rec.js

var SIZE = 40,
    count = 0,
    result = '',
    rec;

(rec = () => {
  var unfilledSide = ((SIZE - count) / 2) | 0;
  result += 
  Array(unfilledSide).join(' ') + 
  Array(count).join('*') +  
  Array(unfilledSide).join(' ') + '\n';
  SIZE > ++count ? rec() : null;
})();

console.log(result);