#timeout-middleware# Express middleware that intercepts the res.status, res.sendStatus and res.send functions and checks for whether the response has timed out or not.

##Why The res object in Express is a subclass of Node.js's http.ServerResponse. It is possible to call res.setHeader(name, value) as often as needed until res.writeHead(statusCode) is called. After writeHead, the headers are baked in and you can only call res.write(data), and finally res.end(data).

Because of the way middleware processing works, once a module passes the request to the next middleware, it can no longer stop the flow, so we need to check if the request has timed out before we can continue to act on the request.

The express module for request timeout connect-timeout does this with a middleware that checks for req.timedout. Which would be used (as top-level middleware) like this:

var express = require('express');
var timeout = require('connect-timeout');

// example of using this top-level; note the use of haltOnTimedout
// after every middleware; it will stop the request flow on a timeout
var app = express();
app.use(timeout('5s'));
app.use(bodyParser());
app.use(haltOnTimedout);
app.use(cookieParser());
app.use(haltOnTimedout);

// Add your routes here, etc.

function haltOnTimedout(req, res, next){
  if (!req.timedout) next();
}

app.listen(3000);

Above codesnippet is copied from: https://github.com/expressjs/timeout

Since we want to set a timeout on the response and try to keep ourselves DRY:
We intercept the usage of res.status, res.sendStatus and res.send with a wrapper that checks for the res.timedout bool that we put on the res object.
If res.timedout is true we return res.
Else we simply return the res function that was called with its arguments:

function wrap(res, fn) {
  return function(...args) {
    if (res.timedout) {
      console.log({res: args, fn: fn.name});
      return res;
    } else {
      return fn.apply(res, args);
    }
  };
}

This will then be used in the actual middleware function:

export default (timeoutValue) => {
    return (req, res, next) => {
      const {send, sendStatus, status} = res;
      res.setTimeout(timeoutValue);
      res.on('timeout', () => {
        res.timedout = true;
        if (!res.headersSent) {
          console.log({req, message: 'Request Timeout'});
          res.statusCode = 503;
          res.type('txt');
          send.apply(res, ['Request Timeout']);
        }
      });
      res.on('error'), error => {
        console.log(error);
      });
      res.send = wrap(res, send);
      res.sendStatus = wrap(res, sendStatus);
      res.status = wrap(res, status);
      next();
    };
};

First we use destructuring to extract the send, sendStatus and status functions from res. Then we set a timeout on the res object (see https://nodejs.org/api/http.html#http_class_http_serverresponse).

On timeout we set a boolean attribute called res.timedout to true. Then we check for the res.headersSent and if it return false we set the res.statusCode to 503, res.type to 'txt' and applies the [Request Timeout] to send.

On error we simply log the error to the console. This is needed since the request object is an EventEmitter and if you don't have a listener for the error event, an error will be thrown, which could crash your program.

Outside of the timeout listener we let the res.send use the wrapper function to intercept any timeouts for that function. We do the same with res.sendStatus and res.status.

Finally we proceed with next().

Now we can use it as a top-level middleware like this:

import express from 'express';
import timeout from 'timeout-middleware';
const app = express();

app.use(timeout(30000)); //timeout in milliseconds
app.use(bodyParser());
app.use(cookieParser());

app.listen(3000);