zhe 有何意义

function parents(elem){
  var matched = [];
  while ( (elem = elem[ 'parentNode' ]) && elem.nodeType !== 9 ) {
    if ( elem.nodeType === 1 ) {
      matched.push( elem );
    }
  }
  return matched;
}

函数中加粗的句子为何！==9，而不是其他，或者省略elem.nodeType !== 9