You could just use CSS without any javascript.
Give your anchor a class:
<a class="anchor"></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;}
By Jan see offsetting an html anchor to adjust for fixed header