How to use the custom parameter in the URL?

I want to add a callback function to format the return post data，for example:

https://dev/wp-json/wp/v2/posts?callback=ABCD

it should return data as follow:

ABCD([{"id":1...},{"id":2...},{"id":3...}])

Check out this documentation for details on writing your own custom endpoint, including accessing request parameters.

The REST API already supports JSONP with ?_jsonp