var s = 0;
var colorDirection = 1;

setInterval(function () {
  document.body.style.backgroundColor = 'hsl(269, ' + s + '%, 50%)'

  s += colorDirection

  if (s > 100) {
    colorDirection = -1
    s = 100
  } else if (s < 0) {
    colorDirection = 1
    s = 0
  }
}, 10)