When I'm sending a POST request to MySql database, it adds the product but it shoots me with that error.

Error [ERR_HTTP_HEADERS_SENT]: Cannot set headers after they are sent to the client at new NodeError (node:internal/errors:387:5) at ServerResponse.setHeader (node:_http_outgoing:603:11) at ServerResponse.header (C:\Users\DELL\Desktop\Small project\node_modules\express\lib\response.js:794:10) at ServerResponse.send (C:\Users\DELL\Desktop\Small project\node_modules\express\lib\response.js:174:12) at Query. (C:\Users\DELL\Desktop\Small project\server.js:47:16) at Query. (C:\Users\DELL\Desktop\Small project\node_modules\mysql\lib\Connection.js:526:10) at Query._callback (C:\Users\DELL\Desktop\Small project\node_modules\mysql\lib\Connection.js:488:16) at Query.Sequence.end (C:\Users\DELL\Desktop\Small project\node_modules\mysql\lib\protocol\sequences\Sequence.js:83:24) at Query._handleFinalResultPacket (C:\Users\DELL\Desktop\Small project\node_modules\mysql\lib\protocol\sequences\Query.js:149:8) at Query.OkPacket (C:\Users\DELL\Desktop\Small project\node_modules\mysql\lib\protocol\sequences\Query.js:74:10) { code: 'ERR_HTTP_HEADERS_SENT'

app.post("/product", (req, res, next) => {
  let product = {
    product_title: "lool",
    product_price: 4444.12,
    product_type: "sneakers",
    product_brand: "lol",
  };
  let sql = `INSERT INTO products SET ?`;

  db.query(sql, product, (err, result) => {
    if (err) {
      throw err;
    }
    console.log(result);
    return res.send("Completed");
  });

  res.send("Added");
});

CodePudding user response：

You can only use res.send() once, when you use it a second time it throws this error. Just remove this res.send("Added").

CodePudding user response：

If you're writing asynchronous code, you must keep in mind the timing involved, and you must schedule your responses accordingly:

app.post("/product", (req, res, next) => {
  // ...
  db.query(sql, product, (err, result) => {
    // ...

    // Runs at some point in the distant future in terms of compute time
    return res.send("Completed");
  });

  // Runs *immediately*
  res.send("Added");
});

You cannot have that immediate res.send(), you must remove that. If you want to handle the error, that error handling happens inside the callback.¹

--

¹ This is why using async functions with await makes your code a lot easier to manage.