( function ( mw ) {
	var pageVisibility = {
		/**
		 * Cached value of the browser specific name of the `document.hidden` property
		 *
		 * Exposed for testing only.
		 *
		 * @type {null|undefined|string}
		 * @private
		 */
		documentHiddenPropertyName: null,
		/**
		 * Cached value of the browser specific name of the `document.visibilitychange` event
		 *
		 * Exposed for testing only.
		 *
		 * @type {null|undefined|string}
		 * @private
		 */
		documentVisibilityChangeEventName: null
	};

	/**
	 * Return the browser specific name of the `document.hidden` property
	 *
	 * Exposed for testing only.
	 *
	 * @see https://www.w3.org/TR/page-visibility/#dom-document-hidden
	 * @see https://developer.mozilla.org/en-US/docs/Web/API/Page_Visibility_API
	 * @private
	 * @param {Object} doc window.document object
	 * @return {string|undefined}
	 */
	pageVisibility.getDocumentHiddenPropertyName = function ( doc ) {
		var property;

		if ( pageVisibility.documentHiddenPropertyName === null ) {
			if ( doc.hidden !== undefined ) {
				property = 'hidden';
			} else if ( doc.mozHidden !== undefined ) {
				property = 'mozHidden';
			} else if ( doc.msHidden !== undefined ) {
				property = 'msHidden';
			} else if ( doc.webkitHidden !== undefined ) {
				property = 'webkitHidden';
			} else {
				// let's be explicit about returning `undefined`
				property = undefined;
			}
			// cache
			pageVisibility.documentHiddenPropertyName = property;
		}

		return pageVisibility.documentHiddenPropertyName;
	};

	/**
	 * Return the browser specific name of the `document.visibilitychange` event
	 *
	 * Exposed for testing only.
	 *
	 * @see https://www.w3.org/TR/page-visibility/#sec-visibilitychange-event
	 * @see https://developer.mozilla.org/en-US/docs/Web/API/Page_Visibility_API
	 * @private
	 * @param {Object} doc window.document object
	 * @return {string|undefined}
	 */
	pageVisibility.getDocumentVisibilitychangeEventName = function ( doc ) {
		var eventName;

		if ( pageVisibility.documentVisibilityChangeEventName === null ) {
			if ( doc.hidden !== undefined ) {
				eventName = 'visibilitychange';
			} else if ( doc.mozHidden !== undefined ) {
				eventName = 'mozvisibilitychange';
			} else if ( doc.msHidden !== undefined ) {
				eventName = 'msvisibilitychange';
			} else if ( doc.webkitHidden !== undefined ) {
				eventName = 'webkitvisibilitychange';
			} else {
				// let's be explicit about returning `undefined`
				eventName = undefined;
			}
			// cache
			pageVisibility.documentVisibilityChangeEventName = eventName;
		}

		return pageVisibility.documentVisibilityChangeEventName;
	};

	/**
	 * Whether `window.document` is visible
	 *
	 * `undefined` is returned if the browser does not support the Visibility API.
	 *
	 * Exposed for testing only.
	 *
	 * @see https://www.w3.org/TR/page-visibility/#dom-document-hidden
	 * @see https://developer.mozilla.org/en-US/docs/Web/API/Page_Visibility_API
	 * @private
	 * @param {Object} doc window.document object
	 * @returns {boolean|undefined}
	 */
	pageVisibility.isDocumentHidden = function ( doc ) {
		var property = pageVisibility.getDocumentHiddenPropertyName( doc );

		return property !== undefined ? doc[ property ] : undefined;
	};

	mw.popups.pageVisibility = pageVisibility;
}( mediaWiki ) );