node fs to json

If this json file won't become too big over the time you should try:

Create a javascript object with the table array in it
var obj = {
   table: []
};
Add some data to it like
obj.table.push({id: 1, square:2});
Convert it from an object to string with stringify
var json = JSON.stringify(obj);
use fs to write the file to disk
fs.writeFile('myjsonfile.json', json, 'utf8', callback);
if you want to append it read the json file and convert it back to an object
fs.readFile('myjsonfile.json', 'utf8', function readFileCallback(err, data){
    if (err){
        console.log(err);
    } else {
    obj = JSON.parse(data); //now it an object
    obj.table.push({id: 2, square:3}); //add some data
    json = JSON.stringify(obj); //convert it back to json
    fs.writeFile('myjsonfile.json', json, 'utf8', callback); // write it back 
}});
This will work for data big as 100 MB max effectively. Over this limit you should use a database engine.

UPDATE:

Create a function which return the current date (year+month+day) as a string. Create the file named this string + .json. 
the fs module has a function which can check for file existance named fs.stat(path, callback). With this you can check if 
the file exists. If it exists use the read function if its not use the create function. Use the date string as the path cuz 
the file will be named as the today date + .json. the callback will contains a stats object which will be null if the file 
does not exists.