DOM要素の配列を結合する

joinDOM.js

// DOM要素を結合する。引数はScalaのmkString風
Array.prototype.joinDOM = function() {
  var [sep, head, tail] = [null, null, null],
      arg = Array.map(arguments, convertToDOM);

  switch(arg.length) {
  case 0: break;
  case 1: [sep] = arg; break;
  case 3: [head, sep, tail] = arg; break;
  default: throw new Error('invalid arguments');
  }

  var df = document.createDocumentFragment();
  function append(e, clone) {
    if (e !== null) df.appendChild(clone ? e.cloneNode(true) : e);
  }

  append(head);
  for (let [i, elem] in Iterator(this)) {
    if (i > 0) append(sep, true);
    append(convertToDOM(elem));
  }
  append(tail);

  return df;
};