code: function findRectangleIndexContainingPoint(rectangles, x, y) {
      let closestIdx;
      rectangles.some((d, idx) => {
        if ((d.x1 <= x && x <= d.x2) && (d.y1 <= y && y <= d.y2)) {
          closestIdx = idx;
          return true;
        }
        return false;
      });
      return closestIdx;
    }
docs: Find the index of the rectangle that contains a given
  coordinate.
  @param {array} points - Array of rectangles, each rectangle is of the form
  {x1: number, x2: number, y1: number, y2: number},
  where x1 and x2 are the minimum and maximum x coordinates
  and y1 and y3 are the minimum and maximum x coordinates.
  @param {number} x     - The x coordinate
  @param {number} y     - The y coordinate