Below line 258

⎗
✓
<input type="range" min="50" max="400" value="50" class="slider" id="zoomSlider" style="width: 100%; margin-left: 50px;"/>

Below line 284

⎗
✓
document.getElementById("zoomSlider").value=curr;

Below line 285

⎗
✓
document.getElementById("zoomSlider").addEventListener("input",function(e){
      console.log(e.target.value);
      curr = e.target.value;
      update();
    })