Cookies are quite useful for certain things, but it is not always clear on how to set or use Cookies. In the documentation there is a short section on how to use the IPS JS Utilities when working with cookies:

ips.utils.cookie.set( 'myKey', 'Hello world', true ); // Sets a sticky cookie
ips.utils.cookie.get( 'myKey' );
// -> Hello world
ips.utils.cookie.unset( 'myKey' );
ips.utils.cookie.get( 'myKey' );
// -> undefined

You can also use a similar way to work with Cookies in PHP using the following format:

{{\IPS\Request::i()->setCookie( 'name', 'value' );}}

This will result in an HTTP only cookie, but if you extend this a bit you can get something useful to fetch in your js calls as well.

\IPS\Request::i()->setCookie('cookie_name', 'value', NULL, TRUE));

Null refer to how long the cookie will remain and by defult NULL means that it will be alive for as long as the session is alive. You can add a time object there to make the cookie persist for a certain amount.

\IPS\Request::i()->setCookie('cookie_name', 'value', \IPS\DateTime::create()->add(new \DateInterval('P7D')));

In this example we add a 7day persistence to the cookie. You can also do this in JavaScript of course:
 

var date = new Date();
date.setDate(date.getDate() + 7);
ips.utils.cookie.set('cookie_name', 'value', date.toUTCString());

Here are some more information on the cookie classes in IPS:

	/**
	 * Set a cookie
	 *
	 * @param	string				$name		Name
	 * @param	mixed				$value		Value
	 * @param	\IPS\DateTime|null	$expire		Expiration date, or NULL for on session end
	 * @param	bool				$httpOnly	When TRUE the cookie will be made accessible only through the HTTP protocol
	 * @param	string|null			$domain		Domain to set to. If NULL, will be detected automatically.
	 * @param	string|null			$path		Path to set to. If NULL, will be detected automatically.
	 * @return	bool
	 */
	public function setCookie( $name, $value, $expire=NULL, $httpOnly=TRUE, $domain=NULL, $path=NULL )

		/**
		 * Set a cookie value
		 *
		 * @param	{string} 	cookieKey 	Key to set
		 * @param 	{mixed} 	value 		Value to set in this cookie
		 * @param 	{boolean} 	sticky 		Whether to make this a long-lasting cookie
		 * @returns {void}
		 */
		set = function( cookieKey, value, sticky )