Array.prototype.copyWithin()

[1, 2, 3, 4, 5].copyWithin(0, 3);
// [4, 5, 3, 4, 5]

[1, 2, 3, 4, 5].copyWithin(0, 3, 4);
// [4, 2, 3, 4, 5]

[1, 2, 3, 4, 5].copyWithin(0, -2, -1);
// [4, 2, 3, 4, 5]

[].copyWithin.call({ length: 5, 3: 1 }, 0, 3);
// {0: 1, 3: 1, length: 5}

//   ES6   Array
var i32a = new Int32Array([1, 2, 3, 4, 5]);

i32a.copyWithin(0, 2);
// Int32Array [3, 4, 5, 4, 5]

//  ,      ES6:
[].copyWithin.call(new Int32Array([1, 2, 3, 4, 5]), 0, 3, 4);
// Int32Array [4, 2, 3, 4, 5]

if (!Array.prototype.copyWithin) {
  Array.prototype.copyWithin = function (target, start /*, end*/) {
    //  1-2.
    if (this == null) {
      throw new TypeError("this is null or not defined");
    }

    var O = Object(this);

    //  3-5.
    var len = O.length >>> 0;

    //  6-8.
    var relativeTarget = target >> 0;

    var to =
      relativeTarget < 0
        ? Math.max(len + relativeTarget, 0)
        : Math.min(relativeTarget, len);

    //  9-11.
    var relativeStart = start >> 0;

    var from =
      relativeStart < 0
        ? Math.max(len + relativeStart, 0)
        : Math.min(relativeStart, len);

    //  12-14.
    var end = arguments[2];
    var relativeEnd = end === undefined ? len : end >> 0;

    var final =
      relativeEnd < 0
        ? Math.max(len + relativeEnd, 0)
        : Math.min(relativeEnd, len);

    //  15.
    var count = Math.min(final - from, len - to);

    //  16-17.
    var direction = 1;

    if (from < to && to < from + count) {
      direction = -1;
      from += count - 1;
      to += count - 1;
    }

    //  18
    while (count > 0) {
      if (from in O) {
        O[to] = O[from];
      } else {
        delete O[to];
      }

      from += direction;
      to += direction;
      count--;
    }

    //  19.
    return O;
  };
}