Source

File: wp-includes/rest-api/endpoints/class-wp-rest-posts-controller.php

protected function handle_featured_media( $featured_media, $post_id ) {

		$featured_media = (int) $featured_media;
		if ( $featured_media ) {
			$result = set_post_thumbnail( $post_id, $featured_media );
			if ( $result ) {
				return true;
			} else {
				return new WP_Error(
					'rest_invalid_featured_media',
					__( 'Invalid featured media ID.' ),
					array( 'status' => 400 )
				);
			}
		} else {
			return delete_post_thumbnail( $post_id );
		}

	}