write-append-file.js

var f='write.txt',
    fs=require('fs');

fs.writeFile(f,'Some text to write.',function(err){
  if(err)
    console.error(err);
  console.log('Written!');
});

fs.appendFile(f,'Some more text to append.',function(err){
  if(err)
    console.error(err);
  console.log('Appended!');
});