JSFiddle - React, Tailwind, and code Playground

HTML

<script src="https://code.jquery.com/jquery-2.2.4.js"></script>
<div class="blueBlock">

</div>
<nav class="menu">
  <ul>
    <li><a href="#accueil">Accueil</a></li>
    <li><a href="#portfolio">Portfolio</a></li>
    <li><a href="#cv">CV</a></li>
    <li><a href="#contact">Contact</a></li>
  </ul>
</nav>


<div class="block" id="accueil">
  Accueil
</div>

<div class="block" id="portfolio">
  portfolio
</div>

<div class="block" id="cv">
  CV
</div>

<div class="block" id="contact">
  Contact
</div>

CSS

nav{
  border: 1px solid green;
}

ul{
  list-style:none;
}

ul li{
  display: inline-block;
}

.menu{
  position: static;
}

.fixed {
  position: fixed;
  top: 0;
  left: 0;
  width: 100%;
}

blueBlock{
  height: 100px;
  background-color: blue;
}

div.block{
  height: 300px;
  border: 1px solid red;
}

JavaScript

$(function(){                            
  $(window).on('scroll', function(){

    var scrollPos = $(window).scrollTop();
    var accueil = $('#accueil').offset().top;
   

    if(scrollPos >= accueil){

      $('nav').addClass('fixed');

    }
    else{

      $('nav').removeClass('fixed');
    }
  });   
});