pyramid.js

/* 
 
height = 6

.....∆      
....∆∆∆     
...∆∆∆∆∆    
..∆∆∆∆∆∆∆   
.∆∆∆∆∆∆∆∆∆  
∆∆∆∆∆∆∆∆∆∆∆ 

*/

function buildPyramid(height, layerCount = 1, pyramid = []) {
  if (layerCount > height) return pyramid.join("\n");

  const padCount = height + layerCount - 1;
  const brickCount = layerCount * 2 - 1; 

  const layer =
    Array(brickCount)
      .fill("∆", 0, brickCount)
      .join("")
      .padStart(padCount, " ");

  return buildPyramid(height, layerCount + 1, pyramid.concat(layer));
}

console.log(buildPyramid(12));