Can I get dom by placing script in front of dom...

I read the question wrong, but it seems there is nothing wrong with it. Can you post a screenshot of the error? Furthermore, it is best to add an existential judgment to such a continuous judgment, otherwise problems may easily arise. Similar to:

if (dom.next) {
    dom.next.next
}

var test1 = document.getElementById('dialogArea');
var test2= test.nextElementSibling.nextElementSibling;

Then your html cannot be line-wrapped. If it is line-wrapped, there will be a nextSibling of text type behind each input. The blank character is also equivalent to a text node

var x=document.getElementById("dialogArea");
x.nextElementSibling.nextElementSibling.value = "30";

<input type="hidden" name="dinwei" class="dinwei" id="dialogArea" value="" /><input name="myInput" type="text" size="20" value="How1 many input elements?"/><input name="myInput" type="text" size="20" value="How2 many input elements?"/><input name="myInput" type="text" size="20" value="How3 many input elements?"/><input type="button"  onclick="getElements()"value="button" />

It’s best to use jquery. If you have to use js, you can write it like this.

var x=document.getElementById("dialogArea");
next(next(x)).value="30";

function next(e){
        e = e.nextSibling;
        if(e.nodeType == 3){ // 3是指text类型
            e = e.nextSibling;
        }
        return e;
}