Javascript to count selected items in CheckBoxList

Protected Sub Page_Load(ByVal sender As Object, ByVal e As System.EventArgs)
    CheckBoxList1.Attributes.Add("onclick", "javascript:CopyNumItemsToTextBox();")
End Sub

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function CopyNumItemsToTextBox() {
//This function updates TextBox1 with the number of items checked when an item is checked/unchecked
    //Get the checkbox object
    var textBox = document.getElementById("TextBox1");
    //Get the checkboxlist object
    var checkBoxList = document.getElementById("CheckBoxList1");
    //Get the number of checkboxes in the checkboxlist
    var numCheckBoxItems = checkBoxList.cells.length;
    var numItemsChecked = 0; 
                   
    for(i=0; i<numCheckBoxItems; i++)
    {
        //Get the checkboxlist item
        var checkBox = document.getElementById(checkBoxList.id + '_' + [i]);
        //Check if the checkboxlist item exists, and if it is checked
        if(checkBox!=null && checkBox.checked){ numItemsChecked = numItemsChecked + 1; }
    }
    //Set the text box to the number of items checked in the checkboxlist               
   textBox.value = numItemsChecked;
}