It would be something like this assuming your passing json in the post request:

app.post('/api/employees', function(req, res) {
    if(req.body) {
      userData.push(req.body)
    }
    else {
      res.statusCode = 500
      return res.json({Error: ['Object Missing']});
    }
});

It would be something like this assuming your passing json in the post request: Your request body would be like this:

{
  "id": "1",
  "firstName": "First Name",
  "lastName": "Last Name"
}

app.post('/api/employees', function(req, res) {
    if(req.body) {
      userData.push(req.body)
    }
    else {
      res.statusCode = 500
      return res.json({Error: ['Object Missing']});
    }
});