Why Isn't My React App Displaying JSON Data from My FastAPI Backend?

React Application Not Displaying JSON Data from FastAPI Backend

Problem

A React application fails to display JSON data fetched from a FastAPI backend at localhost:8000/todo. The data is received but is not rendered in the React UI.

Cause

This issue is caused by a lack of Cross-Origin Resource Sharing (CORS) configuration in the FastAPI backend. By default, browsers restrict cross-origin requests to prevent security risks.

Solution

To resolve this issue, CORS must be enabled in the FastAPI application. This can be achieved using the CORSMiddleware.

Example

The following code demonstrates how to enable CORS in a FastAPI application:

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=["*"],
)

Explanation

The CORSMiddleware allows cross-origin requests from the specified origins (http://localhost:3000 and http://127.0.0.1:3000 in this example). It also allows cookies to be included in the request (allow_credentials=True) and does not restrict HTTP methods or headers (allow_methods=["*"], allow_headers=["*"]).

Additional Information

• CORS Headers: The HTTP headers set by the CORS middleware control how the browser handles cross-origin requests. For example, the Access-Control-Allow-Origin header specifies the allowed origins for a request.
• Origin: Origin refers to the protocol, domain, and port of a request. Requests from different origins are considered cross-origin requests.
• Same-Origin Policy: By default, browsers enforce the same-origin policy, which restricts cross-origin requests to prevent security vulnerabilities. CORS provides a way to relax this policy.

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