JSFiddle - React, Tailwind, and code Playground

HTML

<div>
  <p>Minha opacidade é: <br> <span></span></p>
</div>
<div>
  
</div>

CSS

*{
  margin: 0; padding: 0; border: 0; box-sizing: border-box;
}
div{
  width: 100%;
  background: #333;
  height: 100vh;
  position: relative;
}
div:nth-child(odd){
  width: 100%;
  background: #666;
  height: 100vh;
}
div p{
  display: block;
  width: 100%;
  font-size: 6vw;
  text-align: center;
  opacity: 0.7;
  position: absolute;
  line-height: 40px;
  bottom: 10px;
  color: #fff;
  height: 100px;
}

JavaScript

$(document).on('scroll', function(){
  var max = 0.7;
  var opacity = max * (1 - $(this).scrollTop() / $(window).height());
  $('div p').css('opacity', opacity);
  $('div p span').html($('div p').css('opacity'));
})