Node.js : Write new data to an existing json file - json

Question

I'm trying to add data to an existing json file (codes below). When I access the locahost, the new piece of data shows up, however, when I check the data (users.json), the new piece of data (i.e. user4) isn't there.
Does anyone know what's wrong with the code? Thank you!
var express = require('express');
var app = express();
var fs = require("fs");
var user = {
"user4" : {
"name" : "mohit",
"password" : "password4",
"profession" : "teacher",
"id": 4
}
}
app.get('/addUser', function (req, res) {
// First read existing users.
fs.readFile( __dirname + "/" + "users.json", 'utf8', function (err, data) {
data = JSON.parse( data );
data["user4"] = user["user4"];
console.log( data );
res.end( JSON.stringify(data));
});
})
var server = app.listen(8081, function () {
var host = server.address().address
var port = server.address().port
console.log("Example app listening at http://%s:%s", host, port)
})
EDIT:
I added fs.writeFile(...) (codes below). After running the code, the only content of the uers.json file is:utf8
var express = require('express');
var app = express();
var fs = require("fs");
var user = {
"user4" : {
"name" : "mohit",
"password" : "password4",
"profession" : "teacher",
"id": 4
}
}
app.get('/addUser', function (req, res) {
// First read existing users.
fs.readFile( __dirname + "/" + "users.json", 'utf8', function (err, data) {
data = JSON.parse( data );
data["user4"] = user["user4"];
console.log( data );
// res.end( JSON.stringify(data));
data = JSON.stringify(data);
fs.writeFile(__dirname+"/"+"users.json", "utf8", function(err,data){
if (err){
console.log(err);
};
res.end(data);
});
});
})

Answer 1

To write to a file you should use fs.writeFile.
fs.writeFile(__dirname + "/" + "users.json", user["user4"], 'utf8', function()
{
// do anything here you want to do after writing the data to the file
});

Answer 2

I have passed data to writeFile so that it may write the information in data variable to JSON
fs.readFile( __dirname + "/" + "users.json", 'utf8', function (err, data) {
data = JSON.parse( data );
data["user4"] = user["user4"];
console.log( data );
data = JSON.stringify(data);
fs.writeFile(__dirname + "/" + "users.json", data , 'utf8', function(err,data) {
if (err){
console.log(err);
};
res.end(data);
});
});