Javascript: Triggering "for" loop using "if...else" statements not working

I am trying to make a simple script which automatically blocks the input boxes in the file when I tick a checkbox. For this, I am trying to add/remove the "disabled" attribute by triggering a loop every time the checkbox is clicked. Looks something like this:

function locker() {
  var boxes = document.querySelectorAll("input[type='text']");
  var x = getElementById("lock")
  for (i = 0; i < inputBoxes.length; i++) {
    if (x.checked == true) {
      boxes[i].disabled = true;
    } else {
      boxes[i].disabled = false;
    }
  }

<input type="checkbox" id="lock" onClick="locker()">
<input type="text"></input>
<input type="text"></input>
<input type="text"></input>

However, I can't seem to get it to work. I don't have much experience coding, and I feel like I am making a very basic mistake, but I couldn't find a solution to this problem so far... How can I solve this? Are there any other workarounds to get the same result?

Thanks in advance