look at jQuery... if you have never read the source for it, as a developer, you are doing yourself an injustice. not only has john resig worked on it, but also yehuda katz, so there are some really smart people who coded it to learn from.
contains = hasCompare || rnative.test( docElem.contains ) ?
function( a, b ) {
var adown = a.nodeType === 9 ? a.documentElement : a,
bup = b && b.parentNode;
return a === bup || !!( bup && bup.nodeType === 1 && (
adown.contains ?
adown.contains( bup ) :
a.compareDocumentPosition && a.compareDocumentPosition( bup ) & 16
));
} :
function( a, b ) {
if ( b ) {
while ( (b = b.parentNode) ) {
if ( b === a ) {
return true;
}
}
}
return false;
};