maybe the easiest copy paste version.js

function islCompare() {
  var order = '0123456789aAáÁbBcCdDeEéÉfFgGhHiIíÍjJkKlLmMnNoOóÓpPqQrRsStTuUúÚvVwWxXyYýÝzZþÞæÆöÖ';

  function charOrder(a) {
    var ix = order.indexOf(a);
    return ix === -1 ? a.codePointAt() + order.length : ix;
  }

  return function(a, b) {
    for (var i = 0; i < Math.min(a.length, b.length); i++) {
      var iA = charOrder(a[i]);
      var iB = charOrder(b[i]);

      if (iA != iB) return iA - iB;
    }

    return a.length - b.length;
  };
}

var sorted = ['ab', '0939239', 'HINNI', 'æði', '[takn]', 'abb', 'áb', 'ma', 'æði', 'íris', 'þalli', 'hannes'].sort(islCompare());

console.log(sorted);

// todo: maybe do something different with numbers

ísl collator for sorting.js

var order = '0123456789aAáÁbBcCdDeEéÉfFgGhHiIíÍjJkKlLmMnNoOóÓpPqQrRsStTuUúÚvVwWxXyYýÝzZþÞæÆöÖ';

function charOrder(a) {
  var ix = order.indexOf(a);
  return ix === -1 ? a.codePointAt() + order.length : ix;
}


function customIsCollator(a, b) {
  for (var i = 0; i < Math.min(a.length, b.length); i++) {
    var iA = charOrder(a[i]);
    var iB = charOrder(b[i]);
    
    if (iA != iB) return iA - iB;
  }

  return a.length - b.length;
}

var sorted = ['ab', '0939239', 'HINNI', 'æði', '[takn]', 'abb', 'áb', 'ma', 'æði', 'íris', 'þalli', 'hannes'].sort(customIsCollator);

console.log(sorted);

// todo: maybe do something different with numbers