Visibility API

gistfile1.js

//http://www.w3.org/TR/2011/WD-page-visibility-20110602/
(function () {
	"use strict";

	//no need to shim if it's already there
	if ( 'hidden' in document && 'onvisibilitychange' in document ) {
		return;
	}

	//fail silently if the browser sux
	if (
		!Array.prototype.forEach   ||
		!Object.defineProperty     ||
		!document.addEventListener ||
		!window.addEventListener
	) {
		return;
	}

	//check to see if there's a native implementation
	var prefixes = [ 'webkit', 'moz', 'o', 'ms' ], native = '';
	prefixes.some(function ( prefix ) {
		if ( (prefix + 'Hidden') in document ) {
			native = prefix;
			return true;
		}
	});

        //* Y U NO INLINE
	var hiddenGetter, stateGetter;
	if ( native ) {
		hiddenGetter = function () {
			return document[ native + 'Hidden' ];
		};
	} else {
		hiddenGetter = function () {
			return hidden;
		};
	}
        //* Y U CREATE NEW visibilityStates OBJECT PER FUNCTION CALL
	stateGetter = function () {
		//corresponding values to document.hidden
		var visibilityStates = {
			'true' : 'hidden',
			'false' : 'visible'
		};
		return visibilityStates[ document.hidden ];
	};

	//will be called whenever the visibility has been changed
	var visibilityChange = function () {
		document.hidden = hiddenGetter();
		document.visibilityState = stateGetter();

		//create the visibilitychange event
		var evt = document.createEvent( 'Event' );
		evt.initEvent( 'visibilitychange', true, true );
		//you're FIRED!
		document.dispatchEvent( evt );

		//check for document.onvisibilitychange and call it if it exists
		var vischange = 'onvisibilitychange'; //it's really long to write...
		if ( document[vischange] && document[vischange].call ) {
			document[ vischange ].call( document, evt );
		}
	},
		hidden = false;

	//add the event listeners to tap into native functionality
	if ( native ) {
		document.addEventListener( native + 'visibilitychange', visibilityChange );
	} else {
		//the window.focus and window.blur events are, well, close enough
		window.addEventListener( 'focus', function () {
			hidden = false;
			visibilityChange();
		});
		window.addEventListener( 'blur', function () {
			hidden = true;
			visibilityChange();
		});
	}

	//add document.hidden, document.visibilityState and document.onvisibilitychange
	Object.defineProperties(document, {
		hidden : {
                        //* Y U NO INLINE
			value : hiddenGetter(),
			writable : true,
			configurable : true
		},
		visibilityState : {
                        //* Y U NO INLINE
			value : stateGetter(),
			writable : true,
			configurable : true
		},
		onvisibilitychange : {
			value : null,
			writable : true,
			configurable : true
		}
	});
}());