Why is My React UI Not Displaying POST Response from a FastAPI Backend?

React UI Not Displaying POST Response from FastAPI Backend

In this scenario, a React UI is expected to fetch a JSON file from a FastAPI backend at "localhost:8000/todo" and present the data as part of the UI at "localhost:3000." However, two items from the JSON file ("Read a book." and "Cycle around town.") are not being displayed.

Issue Root Cause:

The issue lies in the lack of Cross-Origin Resource Sharing (CORS) configuration in the FastAPI backend. CORS is a mechanism that allows resources from one origin to be shared with resources from another origin, which is necessary in this case as the React UI and FastAPI backend are running on different domains.

Solution:

To enable CORS in the FastAPI backend, the CORSMiddleware must be configured. This middleware allows specifying the origins that are allowed to access the backend, the methods and headers allowed, and whether credentials are allowed.

Example Implementation:

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=["*"],
)

In this example, the CORSMiddleware is configured to allow requests from the two origins: "http://localhost:3000" and "http://127.0.0.1:3000." Additionally, it allows all methods and headers and enables credential sharing.

Note:

It's important to remember that origins refer to a combination of protocol, domain, and port. Therefore, even localhost URLs with different ports are considered different origins and require CORS configuration.

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