Insert variable script ajax url

functions.php

<?php
add_action( 'wp_head', 'wp_set_ajaxurl' );
function wp_set_ajaxurl() {
  $html  =  '<script>';
  $html .= 'var ajaxurl = "' . esc_url( self_admin_url( 'admin-ajax.php' ) ) . '";';
  $html .= '</script>' . PHP_EOL;
  
  echo $html;
}