Save html-form data in json format in a .json file using node and express with javascript

Newbie in node and express I am taking user input from html-form and trying to append or push it in a .json file. I have used jsonfile npm-package but it is not coming in a array format of json

code for appending-

var express = require('express');
var app = express();

//jade --> ejs -->html 
app.engine('html', require('ejs').renderFile);
app.set('views', path.join(__dirname, 'views'));
app.set('view engine', 'html');

var jsonfile = require('jsonfile');    
var file = './userdata.json'


    //trying to write via form to json
    app.post('/gettingdata', function(req, res) {
        var user_id = req.body.usrid;
        var token = req.body.usrphone;
        var geo = req.body.usrdata;

        //start writing
        var obj = { name: user_id , phone: token, adress: geo }
        jsonfile.writeFileSync(file, obj, {flag: 'a'});

        //default
        //res.send(user_id + ' ' + token + ' ' + geo);
    }); 

html -

<body>
    <form action="/gettingdata" method="post">
            Name:<input type="text" name="usrid" /><br>
            Phone:<input type="text" name="usrphone" /><br>
            RData:<input type=="text" name="usrdata" /><br>
            <input type="submit" value="Submit" >
    </form>
</body>

json appearing as-

{"name":"name1","phone":"8989898989","adress":"random1"}
{"name":"name1","phone":"767656568","adress":"randomdata1"}
{"name":"name1","phone":"767656568","adress":"randomdata1"}

there are no commas appearing between objects and no square brackets. I need them to be able to make parsing possible, so that I can dynamically edit and delete data from my front-end later. Suggest any link,method or npm package to do so.If question repeated share the link of that too