title: How to add to existing data in a JSON file using javascript?
body: <p>I used <code>writeFile()</code> function, but the new data overwrites the existing data. How can I fix this code?
  Here's what I have right now :</p>
  
  <pre><code>var obj = {username:user_name, password:password}; 
    jsonfile.writeFile(file, obj, function (err) {  
        console.error(err);
  }); 
  </code></pre>