class Session {
  #id;
  constructor(id) {
    this.#id = id;
  }
  getId() { return this.#id; }
}

const activeSessions = new WeakSet();

const s1 = new Session("user_42");
const s2 = new Session("user_99");
let s3 = new Session("user_07");

activeSessions.add(s1);
activeSessions.add(s2);
activeSessions.add(s3);

console.log(activeSessions.has(s1));         // line A
console.log(activeSessions.has(s3));         // line B

s3 = null;

console.log(activeSessions.has(s3));         // line C

activeSessions.delete(s2);
console.log(activeSessions.has(s2));         // line D
console.log(activeSessions.size);            // line E