For security reasons and in order to protect end users, most browsers enforce the same-origin policy, which means that the browser will prevent scripts loaded from one origin (for example, http://127.0.0.1:8000) from making calls to a server of a different origin (for example, http://localhost:8080). To demonstrate why the same-origin policy is important, take a look at the following example.
Let's suppose you are logged in to your online banking site, personal.bank.io. Then, you open a malicious site, malicious.io, which runs the following script inside malicious.io:
fetch('personal.bank.io/api/transfer', { method : "POST", body : JSON.stringify({ amount : '999999', to: 'malicious.io' })})
If the same-origin policy was ...
