When building a web service that uses the HTTP protocol for transmitting hypermedia documents, we can upload resources to the same server, that way we can use them in the service we're building. In fact, we can't use resources from another server by default, this is restricted by the same-origin policy.
This is a security measure implemented in browsers, if the petition comes from a CLI there will be no CORS.
Trying to access to another server information/resources is called a Cross-Origin Request. And whenever we want to use them in another origin (server) we need the help of a technology called Cross-Origin Resource Sharing or CORS for short.
What does CORS do?
Using the HTTP headers the main server needs to specify if others domains should be able to fetch data from itself. This is achieved using the
Access-Control-Allow-Origin header, then, you have something like this in the header response:
Acces-Control-Allow-Origin: <other origin(s)>
If you want anyone to have access you can use the
Having this in mind, we can now see an example. Imagine we have a domain called
domain-a.com without any CORS declared, and we want to fetch some data from
domain-a.com in our
domain-b.com. The browser will prevent this fetch to happens with a CORS error, because by default all of these petitions are restricted.
But, when we add
Acces-Control-Allow-Origin: http://domain-b.com to our
domain-a.com server, the fetch petition from the second domain to the first one will succeed, the browser will now allow this fetch request because
domain-a.com told the browser to permit the petitions from
This was a brief description of what is CORS, more information can be found at the MDN. Thanks for reading 🥳.