<input type="range" name="quantity" min="0" max="100">
<output for="quantity"></output>

body, html {
  height: 100%;
}

body {
  display: flex;
  align-items: center;
  justify-content: center;
}

const rangeInput = document.querySelector('input');
const output = document.querySelector('output');

function setDefaultState() {
  output.value = rangeInput.value;
}

rangeInput.addEventListener('change', function(){
  output.value = this.value;
});

document.addEventListener('DOMContentLoaded', function(){
  setDefaultState();
});