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I'm am trying to fetch a serverless function from a react app in development mode with the following code.

let response = await fetch(url,
           method: 'POST',
           mode: 'cors',
           body: "param=" + paramVar,
        .then(response => response.json());

The backend function is a Python Cloud function with the following code:

def main(request):
    # CORS handling
    if request.method == 'OPTIONS':
        # Allows GET requests from any origin with the Content-Type
        # header and caches preflight response for an 3600s
        headers = {
            'Access-Control-Allow-Origin': '*',
            'Access-Control-Allow-Methods': 'GET, POST',
            'Access-Control-Allow-Headers': 'Content-Type',
            'Access-Control-Max-Age': '3600'

        return ('', 204, headers)
    # Set CORS headers for the main request
    headers = {
        'Access-Control-Allow-Origin': '*',
        'Access-Control-Allow-Headers': 'Content-Type',

    # Process request
    return (json.dumps(response), 200, headers)

But I keep getting the following error:

Access to fetch at 'url' from origin 'http://localhost:3000' has been blocked by CORS policy: No 'Access-Control-Allow-Origin' header is present on the requested resource. If an opaque response serves your needs, set the request's mode to 'no-cors' to fetch the resource with CORS disabled.

When I try to perform the same request using curl I get a proper response. Using curl to get the options gives me the following:

HTTP/2 204
access-control-allow-headers: Content-Type
access-control-allow-methods: GET, POST
access-control-allow-origin: *
access-control-max-age: 3600
content-type: text/html; charset=utf-8
date: Sun, 16 Aug 2020 01:29:41 GMT

Anyone can help me understand why I'm not able to get a response at my front-end? The 'Access-Control-Allow-Origin' is present in the headers so I really don't understand what is the cause of this error.

Install the CORS package in the backend. Then open your server.js file or whatever is yours. Then import it to the file

const cors = require('cors');

And then use it


reload the browser and should be done!

1 users liked answer #0dislike answer #01
Atanas Vihrogonov profile pic
Atanas Vihrogonov

add content-type header to your fetch method in the frontend and try again:

let response = await fetch(url,
           method: 'POST',
           mode: 'cors',
           body: "param=" + paramVar,
           headers: {
             'Content-Type': 'application/json'
  }).then(response => response.json());
-1 users liked answer #1dislike answer #1-1
Methkal Khalawi profile pic
Methkal Khalawi

There was actually a bug in the backend that was only triggered by some additional headers added by the browser. In that particular case, the server was returning a 404 error which wouldn't contain my header definitions and would cause the CORS policy block.

I was only able to identify the bug after I used devtools to track the request sent by the browser and replicated all the headers in my curl request. After fixing the function logic the problem was fixed.

-1 users liked answer #2dislike answer #2-1
lspinheiro profile pic

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