const fs = require('fs');

fs.writeFile("/tmp/test", "Hey there!", function(err) {
    if(err) {
        return console.log(err);
    }
    console.log("The file was saved!");
}); 

// Or
fs.writeFileSync('/tmp/test-sync', 'Hey there!');

var fs = require('fs');

fs.open('mynewfile2.txt', 'w', function (err, 
  file) {
  if (err) throw err;
  console.log('Saved!');
});

fs.appendFile('<fileName>',<contenet>, callbackFunction)

fs.writeFile('<fileName>',<contenet>, callbackFunction)

fs.open('<fileName>',<file_open_mode>, callbackFunction)