Check whether request is WP REST API request?

Question

What is the best way to check whether a request is an API request?

Note that the request might be a custom API request, which means it may be as follows:

mysite.com/wp-json/my_namespace/my_version/my_action

Obviously, I can check whether the API route wp-json, but there should be a built-in function to check that.

I need it to do some hooks such as

add_action('init', 'do_something_only_if_api_request');

function do_something_only_if_api_request()
{
   if ( ! wp_api_request() ) {
     return;
   }
   // do stuff
}

Answer 1

You can check defined('REST_REQUEST'). This constant is defined as true in rest_api_loaded() (and not defined otherwise).

There was a discussion of WP Rest API developers about the introduction of a new function like is_rest_request(). In the end they went for this constant.