lib/index.esm.js
function from(arrayLike /* , mapfn = undefined, thisArg = undefined */) {
    var O = _toObject(arrayLike);
    var C = typeof this == 'function' ? this : Array;
    var aLen = arguments.length;
    var mapfn = aLen > 1 ? arguments[1] : undefined;
    var mapping = mapfn !== undefined;
    var index = 0;
    var iterFn = core_getIteratorMethod(O);
    var length, result, step, iterator;
    if (mapping) mapfn = _ctx(mapfn, aLen > 2 ? arguments[2] : undefined, 2);
    // if object isn't iterable or it's array with default iterator - use simple case
    if (iterFn != undefined && !(C == Array && _isArrayIter(iterFn))) {
      for (iterator = iterFn.call(O), result = new C(); !(step = iterator.next()).done; index++) {
        _createProperty(result, index, mapping ? _iterCall(iterator, mapfn, [step.value, index], true) : step.value);
      }
    } else {
      length = _toLength(O.length);
      for (result = new C(length); length > index; index++) {
        _createProperty(result, index, mapping ? mapfn(O[index], index) : O[index]);
      }
    }
    result.length = index;
    return result;
  }
function from(arrayLike /* , mapfn = undefined, thisArg = undefined */) {
    var O = _toObject(arrayLike);
    var C = typeof this == 'function' ? this : Array;
    var aLen = arguments.length;
    var mapfn = aLen > 1 ? arguments[1] : undefined;
    var mapping = mapfn !== undefined;
    var index = 0;
    var iterFn = core_getIteratorMethod(O);
    var length, result, step, iterator;
    if (mapping) mapfn = _ctx(mapfn, aLen > 2 ? arguments[2] : undefined, 2);
    // if object isn't iterable or it's array with default iterator - use simple case
    if (iterFn != undefined && !(C == Array && _isArrayIter(iterFn))) {
      for (iterator = iterFn.call(O), result = new C(); !(step = iterator.next()).done; index++) {
        _createProperty(result, index, mapping ? _iterCall(iterator, mapfn, [step.value, index], true) : step.value);
      }
    } else {
      length = _toLength(O.length);
      for (result = new C(length); length > index; index++) {
        _createProperty(result, index, mapping ? mapfn(O[index], index) : O[index]);
      }
    }
    result.length = index;
    return result;
  }
22.1.2.1 Array.from(arrayLike, mapfn = undefined, thisArg = undefined)