javascript - 多个后代选择器，错误或误解？

Question

这个问题在这里已经有了答案:





Why does querySelector('div span') match even though querySelector('div') does not?

(1 个回答)


去年关闭。




以下两种选择节点的方法不应该产生相同的结果吗？

let tmp = fruits.querySelector("ul:first-of-type li:first-of-type");
tmp = tmp.querySelector("span")    

对比

let tmp = fruits.querySelector("ul:first-of-type li:first-of-type span");

(查看实际操作here)
我已经在 Firefox 和 chrome 中对此进行了测试。两种情况的结果不同。有人可以解释为什么吗？
堆栈片段中的示例:

let fruits = document.querySelector("[data-segment='fruits']");
console.log(fruits);
let tmp = fruits.querySelector("ul:first-of-type li:first-of-type")
tmp = tmp.querySelector("span")
console.log("Works:")
console.log(tmp)
console.log("Does not work:")
console.log(fruits.querySelector("ul:first-of-type li:first-of-type span"))

<main id="app" data-v-app="">
  <section>
    <h2>Tree</h2>
    <ul role="tree">
      <li role="treeitem" data-segment="fruits" aria-level="1" aria-setsize="3" aria-posinset="1" aria-expanded="true"><span tabindex="0">Fruits</span>
        <ul role="group">
          <li role="none" data-segment="oranges" aria-level="2" aria-setsize="5" aria-posinset="1"><span tabindex="-1">Oranges</span>
            <!--v-if-->
          </li>
          <li role="none" data-segment="pineapple" aria-level="2" aria-setsize="5" aria-posinset="2"><span tabindex="-1">Pineapple</span>
            <!--v-if-->
          </li>
          <li role="treeitem" data-segment="apples" aria-level="2" aria-setsize="5" aria-posinset="3" aria-expanded="false"><span tabindex="-1">Apples</span>
            <ul role="group">
              <li role="none" data-segment="macintosh" aria-level="3" aria-setsize="3" aria-posinset="1"><span tabindex="-1">Macintosh</span>
                <!--v-if-->
              </li>
              <li role="none" data-segment="granny_smith" aria-level="3" aria-setsize="3" aria-posinset="2"><span tabindex="-1">Granny Smith</span>
                <!--v-if-->
              </li>
              <li role="none" data-segment="fuji" aria-level="3" aria-setsize="3" aria-posinset="3"><span tabindex="-1">Fuji</span>
                <!--v-if-->
              </li>
            </ul>
          </li>
          <li role="none" data-segment="bananas" aria-level="2" aria-setsize="5" aria-posinset="4"><span tabindex="-1">Bananas</span>
            <!--v-if-->
          </li>
          <li role="none" data-segment="pears" aria-level="2" aria-setsize="5" aria-posinset="5"><span tabindex="-1">Pears</span>
            <!--v-if-->
          </li>
        </ul>
      </li>
      <li role="none" data-segment="vegetables" aria-level="1" aria-setsize="3" aria-posinset="2"><span tabindex="-1">Vegetables</span>
        <!--v-if-->
      </li>
      <li role="none" data-segment="grains" aria-level="1" aria-setsize="3" aria-posinset="3"><span tabindex="-1">Grains</span>
        <!--v-if-->
      </li>
    </ul>
  </section>
</main>

Answer 1

documentation解释它:

element = baseElement.querySelector(selectors);

Return value

The first descendant element of baseElement which matches the specified group of selectors. The entire hierarchy of elements is considered when matching, including those outside the set of elements including baseElement and its descendants; in other words, selectors is first applied to the whole document, not the baseElement, to generate an initial list of potential elements. The resulting elements are then examined to see if they are descendants of baseElement. The first match of those remaining elements is returned by the querySelector method.

(强调我的。)
让我们考虑一个简化的例子:

console.log(document.getElementById("a").querySelector("ul li span"));

<ul>
  <li id="a"><span>A</span>
    <ul>
      <li><span>B</span></li>
    </ul>
  </li>
</ul>

在这里，baseElement是 document.getElementById("a") ; selectors是 "ul li span" .document.querySelector("ul li span")确实包括<span> s，它们都在baseElement里面. <span>A</span>恰好是这个集合中的第一个。
有一个相当新的伪类，叫做 :scope 这可能会有所帮助:

console.log(document.getElementById("a").querySelector(":scope ul li span"));

<ul>
  <li id="a"><span>A</span>
    <ul>
      <li><span>B</span></li>
    </ul>
  </li>
</ul>