How to validate multiple checkboxes in JavaScript?

I have 4 checkboxes, I want to show different alerts for first two when they are selected but not getting the output.Please let me know where it went wrong.

let ReviewCheckBox = checkform.ratings;
if (ReviewCheckBox[0].checked == true) {
  alert("1 Selected");
} else if (ReviewCheckBox[0].checked == true && ReviewCheckBox[1].checked == true) {
  alert("Both Selected");
}

<div>
  <span>CUSTOMER REVIEWS</span><br>
  <form name="checkform" onclick="productFilter()">
    <input type="checkbox" name="ratings" id="rating-checka">
    <label for="rating-checka">4 * & Above</label><br>

    <input type="checkbox" name="ratings" id="rating-checkb">
    <label for="rating-checkb">3 * & Above</label><br>

    <input type="checkbox" name="ratings" id="rating-checkc">
    <label for="rating-checkc">2 * & Above</label><br>

    <input type="checkbox" name="ratings" id="rating-checkd">
    <label for="rating-checkd">1 * & Above</label>
  </form>
</div>