if I modify my code like so:

router.post('/', function(req, res) {
  var movies = JSON.parse(fs.readFileSync('movies.json'));
  var title = req.body.title;
  movies.forEach(function(movie) {
    if (title == movie.title) {
      return res.render('movie', { watched: false });
    }
  });
res.render('movie', { watched: true});
});

Then I only recieve the error when the if statement is false. Why is that the case?

Answer 1

You are returning inside for loop so each time it will send

modify your code like below

router.post('/', function(req, res) {
  var movies = JSON.parse(fs.readFileSync('movies.json'));
  var title = req.body.title;
  var counter =0;
  var arr = [];
  movies.forEach(function(movie) {
    if (title == movie.title) {
      arr.push({ watched: false , movieId:id})      
    } else {
      arr.push({ watched: true , movieId:id})
    }
    counter++;
   if(counter == movies.length){
      return res.send({'movie':arr});
   }
  });
});

Answer 2

Turns out the solution I needed was a lot simpler than I was thinking. I needed to pass a boolean to the render function based on the result of the if statement, so rather than calling res.render() twice, I just created a new variable to store the result of the if statement and then feed that into res.render() so it only gets called once.

router.post('/', function(req, res) {
  var movies = JSON.parse(fs.readFileSync('movies.json'));
  var title = req.body.title;
  var isWatched = false;
  for (let value in movies) {
      if (movies[value].title === title) {
        isWatched = true;
      }
    }
  res.render('movie', { watched: isWatched} );
});