Creating new JSON objects every time [duplicate]

I am trying to create a login form and the functionality I want is the following: every time the user enters his email and password, it should get stored as a new JSON object in a JSON file. Right now, what’s happening is that a single object is getting created and it is being overwritten every time the user logs in. I want a new object for every login attempt. Here is the code:

app.post("/login", validate, (req, res) => {
  let userDataArray = new Array();
  let obj = new Object();
  let email = req.body.email;
  let password = req.body.password;
  obj.email = email;
  obj.password = password;

  // const jsonString = JSON.stringify(user);
  userDataArray.push(obj);
  let jsonString = JSON.parse(JSON.stringify(userDataArray));
  fs.writeFile("data.json", JSON.stringify(jsonString), () => {
    console.log("Written to file");
  });

  res.send(`Email: ${email} Password: ${password}`);
});