[JST] UI Blocking

HTML

<button id=blockUIButton>Block UI</button>
<div id=calculationResult></div>

JavaScript

function calculatePi(numberOfIterations) {
  var pi = 0,
    n = 1;
  for (var i = 1; i < numberOfIterations; i++) {
    pi = pi + (4 / n) - (4 / (n + 2));
    n += 4
  }

  return pi;
};

blockUIButton.onclick = function(){
	calculationResult.innerHTML = calculatePi(1000000000);
}