example.js

//save data sequentially
let counter = 0;
let fpath = path.join(__dirname, 'sampledata');
fs.writeFileSync(path.resolve(fpath, `${counter}.json`), 'utf8');

//get the filenames for concatenation as array
let filenames = fs.readdirSync(fpath);
filenames.forEach(fname => {
	//concatenate the files
});


//let's do this
let rate = 30;
let timestampGroup = 0;
let data = [];
let result = []; //the 'output' structure

//iterate over each file saved from the previous stage
while (filenames.length > 0) {
	//grab the next chunk if we're out of data
	if (data.length === 0) {
		data = JSON.parse(fs.readFileSync(filenames.shift(), 'utf8'));
	}
	
	//ready to move to the next chunk?
	if (data[0].timestamp - timestampGroup > rate) {
		//save the newly created structure
		fs.writeFileSync(path.resolve('outputdata', `${timestampGroup}.json'), JSON.stringify(result), 'utf8');
		result = []; //clear
		timestampGroup += rate;
	}
	
	//move from the front of the 'stream' to the result
	result.push(data.shift());
}