This is my code:

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MongoClient.connect(CONNECTION_STRING, (err, client) => {
       const db = client.db("issues");
       app.post('/api/issues/:project', function (req, res) {
           const project = req.params.project;
           const title = req.body.issue_title;
           const text = req.body.issue_text;
           const creator = req.body.created_by;
           if (!(title && text && creator)) {
               res.status(400).json({err: "missing required field"});
           } 

           const assigned = req.body.assigned_to || '';
           const statusText = req.body.status_text || '';
           const createdOn = new Date();

           db.collection(project).insertOne({
               "issue_title": title, "issue_text": text, "created_by": creator,
               "assigned_to": assigned, "status_text": statusText, "created_on": createdOn,
               "updated_on": createdOn, "open": true
           }, {}, (err, doc) => {
               if (err) {
                   return res.status(500);
               } else res.json(doc.ops[0]);
           })

       });

This is the very interesting error I got:

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

This is because the below piece of code has sent the response to user, but the function keeps executing, and attempts to send another request to user.

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if (!(title && text && creator)) {
  res.status(400).json({err: "missing required field"});
}