Using selectors for droplists in JavaScript

Question

如果我知道下拉列表的ID，我就可以使用javascript来使用下拉菜单来选择一个下拉列表，但是如果该下拉列表没有ID，则无法选择它，所以我想知道是否可以选择页面上所有未使用ID的下拉列表？

document.getElementById("id").selectedIndex = 0;

Answer 1

To select all

const all = document.querySelectorAll('select');

To select the first

const first = document.querySelector('select');
console.log(first.selectedIndex);

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edit:

Here you can see an example how too loop the multiple select-boxes and set the selectedIndex (in my case to 3)

 const all = document.querySelectorAll('select'); [...all].forEach(select => select.selectedIndex = 3); 

 <select> <option value="volvo">Volvo</option> <option value="saab">Saab</option> <option value="opel">Opel</option> <option value="audi">Audi</option> </select> <select> <option value="1">1</option> <option value="2">2</option> <option value="3">3</option> <option value="4">4</option> </select> 

all is a NodeList and with [...all] or optional Array.from(all) you get an Array. This is needed to use the Array-Method forEach