Why do SASS and JS darken color differently (darken)?

Question

JavaScript

Hello.
SASS has a darken() method to make a color darker.
darken(#ff6622, 20%);
If you repeat the same thing with js, it turns out something like this:

var lightenDarkenColor = function (col, amt) {
  var usePound = false;
  if (col[0] == "#") {
    col = col.slice(1);
    usePound = true;
  }
  var num = parseInt(col, 16);
  var r = (num >> 16) + amt;
  if (r > 255) {
    r = 255;
  } else if (r < 0) {
    r = 0;
  }
  var b = ((num >> 8) & 0x00FF) + amt;
  if (b > 255) {
    b = 255;
  } else if (b < 0) {
    b = 0;
  }
  var g = (num & 0x0000FF) + amt;
  if (g > 255) {
    g = 255;
  } else if (g < 0) {
    g = 0;
  }
  return (usePound ? "#" : "") + (g | (b << 8) | (r << 16)).toString(16);
}
var t = document.getElementById('test');
t.style.backgroundColor = lightenDarkenColor('#ff6622', -20);

The output from SASS is the color: #D44100
And for js: #EB520E
Example
The same thing happens with lighten.
Why don't the colors match? Thank you.