純文字版返回頂端的js

<script language="javascript">
var goto_top_type = -1;
var goto_top_itv = 0;

function goto_top_timer() {
	var y = goto_top_type == 1 ? document.documentElement.scrollTop
			: document.body.scrollTop;
	var moveby = 15;
	y -= Math.ceil(y * moveby / 100);
	if (y < 0) {
		y = 0;
	}
	if (goto_top_type == 1) {
		document.documentElement.scrollTop = y;
	} else {
		document.body.scrollTop = y;
	}
	if (y == 0) {
		clearInterval(goto_top_itv);
		goto_top_itv = 0;
	}
}

function goto_top() {
	if (goto_top_itv == 0) {
		if (document.documentElement && document.documentElement.scrollTop) {
			goto_top_type = 1;
		} else if (document.body && document.body.scrollTop) {
			goto_top_type = 2;
		} else {
			goto_top_type = 0;
		}
		if (goto_top_type > 0) {
			goto_top_itv = setInterval('goto_top_timer()', 50);
		}
	}
}
</script>

<a href="javascript:void(0)" onclick="goto_top()">【返回頂部】</a>