WordPress REST API: Add featured image link to posts

add-featured-image-to-post.php

<?php

add_filter( 'rest_prepare_post', array( $this, 'add_featured_image_link' ), 10, 3 );

public function add_featured_image_link( $data, $post, $request ) {

	if ( has_post_thumbnail( $post->ID ) ) {
		$featured_image = wp_get_attachment_image_src( get_post_thumbnail_id( $post->ID ), 'full' );
		$data->add_link( 'featured_image', $featured_image[0], array( 'width' => absint( $featured_image[1] ), 'height' => absint( $featured_image[2] ) ) );
	}

	return $data;

}