const obj = {
  data: ["hello", "world"],
  [Symbol.iterator]() {
    const self = this;
    let index = 0;
    return {
      next() {
        if (index < self.data.length) {
          return {
            value: self.data[index++],
            done: false
          };
        } else {
          return {
            value: undefined,
            done: true
          };
        }
      }
    };
  }
};

for (let i of obj) {
  console.log(i);
}