Add Custom Field to Custom Post Type JSON Endpoint

In this case, I just wanted to make a custom field from ACF visible in the JSON returned for a custom post type which was named faculty.

add_action( 'rest_api_init', 'add_faculty_type_to_json' );
function add_faculty_type_to_json() {

    register_rest_field(
        'faculty', //the post type of your choice
        'faculty_type', //the name for your json element
        array(
            'get_callback'    => 'faculty_return_type', //the function that creates the content 
        )
    );
}

// Return acf field staff_group
function faculty_return_type( $object, $field_name, $request ) {
    global $post;
    $faculty_type = get_field('staff_group', $post->ID); 
    return $faculty_type[0]; //multiple selections are possible here but in this use case I only want the first one
}

That’s all there is to it.