Retrieving data from Wordpress REST API

functions.php

wp_enqueue_script( 'wp-rest', get_template_directory_uri() . '/js/wp-rest.js', null, null, true );

// Posts custom fields
function prepare_rest($data, $post, $request){
	$_data = $data->data;
	$_data['custom_fields'] = get_post_meta($post->ID); // retrieving all post meta fields
	return $_data;
}

add_filter('rest_prepare_post', 'prepare_rest', 10, 3);

// CPT custom fields
function create_api_posts_meta_field() {
	// register_rest_field ( 'name-of-post-type', 'name-of-field-to-return', array-of-callbacks-and-schema() )
	register_rest_field( 'air_tickets', 'post-meta-fields', array(
			'get_callback'    => 'get_post_meta_for_api',
			'schema'          => null,
		)
	);
}

function get_post_meta_for_api( $object ) {
	//get the id of the post object array
	$post_id = $object['id'];
	//return the post meta
	return get_post_meta( $post_id );
}

add_action( 'rest_api_init', 'create_api_posts_meta_field' );

wp-rest.js

// set params
const params = {
  'per_page': '100',
  'order': 'asc'
};

// convert params to string
const apiParams = (function() {
  let res = '', i = 0;
  for (key in params) {
    let sep = i === 0 ? '?' : '&';
    res += sep + key + '=' + params[key];
    i++
  }
  return res;
}());

const apiURL = '/wp-json/wp/v2/';

(function() {
  // retrieving data through fetch API (not for dummy browsers)
  return fetch(  apiURL + 'posts' + apiParams )
  .then( response => {
    if ( response.status !== 200 ) {
      throw new Error( 'Problem! Status Code: ' + response.status );
    }
    response.json().then( data => {
      console.log(data) // put you callback instead this line
    })
  })
  .catch(function(err) {
    console.log( 'Error: ', err )
  })
}());

(function() {
  // retrieving data through HTTP request
  const request = new XMLHttpRequest();
  request.open('GET', apiURL + 'posts' + apiParams, true);
  request.onload = function () {
    if (request.status !== 200) {
      console.log('Problem! Status Code: ' + request.status);
    } else {
      const data = JSON.parse(this.response);
      console.log(data); // put you callback instead this line
    }
  }
  request.send();
}());