blocked by CORS policy: No 'Access-Control-Allow-Origin' header is present on the requested resource

You need to connect the cors service in the services.yml file
cors.config:
class: Drupal\cors\CorsConfig
enabled: true
# Specify allowed headers, like 'x-allowed-header'.
allowedHeaders: [ '*' ]
# Specify allowed request methods, specify ['*'] to allow all possible ones.
allowedMethods: [ '*' ]
# Configure requests allowed from specific origins.
allowedOrigins: ['http://localhost:8081', 'http://localhost:8082','http://localhost:8083'] # This should contain the address of your server from which you will take the data
# Sets the Access-Control-Expose-Headers header.
exposedHeaders: false
# Sets the Access-Control-Max-Age header.
maxAge: false
# Sets the Access-Control-Allow-Credentials header.
supportsCredentials: false

@v.koval,
If that works, that would be great. Can you come up with a patch so we can test it out?

Cheers,
Andrew

I can't do a patch here because it's a services.yml file. Here is a step-by-step instruction to configure Cors:
1) In the default.services.yml or development.services.yml file, you need to add this code:
parameters:
http.response.debug_cacheability_headers: true
# Configure Cross-Site HTTP requests (CORS).
# Read https://developer.mozilla.org/en-US/docs/Web/HTTP/Access_control_CORS
# for more information about the topic in general.
# Note: By default the configuration is disabled.
cors.config:
enabled: true
# Specify allowed headers, like 'x-allowed-header'.
allowedHeaders: [ '*' ]
# Specify allowed request methods, specify ['*'] to allow all possible ones.
allowedMethods: [ '*' ]
# Configure requests allowed from specific origins.
allowedOrigins: [ 'http://localhost:8081', 'http://localhost:8082','http://localhost:8083' ]
# Sets the Access-Control-Expose-Headers header.
exposedHeaders: false
# Sets the Access-Control-Max-Age header.
maxAge: false
# Sets the Access-Control-Allow-Credentials header.
supportsCredentials: false
2) Make sure that the services.yml file is connected to settings.php. For example, I connected cors in the development.services.yml file, so you need to add this to settings.php - $settings['container_yamls'][] = DRUPAL_ROOT . '/sites/development.services.yml';

I want to drop this here because it is something I've had trouble with since CORS was added to Drupal.
1. Your services.yml DOES NOT go in the sites directory, it goes in the site you are using (usually default) and you can have a different one per site.
2. allowedHeaders, allowedMethods and allowedOrigins are all arrays of strings so to allow all use ['*'] (not [*] or *).
3. Most important, a / on the end of the domain is a different domain so add them both to the array to make sure.

Number 3 has caused me years of trouble, hope it helps someone.