[Octane]

Переписал пример реализации выше без лишних проверок в надежде понять нафиг это нужно

if (!Array.prototype.copyWithin) {
	Array.prototype.copyWithin = function(target, start, end) {
		var length = this.length, count, direction,
			to = target < 0 ? Math.max(length + target, 0) : Math.min(target, length),
			from = start < 0 ? Math.max(length + start, 0) : Math.min(start, length);
		end = typeof end == "undefined" ? length : end;
		end = end < 0 ? Math.max(length + end, 0) : Math.min(end, length);
		count = Math.min(end - from, length - to);
		direction = 1;
		if (from < to && to < (from + count)) {
			direction = -1;
			from += count - 1;
			to += count - 1;
		}
		while (count > 0) {
			if (from in this) {
				this[to] = this[from];
			}
			else {
				delete this[from];
			}
			from += direction;
			to += direction;
			count -= 1;
		}
		return this;
	};
}

alert([

	[1, 2, 3, 4, 5].copyWithin(0, 3).toString() == [4, 5, 3, 4, 5].toString(),
	[1, 2, 3, 4, 5].copyWithin(1, 3).toString() == [1, 4, 5, 4, 5].toString(),
	[1, 2, 3, 4, 5].copyWithin(1, 2).toString() == [1, 3, 4, 5, 5].toString(),
	[1, 2, 3, 4, 5].copyWithin(2, 2).toString() == [1, 2, 3, 4, 5].toString(),

	[1, 2, 3, 4, 5].copyWithin(0, 3, 4).toString() == [4, 2, 3, 4, 5].toString(),
	[1, 2, 3, 4, 5].copyWithin(1, 3, 4).toString() == [1, 4, 3, 4, 5].toString(),
	[1, 2, 3, 4, 5].copyWithin(1, 2, 4).toString() == [1, 3, 4, 4, 5].toString(),

	[1, 2, 3, 4, 5].copyWithin(0, -2).toString()      == [4, 5, 3, 4, 5].toString(),
	[1, 2, 3, 4, 5].copyWithin(0, -2, -1).toString()  == [4, 2, 3, 4, 5].toString(),
	[1, 2, 3, 4, 5].copyWithin(-4, -3, -2).toString() == [1, 3, 3, 4, 5].toString(),
	[1, 2, 3, 4, 5].copyWithin(-4, -3, -1).toString() == [1, 3, 4, 4, 5].toString(),
	[1, 2, 3, 4, 5].copyWithin(-4, -3).toString()     == [1, 3, 4, 5, 5].toString(),

].every(function (testResult) {
	return testResult;
}));