Can't uncheck my boxes using Javascript

I have the following JS wich is actually working and Check all my boxes if the one of the top is checked (currently works) therefore I am unable to UNCHECK them by repeating the same process (unchecking the first box is not uncheckable) ._.

I have the following code wich actually call my function each time the checkbox is unchecked or checked:

function refresh_checkbox_list(){
    // document.getElementById('gdr_select_users').setAttribute("checked");

    (function() {
    var aa = document.querySelectorAll("input[type=checkbox]");
    for (var i = 0; i < aa.length; i++){
        aa[i].checked = true;
    }
})()
}

<input type="checkbox" name="gdr_select_all_users" id="gdr_select_all_users" onchange="refresh_checkbox_list()">
<input type="checkbox" name="gdr_select_users" id="gdr_select_users">

Just to say: I am a truly noob of Javascript, I know how I would process it in PHP so it does check and uncheck, but I dont know how does "if" statements work in JS, this is my actual problem..

I was thinking about an if statement that checks if my 1st box is checked then check all others boxes, then another statement wich do the exact reverse. Right?

I must admit that I have also found that JS on this website (an user wich had a similar case)

Thanks forward for any help!