设置下拉框属性

    books = document.getElementById('books');
    console.log(books);
    // 设置第几个被选中
    books.selectedIndex = 1;
    console.log(books.selectedIndex) // 0
Select 对象方法
添加和删除下拉框选项
        <select name="books-option"  id="books">
            <option value="1">Python</option>
            <option id="x" value="2">JavaScript</option>
            <option value="3">Java</option>
        </select>
    </form>
<script>
    books = document.getElementById('books');
    console.log(books);
    var option=document.createElement("option");
    option.text="c++";
    // add()
    books.add(option)
</script>
add不传第二个参数，默认添加到最后

第二个参数，可以指定添加位置, 比如添加到第一个后面
    books = document.getElementById('books');
    console.log(books);
    var option=document.createElement("option");
    option.text="c++";
    books.add(option, '1')
如果添加到第一个位置，第二个参数传 0

books.add(option, 0)
remove() 移除一个选项
selectObject.remove(index) 
参数index是下拉框的索引位置
<div id="demo">
    <p>select 下拉框</p>
        <select name="books-option"  id="books">
            <option value="1">Python</option>
            <option id="x" value="2">JavaScript</option>
            <option value="3">Java</option>
        </select>
    </form>
<script>
    books = document.getElementById('books');
    console.log(books);
    books.remove(1);
</script>
HTMLOptionsCollection()对象
options 属性返回HTMLOptionsCollection()对象，选项集合

HTMLOptionsCollection()属性
            <option value="1">Python</option>
            <option value="2">JavaScript</option>
            <option value="3">Java</option>
        </select>
    </form>
<script>
    books = document.getElementById('books');
    console.log(books);
    // options 获取全部选项
    console.log(books.options) // HTMLOptionsCollection(3) [option, option, option, selectedIndex: 0]
    // length 属性
    console.log(books.length);  // 3
    // selectedIndex 被选中索引
    console.log(books.selectedIndex ); //0
    // 索引取值
    console.log(books.options[0]);
    console.log(books.options.item(0));
</script>