https://eloquentjavascript.net/06_object.html#i_rpYp9Ou4LG

Eloquent JavaScript 0602 Groups.js

class Group {
  // Your code here.
  constructor() {
    this.values = [];
  }
  
  add(elem) {
    if (!this.has(elem)) {
      this.values.push(elem);
    }
  }
  
  delete(elem) {
    const idx = this.values.indexOf(elem);
    if (idx >= 0) {
      this.values.splice(idx, 1);
    }
  }
  
  has(elem) {
    return this.values.indexOf(elem) >= 0;
  }
  
  static from(itr) {
    const g = new Group();
    for (const elem of itr) {
      g.add(elem);
    }
    return g;
  }
}

let group = Group.from([10, 20]);
console.log(group.has(10));
// → true
console.log(group.has(30));
// → false
group.add(10);
group.delete(10);
console.log(group.has(10));
// → false