ajax.php

<?php

//In ajax.php
function sendPublicform(){
    if($_POST['typeform'] == 'sometypeform'){
        //some actions
        wp_send_json(array('success' => true,'message' => __('Saved','themedomain')));
    }
}
add_action('wp_ajax_sendPublicform', 'sendPublicform');
add_action('wp_ajax_nopriv_sendPublicform', 'sendPublicform');

app.js

$('#button').on('click', function(event) {    
  $.ajax({
        type: 'POST',
        url: jsvar.ajaxurl,
        dataType: 'json',
        data: mydata,
        global: true,
        success: function(data){
            console.log(data);
        },
        error: function(e){
            console.log(e);
        }
    });
)};

app.php

<?php 
//In app.php
function add_scripts(){
    wp_register_script('main-js', get_template_directory_uri() . "/src/js/main.js");

    global $messagesajax;
    $paramsLogin = array(
        'msg' => $messagesajax,
        'ajaxurl' => admin_url('admin-ajax.php'),
    );
    wp_localize_script('main-js','jsvar',$paramsLogin);
    wp_enqueue_script('main-js');
}
add_action( 'wp_enqueue_scripts', 'add_scripts' );

functions.php

<?php

//In functions, register the script and enqueue
include_once(get_stylesheet_directory() . '/app.php');
include_once(get_stylesheet_directory() . '/ajax.php');

// I tried to test this but when i need to receive large content (chunks of html) this is not to handy

$messagesajax = array(
	'signup-1' => __( 'We just sent you an email to verify your user. Please check your inbox.', 'growlink' ),
	'Wrong credentials' => __('Wrong credentials','growlink'),
);

wp_register_script('main-js', get_template_directory_uri() . "/src/js/main.js");

global $messagesajax;
$paramsLogin = array(
    'msg' => $messagesajax,
    'ajaxurl' => admin_url('admin-ajax.php'),
);
wp_localize_script('main-js','jsvar',$paramsLogin);
wp_enqueue_script('main-js');