do something if viewport width is low

functions.php

add_action( 'wp_footer', 'hide_model_if_mobile' );

function hide_model_if_mobile() {
	?>
	<script>
	jQuery(document).ready(function( $ ) {
	var width = $(window).width(); 

	if ( width <= 768 ) {
		$( 'check-viewport' ).css('display', 'none');
	} else {
		$( 'check-viewport' ).css('display', 'block');
	}  
	});
	</script>
	<?php
}