JS Form Validation

<script>
function deleteVal() {

        var chk = document.getElementById('delete');

    // If the checkbox has been set to delete
    if (chk.checked == true) {

        var ok=confirm("You are about to delete the selected images below.\nAre you sure you want to do this?");

        if (ok) {

            // Submit
            return true;

        } else {

            // Don't submit
            return false;

        }

    }

    // Delete box was not ticked so return true and submit form
    return true;

}

</script>

<form action="" method="POST" onSubmit="return deleteVal();">
  <label>Delete images?</label><input type="checkbox" name="delete" id="delete"/>
  <input type="submit" name="submit" value="Submit" />
</form>