I can get correct response with Postman but body of the fetch response is empty

Question

I send post request to my API that contains userName and password in the body. When I do that with Postman I can get a response which has a body contains a token.

But when I send the same request with fetch in fronted I get a responce with an empty body.

My feth is here:

async function login(){
    await fetch('/auth/login',
    {
        method: 'POST',
        headers: {
            'Content-Type': 'application/json'
        },
        body: JSON.stringify(
            {
                'userName' : userName,
                'password' : password,
            }
        )
    }
).then(
    res => {
        console.log(res)
        localStorage.setItem('Authorization',JSON.stringify(res))
    }
).catch(
    err => {
        console.log("Erorororor: " + JSON.stringify(err))
    }
)
}

What is the problem?

Answer 1

I changed my code to

.then(res => return res.text()).then(data => console.log(data)).catch(...)
    

and now I can get the result which I want.

Answer 2

TLDR Solution

It's caused by some header settings from the server side (For example, setting Access-Control-Allow-Origin: * twice will likely cause this issue). By tuning the headers you can fix it.

Details

I met the same issue. It's NOT caused by javascript code as the browser itself doesn't get body either, but headers and cookies are got.

By expanding the timing tab you can see a warning showing the request is not finished!

But the postman can get the body:

When

The same code works in the past. It came out recently. I'm not sure what's changed as there are many things changed, for example, the browser version, the nodejs version upgraded, etc.

Fixed

By tunning the headers in the server side fixed the problem.