class makeIterator { constructor(arr) { this.arr = arr this.i = 0; this.j = 0; } next() { if(this.i >= this.arr.length) return undefined let ans = this.arr[this.i][this.j]; this.j++; if(this.j >= this.arr[this.i].length) { this.i++; this.j = 0; } return ans; } } let m = new makeIterator([[1, 4, 7],[2, 9],[8, 7, 6, 3]]); console.log(m.next()); console.log(m.next());