[JS] Please Select a checkbox


        
            
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            http://jsbin.com/?html,js,output

HTML
<!DOCTYPE html>

<html>

<head>

<meta charset="utf-8">

<meta name="viewport" content="width=device-width">

<title>JS Bin</title>

</head>

<body>

<form name="form" onsubmit="return Valida(this)">

<input type="checkbox" value="1" id="chk" name="item1"> Item 1

<input type="checkbox" value="2" id="chk" name="item2"> Item 2

<input type="checkbox" value="3" id="chk" name="item3"> Item 3

<input type="checkbox" value="4" id="chk" name="item4"> Item 4

<input type="checkbox" value="5" id="chk" name="item5"> Item 5

<br><br>

<input type="submit" value="Enviar">

</form>

</body>

</html>
Javascript
function Valida(form){

var ok = false;

var form = document.forms[0].chk;
for (i = 0; i < form.length; i++)   { 
    if (form[i].checked) { 
        ok = true; 
    }
}
if (!ok) {
    alert ("Selecione algum item.");
    return false;
}
}