例子

DOM

 // 如何获取DOM元素
        var app = document.getElementsByTagName("div");
        // app HTMLCollection(HTML的集合),不是数组  类数组对象,可以使用数组的属性,可以for循环
        console.log(Array.isArray(app))
        console.log(app.length)
        for(var i=0;i<app.length;i++){
            console.log(app[i])
        }

getElementsByClassName

  <!-- div.one*4{hello $} -->
    <div class="one">hello 1</div>
    <div class="one">hello 2</div>
    <div class="one">hello 3</div>
    <div class="one">hello 4</div>
    <script>
        var all = document.getElementsByClassName("one");
        console.log(all);
    </script>

getElementById

 <div id="app">
        app
        <div class="one">one1</div>
        <div class="one">one1</div>
        <div class="one">one1</div>
    </div>
    <script>
        var app = document.getElementById("app");
        console.log(app);
        // NodeList
        var all = document.querySelectorAll(".one");
        console.log(all)
    </script>