How to make 2 middleware work together?

Question

JavaScript

I needed to write a middleware to check the size of the POST body, if it is greater than some value, return an error, otherwise go further.
I wrote this function:

function middleware(len = 10) {
  return (req, res, next) => {
    req.len = 0;
    req.on('data', function(chunk) {
      req.push(chunk);
      req.len += chunk.length;
      if (req.len > len) {
        res.write('Error');
        res.end();
      }
    });
    req.on('end', function() {
      if (req.len <= len) {
        next();
      }
    });
  };
}

app.use(middleware(10))
app.post('/',(req,res)=>{res.end('It works'))

Now if you run
curl http://host -d var=foo
Then it displays It works, and if you run

curl http://host -d var=SomeBigVar00000000000000000

it displays an error. Everything seems to be fine, but if you add `app.use(bodyParser.urlencoded());` between the first `app.use` and `app.post` to parse the body, then this middleware does not work.
How to make it work?