[javascript] Check if cookie exists else set cookie to Expire in 10 days

if (/(^|;)\s*visited=/.test(document.cookie)) {
    alert("Hello again!");
} else {
    document.cookie = "visited=true; max-age=" + 60 * 60 * 24 * 10; // 60 seconds to a minute, 60 minutes to an hour, 24 hours to a day, and 10 days.
    alert("This is your first time!");
}

is one way to do it. Note that document.cookie is a magic property, so you don't have to worry about overwriting anything, either.

There are also more convenient libraries to work with cookies, and if you don’t need the information you’re storing sent to the server on every request, HTML5’s localStorage and friends are convenient and useful.