将检索
<a class="class-name">Any</a> <b class="class-name">tag</b> <div class="class-name an-extra-class">with that class.</div>
如果没有现有元素包含给定的类，则将返回一个空集合。
示例
<p class="my-class">I will be matched</p> <p class="my-class another-class">So will I</p> <p class="something-else">I won't</p>
var myClassElements = document.getElementByClassName('my-class');
console.log(myClassElements.length); // 2
var nonExistentClassElements = document.getElementByClassName('nope');
console.log(nonExistentClassElements.length); // 0