The answer is: ‘/’
Sometimes when you change your client-side code, and suddenly you experience a 301 redirect response from your server-side.
In my case, I was using Django as a serverside and use ReactJS as a client-side. The API was implemented in a RESTful style.
Most of the cases, the reason was I forgot to end with ‘/’ in my request URL.
I haven’t really investigated what is behind this rule but, don’t panic and don’t try to dive deep into your serverside code or whatever network related. Just double-check your request URL from the client-side.