在DOM中添加新属性时，如何等待元素？

setTimeout(function() {
  var element = document.getElementById('elementId');
  if (element) {
    element.setAttribute('newAttribute', 'value');
  }
}, 1000); // 延迟1秒执行

var observer = new MutationObserver(function(mutations) {
  mutations.forEach(function(mutation) {
    if (mutation.type === 'childList' && mutation.addedNodes.length > 0) {
      var element = mutation.addedNodes[0];
      element.setAttribute('newAttribute', 'value');
      observer.disconnect(); // 停止观察
    }
  });
});

observer.observe(document, { childList: true, subtree: true });

document.addEventListener('DOMContentLoaded', function() {
  var element = document.getElementById('elementId');
  if (element) {
    element.setAttribute('newAttribute', 'value');
  }
});