Add featured image data to the REST API return

featured-image-rest.php

<?php
add_filter( 'rest_prepare_post', __NAMESPACE__ . '\add_featured_image_data_to_rest', 70, 3 );

/**
 * Format the possible featured image metadata in the REST API response.
 *
 * @param  WP_REST_Response $response  The response object.
 * @param  WP_Post          $item      The original post object.
 * @param  WP_REST_Request  $request   The request used to generate the response.
 *
 * @return void
 */
function add_featured_image_data_to_rest( $response, $post, $request ) {

	// Define our existing response data.
	$set_response_data  = $response->get_data();

	// Don't make any changes if the field is empty.
	if ( empty( $set_response_data['featured_media'] ) ) {
		return $response;
	}

	// Define our media ID.
	$define_media_id    = absint( $set_response_data['featured_media'] );

	// Now add our newer (better) array of featured image data.
	$set_response_data['featured_image'] = [
		'id'    => $define_media_id,
		'file'  => wp_get_attachment_url( $define_media_id ),
		'title' => get_the_title( $define_media_id ),
		'alt'   => get_post_meta( $define_media_id, '_wp_attachment_image_alt', true ),
	];

	// Update the response data.
	$response->set_data( $set_response_data );

	// Return the resulting response.
	return $response;
}