how to fix sticky header on scroll?

Hello i am trying to stick my header in the screen when i scroll from top to bottom. This js function was supposed (following yt tutorial) to stick my header changing my nav color but it isn’t working.

<script type="text/javascript">
    window.addEventListener('scroll', function(){
        var header = document.querySelector('header');
        header.classList.toggle('sticky', window.scrollY > 0);
    });
</script>

this is my CSS

header{
 position: fixed; /*fijamos el header a la pantalla*/
 top:0; /*ayudamos a que el header quede fijo*/
 left:0; /*ayudamos a que el header quede fijo*/
 width: 100%; /*ancho del header*/
 padding: 2.500em 6.250em; /*40px arriba abajo y 100px a los lados*/
 z-index: 100; /*orden de elemento posicionado*/
 display: flex; /*caja flexible*/
 justify-content: space-between; /*separamos los elementos de la caja*/
 align-items:center; /*centramos los elementos*/
 transition: 0.5s;
}
header .sticky{
 background: #fff;
 padding: 20px 100px;
 box-shadow: 0 5px 20px rgba(0,0,0,0.1);
}
header .logo{
 color: #fff;
 font-size: 1.5em; /*24px*/
 text-transform: uppercase;
 text-decoration: none;
 font-weight: 700;
 letter-spacing: 0.125em; /*2px - espacio entre letras*/ 
}
header .sticky .logo{
 color: #111;
}
header ul{
 position: relative;
 display:flex;
}
header ul li{
 position:relative;
 list-style: none;
}
header ul li a{
 position: relative;
 display: inline-block;
 margin: 0 0.938em; /*0 arriba abajo y 15px a los lados*/
 color: #fff; /*color texto*/
 text-decoration: none; /*quitamos subrayado*/
}
header .sticky ul li a{
 color: #111;
}

Originally my header is black with white text so when i scroll from top to bottom it is supposed to stick changing my white text to black so i can appreciate it when i nav in my sections with white background…

Viewing the html dev tools that js function it’s getting called when i scroll image but nothing is happening, it isn’t even changing the text color/background

my HTML

https://pastebin.com/pM1EEU50

Answer

Please notice that element .class and element.class are different. as you are targeting the header element in the function I can assume your problem is that you are looking for an element with .sticky class which is a descendant of header when you select header .sticky; but you need the header itself.

header .sticky {
  background: #fff;
  padding: 20px 100px;
  box-shadow: 0 5px 20px rgba(0,0,0,0.1);
}

consider using header.sticky to select the header with the .sticky class. you have to fix other selectors too.

header.sticky {
  background: #fff;
  padding: 20px 100px;
  box-shadow: 0 5px 20px rgba(0,0,0,0.1);
}

have a look at CSS Selectors for more selector patterns.