you shouldn't use javascript for this work. You should use css media queries. Take a look at this

If you must handle it through JavaScript, you can use the window.screen.width and window.screen.height properties to check the devices height and width. But as a Comment already suggested, you might want to look into media-queries in CSS they are pretty powerful.

Possible usage

if(window.screen.width <= 700) {
    // do mobile logic
} else {
    // do desktop logic
}

You can use media query to handle what will do by its screen size.

Here is to handle it with JS,

Reference:

• https://www.w3schools.com/howto/howto_js_media_queries.asp
• https://css-tricks.com/working-with-javascript-media-queries/

And here is when you want to handle with CSS, you can follow this link

• https://www.w3schools.com/css/css_rwd_mediaqueries.asp