var myInterval;
var interval_delay = 500;
var is_interval_running = false; //Optional

$(document).ready(function () {
    $(window).focus(function () {
        clearInterval(myInterval); // Clearing interval if for some reason it has not been cleared yet
        if  (!is_interval_running) //Optional
            myInterval = setInterval(interval_function, interval_delay);
    }).blur(function () {
        clearInterval(myInterval); // Clearing interval on window blur
        is_interval_running = false; //Optional
    });
});

interval_function = function () {
     is_interval_running = true; //Optional
     // Code running while window is in focus
}