Javascript複選框驗證不起作用

Question

如果我在我的javascript「checkboxlimit（document.forms.world.countries []，0）上添加國家[]」，JavaScript不起作用。如果我刪除[]，那麼複選框驗證工作絕對好。我如何解決這個問題？這裏是我的全碼：

第1頁：

<script type="text/javascript"> 
function checkboxlimit(checkgroup, limit){ 
var checkgroup=checkgroup 
var limit=limit 
for (var i=0; i<checkgroup.length; i++){ 
    checkgroup[i].onclick=function(){ 
    var checkedcount=0 
    for (var i=0; i<checkgroup.length; i++) 
     checkedcount+=(checkgroup[i].checked)? 1 : 0 
    if (checkedcount>limit){ 
     alert("You can only select a maximum of "+limit+" checkboxes") 
     this.checked=false 
     } 
    } 
} 
} 
</script> 

<form id="world" name="world" method="post" action="example2.php"> 
<select name="dropdown" onclick="Calc(world)"> 
<option value=""></option> 
<option value="school">School</option> 
<option value="college">College</option> 
</select> 

<p>Countries:</p> 
<input type="checkbox" name="countries[]" value="USA" /> USA<br /> 
<input type="checkbox" name="countries[]" value="Canada" /> Canada<br /> 
<input type="checkbox" name="countries[]" value="Japan" /> Japan<br /> 
<input type="checkbox" name="countries[]" value="China" /> China<br /> 
<input type="checkbox" name="countries[]" value="France" /> France<br /> 
<input type="submit" value="Submit"> 
</form> 

<script type="text/javascript"> 
function Calc(world){ 
var entersel1 = document.world.dropdown.value; 
if (entersel1 == ''){ 
checkboxlimit(document.forms.world.countries[], 0) 
} 
else if(entersel1 == 'school'){ 
checkboxlimit(document.forms.world.countries[], 2) 
} 
else{ 
checkboxlimit(document.forms.world.countries[], 3) 
} 
} 
</script> 

第2頁：

<?php 
$dropdown = $_POST['dropdown']; 
$countries = implode(',', $_POST['countries']); 
echo $dropdown.'<br>'.$countries; 
?> 

Answer 1

<script type="text/javascript"> 
function Calc(world){ 
var entersel1 = document.world.dropdown.value; 
if (entersel1 == ''){ 
checkboxlimit(document.forms.world['countries[]'], 0) 
} 
else if(entersel1 == 'school'){ 
checkboxlimit(document.forms.world['countries[]'], 2) 
} 
else{ 
checkboxlimit(document.forms.world['countries[]'], 3) 
} 
} 
</script> 

將其放在括號[]，報價之間。