UpTo.js

// Find first ancestor of el with tagName
// or undefined if not found
function upTo(el, tagName) {
  tagName = tagName.toLowerCase();

  while (el && el.parentNode) {
    el = el.parentNode;
    if (el.tagName && el.tagName.toLowerCase() == tagName) {
      return el;
    }
  }

  // Many DOM methods return null if they don't
  // find the element they are searching for
  // It would be OK to omit the following and just
  // return undefined
  return null;
}