Page Visibility API allows site developers to programmatically determine the current visibility of a document and be notified of visibility changes. At this moment Page Visibility API is natively supported in Chrome 13+, Firefox 10+ and IE10. MozVisibility is designed to partially emulate Page Visibility API in older Firefox versions (5—9).

Functioning principle of this library is based on the following feature, introduced in Firefox 5.0: when the page is in inactive tab Firefox clamps setTimeout/setInterval that lesser than 1 second to 1 second.

Therefore, by setting timeout with 500ms delay and calculating the actual delay between setting and firing timeout, we can determine whether page visible or not.

Add mozvisibility.js to your project.

You can use the following properties to determine whether page visible or not:

document.mozVisibilityState 	// can be ‘visible’ or ‘hidden’

document.mozHidden		// can be true or false

document.addEventListener("mozvisibilitychange", function () {
	console.log(‘Page became', document.mozHidden ? 'invisible' : 'visible');
}, false);

You can also use this library in conjunction with Visibility.js. In this case include mozvisibility.js BEFORE visibility.js.

• Only ‘visible’ and ‘hidden’ states are supported.
• The library determines page as ‘hidden’ ONLY when it is in the background tab. Other types of invisibility cannot be detected at this time.
• There is a 1 second delay between the moment when page became invisible and firing “mozvisibilitychange” event.
• Doesn’t work in IFRAME.

• Page Visibility Editor’s Draft
• Bug 633421 — Clamp setTimeout/setInterval to something higher than 10ms in inactive tabs
• MDC — Using the Page Visibility API
• Google code — Using the Page Visibility API
• IE9 Test Drive — Page Visibility API

Copyright © 2011 Vladimir Zhuravlev

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