How to get multiple select value both in javascript and php

For PHP, we must use test[] for the select name, but it doesn't work in javascript. document.form1.test is null.

So we should use document.getElementById('test[]') or document.form1['test[]']. On this way, we are able to get multiple select value in both PHP and JavaScript.

<form name="form1">
<select name="test[]" multiple="multiple" onchange="alert(document.getElementById('test[]').value);">
<option value="one">one</option>
<option value="two">two</option>
<option value="three">three</option>
<option value="four">four</option>
<option value="five">five</option>
</select>
</form>