Node.js program that saves data to an external json file

SaveToJSON.js

var fs = require('fs');

var obj = {
  date: date,
  speed: speed
}

data.push(obj)

fs.writeFile ("data.json", JSON.stringify(data, null, 4), function(err) {
    if (err) throw err;
      console.log('Data saved.');
    }
);