React not showing POST response from FastAPI backend application

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You would need to enable CORS (Cross-Origin Resource Sharing) in the FastAPI backend. You can configure it in your FastAPI application using the CORSMiddleware.

Note

Origin

An origin is the combination of protocol (http, https), domain
(myapp.com, localhost, localhost.tiangolo.com), and port (80,
443, 8080).

So, all these are different origins:

  • http://localhost
  • https://localhost
  • http://localhost:8080

Even if they are all in localhost, they use different protocols or
ports, so, they are different “origins”.

Example

Have a look at this related answer as well.

from fastapi import FastAPI
from fastapi.middleware.cors import CORSMiddleware

app = FastAPI()

origins = ["http://localhost:3000", "http://127.0.0.1:3000"]

app.add_middleware(
    CORSMiddleware,
    allow_origins=origins,
    allow_credentials=True,
    allow_methods=["*"],
    allow_headers=["*"],
)

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