Show two simple approaches to determining if the user is on a given tab/window.

windowfocusblur.js

/*
* Using HTML5 Visibility State
* Triggers when user goes to a different tab
* Triggers when user minimizes browser
* Does not trigger when user opens another window on top of browswer
*/

document.visibilityState;

setInterval(function(){
    console.log(document.visibilityState);
}, 500);

/*
* Using JQuery Events
* Triggers when user goes to a different tab
* Triggers when user minimizes browser
* Triggers when user opens another window on top of browswer
*/

$(window).focus(function(){
    console.log('focus');
});

$(window).blur(function(){
    console.log('blur');
});