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;