How to check in Javascript if one element is contained within another

You should use Node.contains, since it's now standard and available in all browsers.

https://developer.mozilla.org/en-US/docs/Web/API/Node.contains

Update: There's now a native way to achieve this. Node.contains(). Mentioned in comment and below answers as well.

Old answer:

Using the parentNode property should work. It's also pretty safe from a cross-browser standpoint. If the relationship is known to be one level deep, you could check it simply:

if (element2.parentNode == element1) { ... }

If the the child can be nested arbitrarily deep inside the parent, you could use a function similar to the following to test for the relationship:

function isDescendant(parent, child) {
     var node = child.parentNode;
     while (node != null) {
         if (node == parent) {
             return true;
         }
         node = node.parentNode;
     }
     return false;
}