wp_localize_script is used to pass dynamic values to custom js files. The version is used in a way that it adds a timestamp to the modified time of js and this way the sites with heavy caching like WPEngine can automatically pick up latest code and end users wont face much caching issues.

This is how it looks on functions.php

$theme_js_path = get_template_directory().'/js/developer.js';
    $js_ver        = filemtime( $theme_js_path );
	wp_register_script( 'developerjs', get_template_directory_uri() . '/js/developer.js', array('jquery'), $js_ver, true );
	$yup = array(
		'siteurl'     => site_url(),
		'adminurl'    => admin_url(),
		'ajaxurl'     => admin_url('admin-ajax.php'),
		'templateurl' => get_template_directory_uri(),
		'search_page' => get_field('search_page','option')
	);
	wp_localize_script( 'developerjs', 'yup', $yup );
	wp_enqueue_script( 'developerjs'); 

This is how it looks on developer.js

        jQuery.ajax({
            url    : yup.ajaxurl,
            type   : 'POST',
            data   : registrationdata,
                beforeSend: function() {
   
                },
                success: function(response) {              
                    responseobj = jQuery.parseJSON(response);
                    if (responseobj['flag'] == 'success') {
                        jQuery('#step-11').show();
                    }
                }
        });