calc_client.js

var Calculator = require('./calc'); // considering we named the Calculator file 'calc.js' and it's on the same dir.

Calculator.add(1, 2, function (response) {

  response.on('data', function (data) {
    console.log('the sum is', data);
  });

  response.on('error', function (error) {
    console.error('something went wrong', err);
  });
});

evented_calc.js

var Calculator = {

  // performs the complex computation of summing two integers :P
  // returns 'error' and 'data'.
  add : function (a, b, callback) {
  
    // we define the handle for our client to attach events and we return it right away.
    var event = new EventEmitter();
    callback(event);

    // now we perform the computation and emit the corresponding events.
    try {
      event.emit("data", parseInt(a,10) + parseInt(b,10));
    } catch (e) {
      event.emit("error", e);
    } 
  }
};

// export the Calculator object
module.exports = Calculator;