HTML a href onlick and parameter substitution without JQuery

I have the following HTML:

<a href="https://datasite.com/data/?search=Datapoints+3-4%3B+Datapoints+4-5&version="
  onclick="location.href=this.href + selectedVersion; return false;">Datapoints 3-4; Datapoints 4-5</a>

A <select> tag returns a string for “selectedVersion”.

This is working. I need to add another OPTIONAL parameter to the url when a CheckBox is checked.

I added an <input type=”checkbox” onclick=”printClick()”> which returns a string for “printableVersion”

  var printableVersion;
  function printClick() {
    var checkBox = document.getElementById("printCheckbox");
    if (checkBox.checked == true){
      printableVersion = "&interface=print";
    } else {
      printableVersion = "";
    }
  }
</script>

I modified the href as follows:

<a href="https://datasite.com/data/?search=Datapoints+3-4%3B+Datapoints+4-5"
  onclick="location.href=this.href + printableVersion + "&version=" + selectedVersion; return false;">Datapoints 3-4; Datapoints 4-5</a>

But neither parameter are added. On this site, I can only use Javascript, not JQuery, ajax, etc.