XMLHttpRequest error in flutter web [Enabling CORS AWS API gateway]

Note: It turns out that this had nothing to do with flutter and everything to do with the fact that I had set the API gateway to a Lambda Proxy

I am trying to hit an API endpoint from a Flutter web application, every time it errors out and gives me the following error.

Error getting sensor data: DioError [DioErrorType.RESPONSE]: XMLHttpRequest error.

I know there are several questions here on SO(like this and this) discussing this issue and the solution seems to be to enable CORS support on the server-side. I am using the AWS API gateway to build the API, I followed these instructions to enable CORS support from my API. Here are my CORS setting from the API gateway console.

The text in the "Access-Control-Allow-headers" is

'Content-Type,X-Amz-Date,Authorization,X-Api-Key,X-Amz-Security-Token'

Enabling CORS on the API gateway didn't seem to help, I am still getting the same error on my flutter web app when I try to hit an API.

The funny thing is, the API work perfectly fine if I hit the API from chrome (i.e. pasting the API URL on the browser and hitting enter). It only fails when I try to hit the API from the flutter web app.

Question: How do I enable CORS support in my API gateway so my flutter web app can use the API ?

this worked for me, I added the below header on the lambda function

return {
    statusCode: 200,
     headers: {
  "Access-Control-Allow-Origin": "*", // Required for CORS support to work
  "Access-Control-Allow-Credentials": true, // Required for cookies, authorization headers with HTTPS
  "Access-Control-Allow-Headers": "Origin,Content-Type,X-Amz-Date,Authorization,X-Api-Key,X-Amz-Security-Token,locale",
  "Access-Control-Allow-Methods": "POST, OPTIONS"
},
    body: JSON.stringify(item)
};

My server was using nginx so I solved the problem by adding the following two lines to the server block of the sites-enabled config file for my API server:

add_header Access-Control-Allow-Origin "*";
add_header Access-Control-Allow-Methods "GET, HEAD";

My app only uses GET and HEAD so you may need to add other methods depending on your situation.

See also: How to Enable CORS in Apache and Nginx?

I was using Nodejs for my backend. When I sent a post request from Dio there was this error :"XMLHttpRequest error.".

Reason Behind this error: Suppose your flutter is running at localhost:5500 and nodejs server on localhost:3000. So, your browser fails to handle the request as they are running on different ports. That's we use CORS or Proxies to solve these problems.

Remember, this is a problem mainly associated with your browser. If you will use postman and send the same request, you will find everything working.

To solve this: I installed a NPM Package called CORS.

npm i  cors

Then, start using it....

const cors = require("cors");
app.use(cors());

By just doing that your error will get resolved. And you don't need to add anything more.