Express API: Error [ERR_HTTP_HEADERS_SENT]

Question

I know this question has been asked a lot but none of the answers served my case. I keep getting this error even though I send a response to client only once. Please could anyone point to me what I'm missing? I appreciate any help. Here is my router code:

router.get("/trainers/:page/:limit", verifyToken, (req, res) => {

  const numPerPage = req.params.limit; // number of items per page
  const page = req.params.page;
  let result; //final object to be sent to client

  connection.query("Select count(*) as totalCount from trainers", (err, rows, fields) => {
    if (err) {
      return res.sendStatus(500).json({error: err})
    }
    var totalCount = rows[0].totalCount;
    var numPages = Math.ceil(totalCount / numPerPage);

    connection.query("SELECT id FROM trainers LIMIT ? OFFSET ?", [numPerPage, (page-1) * numPerPage], (err, rows, fields) => {
      if (err) {
        return res.sendStatus(500).json({error: err})
      } else {
        result = { rows, numPages }
        res.json()
      }
    })
  })
})

here is the error log:

throw err; // Rethrow non-MySQL errors ^

Error [ERR_HTTP_HEADERS_SENT]: Cannot set headers after they are sent to the client at ServerResponse.setHeader (_http_outgoing.js:526:11) at ServerResponse.header (/Users/diego/Documents/DT Personal Training App/express-api/node_modules/express/lib/response.js:771:10) at ServerResponse.send (/Users/diego/Documents/DT Personal Training App/express-api/node_modules/express/lib/response.js:170:12) at ServerResponse.json (/Users/diego/Documents/DT Personal Training App/express-api/node_modules/express/lib/response.js:267:15) at Query. (/Users/diego/Documents/DT Personal Training App/express-api/routes/admin/trainers.js:53:36) at Query. (/Users/diego/Documents/DT Personal Training App/express-api/node_modules/mysql/lib/Connection.js:526:10) at Query._callback (/Users/diego/Documents/DT Personal Training App/express-api/node_modules/mysql/lib/Connection.js:488:16) at Query.Sequence.end (/Users/diego/Documents/DT Personal Training App/express-api/node_modules/mysql/lib/protocol/sequences/Sequence.js:83:24) at Query.ErrorPacket (/Users/diego/Documents/DT Personal Training App/express-api/node_modules/mysql/lib/protocol/sequences/Query.js:92:8) at Protocol._parsePacket (/Users/diego/Documents/DT Personal Training App/express-api/node_modules/mysql/lib/protocol/Protocol.js:291:23) { code: 'ERR_HTTP_HEADERS_SENT' }

Yes can you try using res.status(500); res.json({error: err}) — Owner
replace: return res.sendStatus(500).json({error: err}) to return res.status(500).json({error: err}) I suspect sendStatus is sending headers and closing connection before sending content-type headers (for json method) — num8er
@num8er i'm getting this result: says its syntax error: "error": { "code": "ER_PARSE_ERROR", "errno": 1064, "sqlMessage": "You have an error in your SQL syntax; check the manual that corresponds to your MySQL server version for the right syntax to use near ''2' OFFSET 0' at line 1", "sqlState": "42000", "index": 0, "sql": "SELECT id FROM trainers LIMIT '2' OFFSET 0" } — Diego
Ah so the main error for which the question was raised is resolved. It was because of using setStatus and res.json together — Owner
const numPerPage = parseInt(req.params.limit); const page = parseInt(req.params.page); — num8er

Syed Zubair Ahmed Syed Zubair Ahmed · Accepted Answer · 2020-05-11T09:55:43

As per express docs

res.sendStatus(500) // equivalent to res.status(500).send('Internal Server Error')

so you need to use res.status(500).json({error: err}) to avoid this error :)

Express API: Error [ERR_HTTP_HEADERS_SENT]

1 Answers