Nodejs json file writing

I am making a program that reads through an array containing hundreds of links, reads through each, and scrapes some text, then writes it in an output file as json.

I am having trouble formatting the json file, since .map() returns the json object like this:

{
   "id": "wajnh3ivnydeegrr",
   "lorem ipsum"
},{
   "id": "6yuyz57cmrgo5fbe",
   "message": "lorem ipsum"
},

Instead of a json array like this:

[{
   "id": "wajnh3ivnydeegrr",
   "lorem ipsum"
},{
   "id": "6yuyz57cmrgo5fbe",
   "message": "lorem ipsum"
}]

How can I push each json object into the array while formatted correctly? (the last trailing comma would have to be excluded, somehow). Really appreciate your help! The full request is below:

app.get('/scrape', function(req, res) {    

   dataDocument.map(function(item, err) {

    request(item, function(err, response, html) {
        if (!err) {

            //Variables
            var message, id, date;
            var $ = cheerio.load(html);
            var message = $(".messagebody").text().trim();

            //Get Id
            item = item.split('/');
            id = item[4];

            //Make json entry
            var json = {
                "id": id,
                "message": message,
                "date": date
            };

            json.id = id;
            json.message = message;

            string = JSON.stringify(json, null, 4, function(json) {

                console.log('Successfully written ' + json.id);

            });

            fs.appendFile('./output', string + ',', function(){

            });

        }
    });
});

via agomez