functions.php

sample1

function view_sitename(){
    echo get_bloginfo( 'name' );
    die();
}
add_action( 'wp_ajax_view_sitename', 'view_sitename' );
add_action( 'wp_ajax_nopriv_view_sitename', 'view_sitename' );

template

<button id="submit">View Sitename</button>

js

$( '#submit' ).on( 'click', function(){
    $.ajax({
        type: 'POST',
        url: ajaxurl,
        data: {
            'action' : 'view_sitename',
        },
        success: function( response ){
            alert( response );
        }
    });
    return false;
});

simple sample

js

$( '#submit' ).on( 'click', function(){
     
    var mes = 'Hello World!!';
         
    $.ajax({
        type: 'POST',
        url: ajaxurl,
        data: {
            'action' : 'view_mes',
            'mes' : mes,
        },
        success: function( response ){
            alert( response );
        }
    });
    return false;
});