Javascript textbox value incrementer

A pure javascript code to increment the textbox value by 1

HTML

<input type='text' value='0' id='txtNum' /><div id="up" class="addbtn"> +</div><div id="down" class="addbtn">-</div>

CSS

.addbtn {
   display:inline-block;
   font-weight: bold;
   background:pink;
   border-radius:4px;
   border:2px solid red;
   width:1em;
   height:1em;
   cursor:pointer;
   line-height:1em;
   text-align: center;
   vertical-align: middle;
   user-select:none;
}

JavaScript

var tbox;
var valueAsNumber = 0
function incrementValue(){
 
 value = parseInt(txtBox.value, 10);
 value++;
 alert(value);
 txtBox.value = value;
}



function init(evt) {
  let tbox = document.getElementById('txtNum');
  tbox.value = valueAsNumber;
  let up = document.getElementById('up');
  let down = document.getElementById('down');
  up.addEventListener('click', evt => {
  	valueAsNumber++;
    tbox.value = valueAsNumber;
  } );
  down.addEventListener('click', evt => {
  	valueAsNumber--;
    tbox.value = valueAsNumber;
  } );
}

init();