Request fails in browser, but returns 200 in fiddler with correct results, blocked by CORS policy

Question

I am trying to set up an AWS lambda function, however when I call the endpoint, I am getting this error message back in the console.

Access to fetch at 'https://* .execute-api.eu-west-1.amazonaws.com/default/ ' 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.

I have set the CORS policy on the AWS Gateway

Here is my request code... I am just trying to print out the body of the response for testing purposes.

const response = await fetch(
  'https://**********.execute-api.eu-west-1.amazonaws.com/default/**************',
  {
    method: 'POST',
    body: "{'test': 'test'}",
    headers: {
      'X-Api-Key': '*****************************',
    },
  }
);
const text: any = await response.text();
console.log(text);

Weirdly when I look in fiddler, it is sending the OPTIONS and also returning a correct response, which is currently just printing out the different headers passed to the function.

Answer 1

If you've lambda proxy integration enabled you'll have to add headers from lambda.

From AWS documentation CORS section

For a Lambda proxy integration or HTTP proxy integration, you can still set up the required OPTIONS response headers in API Gateway. However, your backend is responsible for returning the Access-Control-Allow-Origin and Access-Control-Allow-Headers headers, because a proxy integration doesn't return an integration response.

exports.handler = async (event) => {
    const response = {
        statusCode: 200,
        headers: {
            "Access-Control-Allow-Headers" : "Content-Type",
            "Access-Control-Allow-Origin": "https://www.example.com",
            "Access-Control-Allow-Methods": "OPTIONS,POST,GET"
        },
        body: JSON.stringify('Hello from Lambda!'),
    };
    return response;
};

you can read more here