Iterator exmp

class Matrix {
  constructor (width, height, element = (x, y) => undefined) {
    this.width = width;
    this.height = height;
    this.content = [];

    for (let y = 0; y < height; y++) {
      for (let x = 0; x < width; x++) {
        this.content[y * width + x] = element(x, y);
      }
    }
  }

  get (x, y) {
    return this.content[y * this.width + x];
  }

  set (x, y, value) {
    this.content[y * this.width + x] = value;
  }

  [Symbol.iterator] = function () {
    return new MatrixIterator(this);
  }
}

class MatrixIterator {
  constructor (matrix) {
    this.x = 0;
    this.y = 0;
    this.matrix = matrix;
  }

  next () {
    if (this.y === this.matrix.height) {
      return {
        done: true
      }
    }

    let value = {
      x: this.x,
      y: this.y,
      value: this.matrix.get(this.x, this.y)
    };

    this.x++;

    if (this.x === this.matrix.width) {
      this.x = 0;
      this.y++;
    }

    return {
      value,
      done: false
    };
  }
}

let matrix = new Matrix(3, 3, (x, y) => null);