Calculate the amount of rows in a borderless square (x/y angles) to draw a triangle

sequence2D.js

function sequence2D(n) {
  if (n > 1) {
    return this.sequence2D(n - 1) + (n + 1);
  } else if (n == 1) {
    return 3;
  } else {
    throw new RangeError("A sequence operates on real numbers.");
  }
}