generating pyramid with tail recursion

recursive_pyramid.js

var readline = require('readline');

var rl = readline.createInterface({
  input: process.stdin,
  output: process.stdout
});

rl.question("What's the pyramid size? ", function(answer) {
  var size = parseInt(answer);
  if (size) {
  	console.log(generatePyramid(size));
  } else {
  	console.log("Warning: pyramid size should be an Int.");
  }
  rl.close();
});

function generatePyramid(size, numberOfx = 1, numberOfSpaces = size, result = []) {
	if (size === 0) {
		return result.join('\n');
	} else {
		result.push(new Array(numberOfSpaces).join(" ") + new Array(numberOfx).fill("X").join(""));
		return generatePyramid(size - 1, numberOfx + 2, numberOfSpaces - 1, result);
	}
}