В современных браузерах можно так:
<form>
<select class="test" id="test" name="mytest" multiple>
<option>1</option>
<option>2</option>
<option>3</option>
<option>4</option>
</select >
</form>
<input type="button" value="show" onclick="alert(JSON.stringify(
new FormData(document.forms[0]).getAll('mytest')
,0,4))">
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