JS Map WeakMap
A WeakMap is useful for metadata associated with an object. Its object keys do not become strongly retained just by being keys.
Where to run this
This example uses the JavaScript console playground. Select Run to execute it; the example does not run on this lesson page.
A practical example
const metadata = new WeakMap();
const member = {name: "Mira"};
metadata.set(member, {visits: 2});
console.log(metadata.get(member).visits);
What happens
The member metadata records two visits.