Back and Forward buttons not behaving as expected. the buttons cycle correctly but when using only buttons places text at the end. using keyboard ok

function getInputSelection(el) {
  var start = 0,
    end = 0,
    normalizedValue, range,
    textInputRange, len, endRange;
  if (typeof el.selectionStart == "number" && typeof el.selectionEnd == "number") {
    start = el.selectionStart;
    end = el.selectionEnd;
  } else {
    range = document.selection.createRange();
    if (range && range.parentElement() == el) {
      len = el.value.length;
      normalizedValue = el.value.replace(/rn/g, "n");
      textInputRange = el.createTextRange();
      textInputRange.moveToBookmark(range.getBookmark());
      endRange = el.createTextRange();
      endRange.collapse(false);
      if (textInputRange.compareEndPoints("StartToEnd", endRange) > -1) {
        start = end = len;
      } else {
        start = -textInputRange.moveStart("character", -len);
        start += normalizedValue.slice(0, start).split("n").length - 1;
        if (textInputRange.compareEndPoints("EndToEnd", endRange) > -1) {
          end = len;
        } else {
          end = -textInputRange.moveEnd("character", -len);
          end += normalizedValue.slice(0, end).split("n").length - 1;
        }
      }
    }
  }
  return {
    start: start,
    end: end
  };
}

function setBack() {
  document.getElementById('user').value =
    document.getElementById('user').value.substring(0,
      document.getElementById('user').value.length - 1);
}


function moveBack(elId) {
  var elem = document.getElementById(elId);
  elem.setSelectionRange(elem.value.length,
    getInputSelection(elem).start - 1);
  elem.focus();
}

function moveForward(elId) {
  var elem = document.getElementById(elId);
  elem.setSelectionRange(elem.value.length,
    getInputSelection(elem).start + 1);
  elem.focus();
}
function focusElem(){
var foo =document.getElementById('user');
foo.focus();
}

function setA(){ document.getElementById('user').value += "A"; focusElem();}
function setB(){ document.getElementById('user').value += "B"; focusElem();}
function setC(){ document.getElementById('user').value += "C"; focusElem();}
function setD(){ document.getElementById('user').value += "D"; focusElem();}

<input id="user" type="text">
<button type="button" onclick="setBack();">backspace</button>
<button type="button" onclick="moveBack('user')">back</button>
<button type="button" onclick="moveForward('user')">forward</button>


<div>
<button type="button" onclick="setA()">A</button>
<button type="button" onclick="setB()">B</button>
<button type="button" onclick="setC()">C</button>
<button type="button" onclick="setD()">D</button>
<div>