https://eloquentjavascript.net/06_object.html#i_djD3XDJ27V

Eloquent JavaScript 0603 Iterable Groups.js

// Your code here (and the code from the previous exercise)
class GroupIterator {
  constructor(group) {
    this.group = group;
    this.i = 0;
  }
  
  next() {
    if (this.i === this.group.values.length) {
      return { done: true };
    }
    
    const value = this.group.values[this.i];
    this.i += 1;
    return { value, done: false };
  }
}

class Group {
  // Your code here.
  constructor() {
    this.values = [];
  }
  
  add(elem) {
    if (!this.has(elem)) {
      this.values.push(elem);
    }
  }
  
  delete(elem) {
    this.values = this.values.filter(v => v !== elem);
  }
  
  has(elem) {
    return this.values.includes(elem);
  }
  
  static from(itr) {
    const g = new Group();
    for (let elem of itr) {
      g.add(elem);
    }
    return g;
  }
  
  [Symbol.iterator]() {
    return new GroupIterator(this);
  }
}

for (let value of Group.from(["a", "b", "c"])) {
  console.log(value);
}
// → a
// → b
// → c