Get a list of countries available for shipping with Ajax in Craft Commerce

You need to provide a CSRF token, alternatively turn it off in your config/general.php file.

Set a token in your twig template:

{% set csrfToken = {
    csrfTokenName: craft.app.config.general.csrfTokenName,
    csrfTokenValue: craft.app.request.csrfToken,
} %}

<script type="text/javascript">
    window.Craft = {{ csrfToken|json_encode|raw }};
</script>

Then include the CSRF token name and value in your post array (data):

data[window.Craft.csrfTokenName] = window.Craft.csrfTokenValue;

Alternatively you can set enableCsrfProtection to false in your general config file.

The controller action you are hitting returns a template and I think that it is why you are getting this error.

If you don't have headlessMode on (docs) you could make a quick template, add:

{% header "Content-Type: application/json"%}
{{ craft.commerce.countries.getAllCountries() | json_encode() | raw }}

Which would give you an endpoint you can then reach from your front end.

Alternatively, you could make a small module/plugin with a controller.

After enabling the headless mode in Craft I'm getting the same error in JSON format:

Anyway I guess I have to write some module for that extra functionality. Thank you for the tips!