6.3 Iterable Groups (Eloquent JavaScript Solutions)

6-3 Eloquent Javascript Solutions.js

class Group {
  constructor() {
    this.group = [];
  }
  add(item) {
    if (!this.group.includes(item)) {
      this.group.push(item);
    }
  }
  delete(item) {
    let index = this.group.indexOf(item);
    if (index !== -1) {
      this.group.splice(index, 1);
    }
  }
  has(item) {
    return this.group.includes(item);
  }
  static from(a) {
    let g = new Group();
    for (let item of a) {
      g.add(item);
    }
    return g;
  }
  [Symbol.iterator]() {
    return new GroupIterator(this);
  };
}

class GroupIterator {
  constructor(o) {
    this.i = 0;
    this.group = o.group;
  }

  next() {
    if (this.i == this.group.length || this.i > 10) return {done: true};

    let value = this.group[this.i];
    this.i++;
    return {value, done: false};
  }
}

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

6.3 Iterable groups

Make the Group class from the previous exercise iterable. Refer back to the section about the iterator interface earlier in the chapter if you aren’t clear on the exact form of the interface anymore.

If you used an array to represent the group’s members, don’t just return the iterator created by calling the Symbol.iterator method on the array. That would work, but defeats the purpose of this exercise.

It is okay if your iterator behaves strangely when the group is modified during iteration.

Hints

It is probably worthwhile to define a new class GroupIterator. Iterator instances should have a property that tracks the current position in the group. Every time next is called, it checks whether it is done, and if not, moves past the current value and returns it.

The Group class itself gets a method named by Symbol.iterator which, when called, returns a new instance of the iterator class for that group.