multiply.js

var first = document.querySelector('#number1');
var second = document.querySelector('#number2');

var result1 = document.querySelector('.result1');

if (!!window.SharedWorker) {
  var myWorker = new SharedWorker("worker.js");

  first.onchange = function() {
    myWorker.port.postMessage([first.value, second.value]);
    console.log('Message posted to worker');
  }

  second.onchange = function() {
    myWorker.port.postMessage([first.value, second.value]);
    console.log('Message posted to worker');
  }

  myWorker.port.onmessage = function(e) {
    result1.textContent = e.data;
    console.log('Message received from worker');
    console.log(e.lastEventId);
  }
}