class Iterator {
  constructor(list) {
    this.list = list;
    this.index = 0
  }
  next() {
    if (this.hasNext()) {
      return this.list[this.index++]
    }
    return null
  }
  hasNext() {
    if (this.index >= this.list.length) {
      return false
    }
    return true
  }
}

const iterator = new Iterator([1,2,3,4,5]);
while(iterator.hasNext()){
  console.log(iterator.next());
}

// each 封装each以便能手动终止循环
function each(arr, cb) {
  for(let i = 0, l = arr.length; i < l; i++) {
    if (cb.call(arr[i], arr[i], i, arr) === false) {
      break;
    }
  }
}

each([1, 2, 3, 4, 5], function (v, i, a) {
  if (v > 3) {
    return false; // 手动终止循环
  }
  console.log(v)
})