Using ajax

Server side :

function fn(){

	header( 'Content-Type: application/json; charset=utf-8' );
	echo json_encode( 'some response' );

	// don't forget this one below :
	exit();

}

add_action( 'wp_ajax_actionname', 'fn' );
// add below for unlogged user add
add_action( 'wp_ajax_nopriv_actionname', 'fn' );

Client side :

const url = siteURL + '/wp-admin/admin-ajax.php?action=actionname';

fetch(url)
	.then(response => response.json())
	.then(result => console.log(result));