xpath.js

function evaluateXPath(prefix, aNode, aExpr) {
  //
  // prefix : namespace
  // aNode  : DOM node
  // aExpr  : XPath text
  //
  // return value : DOM element or DOM node
  //
  // See URL for xpath expressions
  // https://developer.mozilla.org/ja/docs/Web/XPath/Introduction_to_using_XPath_in_JavaScript#implementing_a_user_defined_namespace_resolver
  //
  var resolver = function(prefix) {
    var ns = {
      'html'        : 'http://www.w3.org/1999/xhtml',
      'regular'     : 'http://www.w3.org/2005/Atom',
      'overallhead' : 'http://xml.kishou.go.jp/jmaxml1/informationBasis1/',
      'overall'     : 'http://xml.kishou.go.jp/jmaxml1/body/meteorology1/',
      'rtd'         : 'http://xml.kishou.go.jp/jmaxml1/',
      'bdd'         : 'http://xml.kishou.go.jp/jmaxml1/body/meteorology1/',
      'jmx_eb'      : 'http://xml.kishou.go.jp/jmaxml1/elementBasis1/',
      'rtw'         : 'http://xml.kishou.go.jp/jmaxml1/',
      'bdw'         : 'http://xml.kishou.go.jp/jmaxml1/body/meteorology1/'
    };
    return ns[prefix] || null;
  };
  var result = aNode.evaluate(
    aExpr,
    aNode,
    resolver,
    XPathResult.ANY_TYPE,
    null
  );

  var found = [];
  var res;
  while (res = result.iterateNext()){
    found.push(res);
  }
  return found;
}