iterator-protocol.js

class Range {
  constructor(from, to, step = 1) {
    this.value = from;
    this.to = to;
    this.step = step;
  }

  next() {
    if (this.value > this.to) {
      return { value: undefined, done: true };
    }
    const value = this.value;
    this.value = value + this.step;
    return { value, done: false };
  }

  [Symbol.iterator]() {
    return this;
  }
}

for (const data of new Range(3, 4)) {
  console.log(data);
}