How to sure at least on checkbox checked in checkbox arra

• How to check checkbox array at least one checkbox select using javascript 

• Making sure at least one checkbox is checked

<script language="javascript">

function valthisform()

{

    var checkboxs=document.getElementsByName("keyword1");

    var okay=false;

    for(var i=0,l=checkboxs.length;i<l;i++)

    {

        if(checkboxs[i].checked)

        {

            okay=true;

            break;

        }

    }

    if(okay)

{

alert("Thank you for checking a checkbox");

}

    else

{

alert("Please check a checkbox");

}

}

</script>

<form name="f1" id="f1" method="post">

<input type="text" name="name_signup" style="border:solid; border-color:#999999;" id="name_signup" class="text-field12" placeholder="Your Name" required />

<input name="keyword1[]" id="keyword1" type="checkbox"  value="1" />

<input name="keyword1[]" id="keyword1" type="checkbox"  value="2" />

<input name="keyword1[]" id="keyword1" type="checkbox"  value="3" />

<input type="submit" onClick = "valthisform();">

</form>