Sets the value of the pair identified by key to value, creating a new key/value pair if none existed for key previously.
Throws a "QuotaExceededError" DOMException exception if the new value couldn't be set. (Setting could fail if, e.g., the user has disabled storage for the site, or if the quota has been exceeded.)
Dispatches a storage event on Window objects holding an equivalent Storage object.
Returns the number of key/value pairs.
MDN Reference