Validating Multiple Checkboxes In Form?

I have some checkboxes in a form that I'd like to validate (check to ensure that at least one has been ticked):
HTML Code:
<input type="checkbox" name="response[]" value="answer1" />answer1
<input type="checkbox" name="response[]" value="answer2" />answer2
<input type="checkbox" name="response[]" value="answer3" />answer3
<input type="checkbox" name="response[]" value="answer4" />answer4

The javascript below only works if I change my checkbox names to name="response" rather than name="response[]".
Code:
function validateform(){
var success = false;
for (i = 0; i < document.surveyform.response.length; i++){
if (document.surveyform.response[i].checked){
success = true;
}} return success;
}

I need to pass along multiple checkbox responses to my PHP script and therefore need to keep checkboxes named name="response[]". When I do this, I get the error "document.surveyform.response has no properties"
What change I can make to the javascript so that I can name my checkboxes name="responses[]" ?