Fit text to a container width

function fitText() {
  var numberContainer = document.querySelector('.number-container');
  var blockCounter = document.querySelector('.block-counter');
  var number = document.querySelector('.number');

  var fontSize = 200;
  var containerWidth = blockCounter.offsetWidth - 20;
  var numberWidth = number.offsetWidth;
  var scale = containerWidth / numberWidth;

  fontSize = Math.floor(fontSize * scale);

  numberContainer.style.fontSize = fontSize + 'px';
}

window.addEventListener('load', fitText);
window.addEventListener('resize', fitText);