Using default parameter values will break compatibility with a bunch of browsers that Store.js is supposed to support.

what about something like this:

set: function(key, value, options) { ...

this.storage.write(this._namespacePrefix + key, this._serialize(value), options)

and in cookieStorage.js:
function write(key, data, options) {
if(!key) { return }
var opts = options ? options : ""
doc.cookie = escape(key) + "=" + escape(data) + "; expires=Tue, 19 Jan 2038 03:14:07 GMT; path=/" + options
} }`

the user could then set a cookie like:
store.set('myName', 'data', ';Secure;SameSite=strict')