how to change css file in html page using js if the user has a page width below a certain threshold

I want to change the css file depending on if the user is on mobile or on desktop. I couldnt find anything that actually works so i just restorted to using the js below to redirect the user to a diff html page with a diff css sheet. But i find this super inefficient. Does anyone know how I can make the css stylesheet change depending on the screen width property in the page?

<script type="text/javascript">
<!--
if (screen.width <= 699) {
document.location = "m_index.html";
}
//-->
</script>


       <!-- main css -->
       <link rel="stylesheet" href="master.css">
 
       <!-- CSS with media query -->
       <link rel="stylesheet"
             media="screen and (max-width: 360px)"
             href="small-screen.css">
       <link rel="stylesheet"
             media="screen and (min-width: 360px) and (max-width: 550px)"
             href="large-screen.css">

Solution

Good idea to have responsive design, but that's a weird way of doing it. Unless there's a really good reason why you're using JS for this, you can do it all in CSS, check this out for example: https://www.w3schools.com/howto/howto_css_media_query_breakpoints.asp

Open the above snippet in fullscreen and resize your window to see the demonstration.

See the MDN page for more detailed documentation.

<!DOCTYPE html>
<!--copied from https://www.w3schools.com/howto/howto_css_media_query_breakpoints.asp-->
<html>
<head>
<meta name="viewport" content="width=device-width, initial-scale=1.0">
<style>
.example {
  padding: 20px;
  color: white;
}
/* Extra small devices (phones, 600px and down) */
@media only screen and (max-width: 600px) {
  .example {background: red;}
}

/* Small devices (portrait tablets and large phones, 600px and up) */
@media only screen and (min-width: 600px) {
  .example {background: green;}
}

/* Medium devices (landscape tablets, 768px and up) */
@media only screen and (min-width: 768px) {
  .example {background: blue;}
} 

/* Large devices (laptops/desktops, 992px and up) */
@media only screen and (min-width: 992px) {
  .example {background: orange;}
} 

/* Extra large devices (large laptops and desktops, 1200px and up) */
@media only screen and (min-width: 1200px) {
  .example {background: pink;}
}
</style>
</head>
<body>

<h2>Typical Breakpoints</h2>
<p class="example">Resize the browser window to see how the background color of this paragraph changes on different screen sizes.</p>

</body>
</html>