wp get featured image

functions.php

add_action('rest_api_init', 'register_rest_images' );

function register_rest_images(){

    register_rest_field( array('post'),

        'fimg_url',

        array(

            'get_callback'    => 'get_rest_featured_image',

            'update_callback' => null,

            'schema'          => null,

        )

    );

}

function get_rest_featured_image( $object, $field_name, $request ) {

    if( $object['featured_media'] ){

        $img = wp_get_attachment_image_src( $object['featured_media'], 'app-thumb' );

        return $img[0];

    }

    return false;

}