In JavaScript you have the object localStorage.
How long this object will be active for? How long is the the data in it kept for?
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Answer
The most correct answer to this question is: You don’t know.
The user could wipe his/her local data at any time, and any type of local storage is subject to user preferences and to be considered extremely volatile. However, there is no defined expiration time, according to Web Storage specifications:
Expiring stored data
User agents may, if so configured by the user, automatically delete stored data after a period of time.
For example, a user agent could be configured to treat third-party local storage areas as session-only storage, deleting the data once the user had closed all the browsing contexts that could access it.
This can restrict the ability of a site to track a user, as the site would then only be able to track the user across multiple sessions when he authenticates with the site itself (e.g. by making a purchase or logging in to a service).
However, this also reduces the usefulness of the API as a long-term storage mechanism. It can also put the user’s data at risk, if the user does not fully understand the implications of data expiration.