Update the value of a user based on the click of a checkbox
So chalk this one up to me not giving it enough thought (although I've had this in the back of my mind for about two weeks and today just decided to use my Saturday to actually sit down and refactor the entire application):
The answer is that I put the state of the checkbox in a variable and I don't need to check for it at all, I can just pass it through into my postingValues object. The code now looks like this:

const chkBox = document.querySelectorAll("input[type=checkbox]");

for (box of chkBox) {
    box.addEventListener("change", updateUser, false);
}

function updateUser() {

// Getting the state of the checkbox and the DOM element to display the message
    const messageBox = document.getElementById("message");
    const validated = this.checked;

// Getting the values to post to the server and the initialization requirements
    const postingValues = JSON.stringify({id:this.value, validated:validated});
    const fetchInit = {method: "POST", headers: {"Content-type": "application/json"}, body: postingValues};

// Making the fetch call and returning the message to the user
    fetch("index.php?action=updateLogin", fetchInit).then(function(response) {
        response.text().then(function(value) {
            messageBox.innerHTML = value;
        });
    });
}