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

Question

Node express crashes when the response is in a setInterval. I want to test some things in the frontend when the response is delayed a bit. For this I have passed the response as a callback to the setInterval function in my nodeJs Express:

router.post('/survey', userMiddleware.isLoggedIn, (req, res ,next) => {
  setInterval(() => {
     res.status(200).send({
      message: 'Upload success!'
    });
  }, 3000 );
});

However, I then receive an error:

node:_http_outgoing:576
    throw new ERR_HTTP_HEADERS_SENT('set');
    ^

Error [ERR_HTTP_HEADERS_SENT]: Cannot set headers after they are sent to the client
    at new NodeError (node:internal/errors:371:5)
    at ServerResponse.setHeader (node:_http_outgoing:576:11)
    at ServerResponse.header (/node/node-express-jwt-test/node_modules/express/lib/response.js:794:10)
    at ServerResponse.send (/node/node-express-jwt-test/node_modules/express/lib/response.js:174:12)
    at ServerResponse.json (/node/node-express-jwt-test/node_modules/express/lib/response.js:278:15)
    at ServerResponse.send (/node/node-express-jwt-test/node_modules/express/lib/response.js:162:21)
    at Timeout._onTimeout (/node/node-express-jwt-test/routes/router.js:83:22)
    at listOnTimeout (node:internal/timers:559:17)
    at processTimers (node:internal/timers:502:7) {
  code: 'ERR_HTTP_HEADERS_SENT'
}

Node.js v17.5.0
[nodemon] app crashed - waiting for file changes before starting...

What's going on?

Answer 1

You keep sending the response at an interval of 3000ms _ the delay is done but the it's repeated

Use the 'setTimeout(function, 3000)' to send once

Read more about set timeout here https://www.w3schools.com/jsref/met_win_setTimeout.asp

So your code Should be like this

 router.post('/survey', userMiddleware.isLoggedIn, (req, res ,next) => {
      setTimeout(() => {
          res.status(200).send({
           message: 'Upload success!'
     });
   }, 3000 );
 });