Validating checkbox with javascript

Validating checkbox with javascript

I have problem to validate multi checkbox , I want to check if the user has selected at least one checkbox. Im try with document.getElementByClassName but does not work
HTML:
<form id="formupload" method="post" enctype="multipart/form-data" action="upload.php" name="formupload">
    <input onKeydown="javascript:validateForm();" id="txtFileName" name="txtFileName" />
    <input onKeydown="javascript:validateForm();" id="title" name="title" />
    <input onKeydown="javascript:validateForm();" id="keywords" name="keywords" />
    <input onKeydown="javascript:validateForm();" id="description" name="description" />


        <span class="niche">
        <input type="checkbox" value="1" name="channel[]" class="css-checkbox" id="box_1">
        <label class="css-label" name="lbl_1" for="box_1">Amateur</label>
        </span>

        <span class="niche">
        <input type="checkbox" value="2" name="channel[]" class="css-checkbox" id="box_2">
        <label class="css-label" name="lbl_2" for="box_2">Amateur</label>
        </span>

        <span class="niche">
        <input type="checkbox" value="3" name="channel[]" class="css-checkbox" id="box_3">
        <label class="css-label" name="lbl_3" for="box_3">Amateur</label>
        </span>

<button id="btnSubmit" class="btn lbtn upBtn" type="submit">Upload</button>
</form>
And Here is javascript:
function validateForm() {
    var txtFileName = document.getElementById("txtFileName");
    var titleTxt = document.getElementById("title");
    var tagsTxt = document.getElementById("keywords");
    var descTxt = document.getElementById("description");

    var isValid = true;
    if (txtFileName.value === "" || titleTxt.value === "" || tagsTxt.value === "" || descTxt.value === "" ) {
        isValid = false;
    }

    document.getElementById("btnSubmit").disabled = !isValid;

}