Question: I created an API endpoint using Google Cloud Functions and am trying to call it from a JS fetch function. I am running into errors that I am pretty sure are related to either CORS or the output format, but I'm not really sure what is going on. A few other SO questions are similar, and helped me realize I needed to remove the mode: "no-cors". Most mention enabling CORS on the BE, so I added response.headers.set('Access-Control-Allow-Origin', '*') - which I learned of in this article - to ensure CORS would be enabled... But I still get the "Failed to fetch" error. The Full Errors (reproducible in the live demo linked below) are:  Uncaught Error: Cannot add node 1 because a node with that id is already in the Store. (This one is probably unrelated?) Access to fetch at 'https://us-central1-stargazr-ncc-2893.cloudfunctions.net/nearest_csc?lat=37.75&lon=-122.5' from origin 'https://o2gxx.csb.app' has been blocked by CORS policy: Request header field access-control-allow-origin is not allowed by Access-Control-Allow-Headers in preflight response. GET https://us-central1-stargazr-ncc-2893.cloudfunctions.net/nearest_csc?lat=37.75&lon=-122.5 net::ERR_FAILED Uncaught (in promise) TypeError: Failed to fetch  See Code Snippets below, please note where I used <---- *** Message *** to denote parts of the code that have recently changed, giving me one of those two errors. Front End Code: function getCSC() { let lat = 37.75; let lng = -122.5; fetch( `https://us-central1-stargazr-ncc-2893.cloudfunctions.net/nearest_csc?lat=${lat}&lon=${lng}`, { method: "GET", // mode: "no-cors", <---- **Uncommenting this predictably gets rid of CORS error but returns a Opaque object which seems to have no data** headers: { // Accept: "application/json", <---- **Originally BE returned stringified json. Not sure if I should be returning it as something else or if this is still needed** Origin: "https://lget3.csb.app", "Access-Control-Allow-Origin": "*" } } ) .then(response => { console.log(response); console.log(response.json()); }); }  Back End Code: import json import math import os import flask def nearest_csc(request): """ args: request object w/ args for lat/lon returns: String, either with json representation of nearest site information or an error message """ lat = request.args.get('lat', type = float) lon = request.args.get('lon', type = float) # Get list of all csc site locations with open(file_path, 'r') as f: data = json.load(f) nearby_csc = [] # Removed from snippet for clarity: # populate nearby_csc (list) with sites (dictionaries) as elems # Determine which site is the closest, assigned to var 'closest_site' # Grab site url and return site data if within 100 km if dist_km < 100: closest_site['dist_km'] = dist_km // return json.dumps(closest_site) <--- **Original return statement. Added 4 lines below in an attempt to get CORS set up, but did not seem to work** response = flask.jsonify(closest_site) response.headers.set('Access-Control-Allow-Origin', '*') response.headers.set('Access-Control-Allow-Methods', 'GET, POST') return response return "No sites found within 100 km"  Fuller context for code snippets above:  Here is a Code Sandbox Demo of the above. Here is the full BE code on GitHub, minus the most recent attempt at adding CORS. The API endpoint.  I'm also wondering if it's possible that CodeSandbox does CORS in a weird way, but have had the same issue running it on localhost:3000, and of course in prod would have this on my own personal domain. The Error would appear to be CORS-related ( 'https://o2gxx.csb.app' has been blocked by CORS policy: Request header field access-control-allow-origin is not allowed by Access-Control-Allow-Headers in preflight response.) but I thought adding response.headers.set('Access-Control-Allow-Origin', '*') would solve that. Do I need to change something else on the BE? On the FE? TLDR; I am getting the Errors "Failed to fetch" and "field access-control-allow-origin is not allowed by Access-Control-Allow-Headers" even after attempts to enable CORS on backend and add headers to FE. See the links above for live demo of code.

Answer: Drop the part of your frontend code that adds a Access-Control-Allow-Origin header. Never add Access-Control-Allow-Origin as a request header in your frontend code. The only effect that’ll ever have is a negative one: it’ll cause browsers to do CORS preflight OPTIONS requests even in cases when the actual (GET, POST, etc.) request from your frontend code would otherwise not trigger a preflight. And then the preflight will fail with this message:  Request header field Access-Control-Allow-Origin is not allowed by Access-Control-Allow-Headers in preflight response  …that is, it’ll fail with that unless the server the request is being made to has been configured to send an Access-Control-Allow-Headers: Access-Control-Allow-Origin response header. But you never want Access-Control-Allow-Origin in the Access-Control-Allow-Headers response-header value. If that ends up making things work, you’re actually just fixing the wrong problem. Because the real fix is: never set Access-Control-Allow-Origin as a request header. Intuitively, it may seem logical to look at it as “I’ve set Access-Control-Allow-Origin both in the request and in the response, so that should be better than just having it in the response” — but it’s actually worse than only setting it in the response (for the reasons described above). So the bottom line: Access-Control-Allow-Origin is solely a response header, not a request header. You only ever want to set it in server-side response code, not frontend JavaScript code.  The code in the question was also trying to add an Origin header. You also never want to try to set that header in your frontend JavaScript code. Unlike the case with the Access-Control-Allow-Origin header, Origin is actually a request header — but it’s a special header that’s controlled completely by browsers, and browsers won’t ever allow your frontend JavaScript code to set it. So don’t ever try to.