node.js/express: Can't set headers after they are sent

When using res.writeHead() the headers are immediately sent to the requester. Since you're using the Express function res.send(), which also sends over your headers, they have already been sent and you will get an error saying the headers have already been sent. Alternatively, you could use res.end() which will not cause a headers sent error since it will finalize the response and send whatever remaining chunks are passed to it.

how can I ensure that res.writeHead is called before res.send?

If you're going to use res.send(), change your res.writeHead() calls to res.setHeader() to set the header values on your response object. Additionally, when using Express you can set the status of the response using res.status(<statusCode>).

Another thing I noticed is you usage of Buffer. You should change your Buffer creation in the success response handler from Buffer() to Buffer.from() as Buffer() is deprecated.

router.get('/getFile', (req, res) => {    console.log("Calling getFile for file " + req.query.serialNumber + ".")    let serialNumber = req.query.serialNumber    let request = new sql.Request(conn)    let query = 'SELECT FileName + \'.\' + FileExtension AS \'File\', FileType, ContentType, SerialNumber, Chart ' +        'FROM dbo.ChangeFiles ' +        'WHERE SerialNumber = ' + serialNumber    request.query(query)        .then(recordset =>{            console.log("Successfully retrieved file " + recordset[0].SerialNumber + " from database.")            let {ContentType, Chart File} = recordset[0]            // Set Response Status and Headers            res.status(200)            res.setHeader('Content-Type', ContentType)            res.setHeader('Content-Disposition', `attachment;filename=${File}`)                        return res.send(Buffer.from(Chart))        }).catch(err => {            console.log(err);            return res.status(500).send("Issue querying database!");        })})