block9.js

var user = {
	name: "Kushan Joshi"
}

var websites = ['Facebook', 'Github', 'Twitter', 'Dev.to', 'Medium'];

var memberOf = (user) => websites.filter(website => isUser(user));

// either save the websites a user is part of
user.memberOf = memberOf(user);

cachedMemberOf = weakCache(memberOf); // avoid recomputing everytime
// or derive it everytime you need it
console.log(cachedMemberOf(user)); 
render(cachedMemberOf(user))