Multiple Checkbox validation at javascript

i have multiple checkbox, and all of it must be checked. i write down the code but it doesnt work.

this is the code html sample::

    <form name="pembres" id="pembres" method="POST" onSubmit="return validateform()"   style="margin:0;"> 
    <input type="checkbox" name="lanjut[]" value="setuju2"  />
    <input type="checkbox" name="lanjut[]" value="setuju3"  />
    <input type="checkbox" name="lanjut[]" value="setuju4"  />
    <input type="checkbox" name="lanjut[]" value="setuju5"  />
    <input type="submit" value="Next Step" name="next" />
    </form>

1st script at head tag

<script type="text/javascript">
        function validateform(){
            var success = false;
                for (i = 0; i < document.pembres.elements['lanjut[]'].length; i++){
                    if (document.pembres.elements['lanjut[]'][i].checked){
                        success = true;
                    }
                }
            return success;
        }
    </script>

2nd script before /body

    <script type="text/javascript">  
var form = document.getElementById('pembres');
form.onsubmit = validateForm;

function validateForm() {
    var isValid = false,
        form = this,
        els = form.elements['lanjut[]'];
        i;
    for (i = 0; i < els.length; i += 1) {
        if (els[i].checked) {
            isValid = true;
        }
    }
    return isValid;
}
</script>