Json Extended Fields for json api

json_extended_fields.php

<?php

/*

Plugin Name: Json Extended Fields for json api
Author: Tuarisa
Description: Extends json functionality for wordpress REST API

*/


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

remove_filter( 'the_content', 'removeFirstImg' ); // remove filter, that added in functions php
register_rest_field(
	'post',
	'image_src',
	array(
		'get_callback'    => 'get_image_src',
		'update_callback' => null,
		'schema'          => null,
		 )
	);
}

function get_image_src( $object, $field_name, $request ) { // first image src (exaclty like now)
	$first_img = '';
	$output = preg_match_all('/<img.*src="(.*)".*>/isU', $object['content']['raw'], $matches);
	$first_img = $matches [1] [0];
	if(empty($first_img) || file_exists($first_img)){
		$first_img = $object['content']['raw'];
	}
	return $first_img;
}

?>