I agree with above answers, Here is a complete read and write sample for anyone who needs it.
router.post('/', function(req, res, next) {
console.log(req.body);
var id = Math.floor((Math.random()*100)+1);
var tital = req.body.title;
var description = req.body.description;
var mynotes = {"Id": id, "Title":tital, "Description": description};
fs.readFile('db.json','utf8', function(err,data){
var obj = JSON.parse(data);
obj.push(mynotes);
var strNotes = JSON.stringify(obj);
fs.writeFile('db.json',strNotes, function(err){
if(err) return console.log(err);
console.log('Note added');
});
})
});