Javascript - Unorthodox way to get value of checkbox's checked attribute?

I am currently utilizing a third party web-based application that allows for UI script injection.

Within the page I am trying to inject my UI script into are a list of elements with associated checkboxes. The following are checkboxes generated by the webpage by the original developers, I cannot change these:

<input type="checkbox" value="1" class="selectionCheckbox" checked="checked" onclick="foo(this)"> // For True (Checked)
<input type="checkbox" value="1" class="selectionCheckbox" onclick="foo(this)"> // For False (Unchecked)

I am trying to load their values into my own UI Script's checkboxs after the document is ready. The nature of my script is basically to inject a X/Y Grid of combinations instead of a list.

I have tried:

function loadCheckOne(vobj, objValue) {
    var containerObj = document.getElementById(objValue);
    var row = containerObj.getElementsByClassName("checkbox");
    // row[0] === the above mentioned input element

    if (row[0].checked){
        vobj.setAttribute("checked", "true");
        console.log("Marked.");
    }
    console.log(row[0].checked); // Prints: "undefined"
}

function loadCheckTwo(vobj, objValue) {
    var containerObj = document.getElementById(objValue);
    var row = containerObj.getElementsByClassName("checkbox");
    // row[0] === the above mentioned input element

    if (row[0].hasAttribute("checked")){
        vobj.setAttribute("checked", "true");
        console.log("Marked.");
    }
    console.log(row[0].hasAttribute("checked")); // Prints: "false"
}


function loadCheckThree(vobj, objValue) {
    var containerObj = document.getElementById(objValue);
    var row = containerObj.getElementsByClassName("checkbox");
    // row[0] === the above mentioned input element

    if (row[0].getAttribute("checked")){
        vobj.setAttribute("checked", "true");
        console.log("Marked.");
    }
    console.log(row[0].getAttribute("checked")); // Prints: "null"
}

The problem is all methods return false or null in some fashion, even for checkboxes that are checked at runtime of the script. Not sure where to go from here.