How to add text sequentialy to a file with javascript

I have this code:

const fs = require("fs");

const saveFile = (fileName, data) => {
  return new Promise((resolve) => {
    fs.writeFile(fileName, data, (err) => {
      resolve(true);
    });
  });
};

const readFile = (fileName) => {
  return new Promise((resolve) => {
    fs.readFile(fileName, "utf8", (err, data) => {
      resolve(data);
    });
  });
};

const filename = "test.txt";

saveFile(filename, "first");

readFile(filename).then((contents) => {
  saveFile(filename, contents + " second");
});

readFile(filename).then((contents) => {
  saveFile(filename, contents + " third");
});

I’m hoping to obtain in ‘test.txt’

first second third

but instead, I get

first thirdd

The idea is that every time I receive a certain post request. I have to add more text to the file

Does someone have any solution for this?

Thank you so much!