JSON.stringify's third parameter defines white-space insertion for pretty-printing. It can be a string or a number (number of spaces). Node can write to your filesystem with fs. Example:

var fs = require('fs');

fs.writeFile('test.json', JSON.stringify({ a:1, b:2, c:3 }, null, 4));
/* test.json:
{
     "a": 1,
     "b": 2,
     "c": 3,
}
*/

See the JSON.stringify() docs at MDN, Node fs docs

Use JSON.stringify(data[, replacer[, indent]]):

jsonfile.writeFile(file, JSON.stringify(JSON.parse(req.body.data), 0, 4), function (err) {
    console.error(err);
});

You also may or may not need to parse the response body though -- I believe responseType: "json" in the builder will automatically parse it for you:

jsonfile.writeFile(file, JSON.stringify(req.body.data, 0, 4), function (err) {
    console.error(err);
});

Here's a complete working isolated example of how it works:

var fs = require('fs');
var req = JSON.parse('{"body": {"data": "Some text."}}');
fs.writeFile('test.json', JSON.stringify(req.body, 0, 4), function (err) {

});

Assuming you're handling POST requests via an HTTP listener or framework like express? Are you using a body parser to parse the incoming JSON?

Let's try:

1. console.log(req.body) to make sure your server recognizes the request.
2. Is it a JSON string? Is it valid? Or is it a javascript object (i.e. already been parsed)?
3. If it's been parsed, then my second suggestion will work. If it hasn't been parsed, parse it as I did in the first example.

Use JSON.stringify(object, null, 4) where 4 is the number of spaces to use as the unit of indentation. You can also use "\t" if you want tabs. This is actually part of the ECMAScript 5 specification, and is documented on MDN.

If this JSON file won't become too big over time, you should try:

1. Create a JavaScript object with the table array in it

   var obj = {
      table: []
   };
   

2. Add some data to it, for example:

   obj.table.push({id: 1, square:2});
   

3. Convert it from an object to a string with JSON.stringify

   var json = JSON.stringify(obj);
   

4. Use fs to write the file to disk

   var fs = require('fs');
   fs.writeFile('myjsonfile.json', json, 'utf8', callback);
   

5. 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 that is up to 100 MB effectively. Over this limit, you should use a database engine.

UPDATE:

Create a function which returns 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 existence named fs.stat(path, callback). With this, you can check if the file exists. If it exists, use the read function if it's 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 contain a stats object which will be null if the file does not exist.