worker_testing.js

function messageHandler(event) {
    // Accessing to the message data sent by the main page
    var messageSent = event.data;
    // Preparing the message that we will send back
    var messageReturned = "Hello " + messageSent + " from a separate thread!";
    // Posting back the message to the main page
    this.postMessage(messageReturned);
}

// Defining the callback function raised when the main page will call us
this.addEventListener('message', messageHandler, false);