You could just use CSS without any javascript.
Give your anchor a class:
<a class="anchor" id="top"></a>
You can then position the anchor an offset higher or lower than where it actually appears on the page, by making it a block element and relatively positioning it. -250px will position the anchor up 250px
a.anchor {
display: block;
position: relative;
top: -250px;
visibility: hidden;
}