Why Isn't My React Frontend Displaying POST Response from My FastAPI Backend?

React Not Displaying POST Response from FastAPI Backend: Resolving CORS Issue

In React, you're trying to retrieve JSON data from an HTTP POST request to a FastAPI backend running on a different port. However, the frontend is not rendering the response as expected.

Understanding CORS

The reason for this is Cross-Origin Resource Sharing (CORS). CORS is a security mechanism that restricts browsers from sending requests to resources (such as JSON data) from different origins, which are defined by their protocol, domain, and port. In your case, the frontend (localhost:3000) is trying to access a resource from the backend (localhost:8000), which is a different origin.

Enabling CORS in FastAPI

To resolve this issue, you need to enable CORS in your FastAPI application. This can be achieved using the CORSMiddleware configured as follows:

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

app = FastAPI()
app.add_middleware(
    CORSMiddleware,
    allow_origins=["YOUR_FRONTEND_ORIGIN"],
    allow_credentials=True,
    allow_methods=["*"],  # Allow all HTTP methods
    allow_headers=["*"],  # Allow all HTTP headers
)

Replace "YOUR_FRONTEND_ORIGIN" with the origin of your React frontend, typically "http://localhost:3000". This will allow your backend to respond to POST requests from your frontend without CORS errors.

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