Listen to res.sendFile errors in Express

Try this one, refer to the document res.sendFile

var options = {
    root: __dirname + '/public/',
    dotfiles: 'deny'
  };

  var fileName = path.resolve(folder, runId, 'temp.html');
  res.sendFile(fileName, options, function (err) {
    if (err) {
      console.log(err);
      res.status(err.status).end();
    }
    else {
      console.log('Sent:', fileName);
    }
  });

Please have a detailed look at the documentation:
http://expressjs.com/en/4x/api.html#res.sendFile

According to the documentation, your code should look like this:

router.post('/foo', function(req,res,next){
    var file = path.resolve(folder, runId, 'temp.html');
    res.sendFile(file, options, function(err) {
        if(err) {
            res.status(err.status).end()
        }
    });
});

Hope this helps