diamonds #

diamond.js

const H = 13;
const P = (H - 1) / 2;
const idx = (curr) => (curr - Math.max(0, 2 * (curr - P)));
const r = [...Array(H).keys()]
  .map(idx)
  .map(i => {
    // any shorter?
    const line = [..." ".repeat(H)];
    line[P-i] = "#";
    line[P+i] = "#";
    return line.join("");
  })
  .join("\n");

  console.log(r);