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Fetch not receiving headers JS from Flask send_file()

I'm writing a full stack app. I have a python backend using flask that sends a file and a Vue client that receives. Its been working fine up until the point when I try to send the filename over using a Content-Disposition header.
On the backend I've tried:
return send_file(base_path + filename, as_attachment=True, download_name=filename)
And to set the headers manually,
response = make_response(send_file(base_path + filename))
response.headers['Content-Disposition'] = f"attachment; filename=\"{filename}\""
return response
I've also tried to put in headers that would not be blocked by CORS just to see if the request would receive the header but to no avail,
response = make_response(send_file(base_path + filename))
response.headers['Content-Type'] = "sample/info"
return response
I'm printing the header to the console by doing
fetch('http://localhost:4999/rdownload/' + this.$route.params.id, {
method: 'GET'
}).then(res =\> {
if (res.status == '500') { }
console.log(res.headers)
//const header = res.headers.get('Content-Disposition');
//console.log(header)
res.blob().then((blob) => {
/* ... */
})
})
Any help would be appreciated! Thanks :)

Research
In the interest of logging the solution I found and helping out anyone in the future who may be interested in knowing the answer here's what I discovered:
There is a restriction to access response headers when you are using Fetch API over CORS.
https://stackoverflow.com/a/44816592/20342081
So, no matter what using the JS fetch-api you will be unable to access all headers (outside of Cache-Control, Content-Language, Content-Type, Expires, Last-Modified, and Pragma) unless you expose them by specifying them in a request header. That would look something like this:
fetch('https://myrequest/requestend/', {
headers: {
'Access-Control-Expose-Headers': 'Content-Disposition'
}
})
When a cross-origin source accesses your API you will have to expose the header from the back end as well. https://stackoverflow.com/a/66291644/20342081
I was also confused about how the differences between Access-Control-Expose-Headers and Access-Control-Allow-Headers. In my case the solution was use "expose headers" on both the frontend and the backend (and allow wouldn't work). However, Allow has its own applications which I have yet to understand fully. For those endeavoring check out: https://stackoverflow.com/a/28108431/20342081
Solution
I implemented these things in my code by doing:
class RequestResult(Resource):
def get(self, index):
base_path = f"Requests/{index}/"
filename = os.listdir(base_path)[0]
response = make_response(send_file(base_path + filename, as_attachment=True, download_name=filename))
response.headers['Access-Control-Expose-Headers'] = "Content-Disposition"
return response
And on the front end exposing the header as well on the fetch request:
fetch('http://localhost:4999/rdownload/' + this.$route.params.id, {
method: 'GET',
mode: 'cors',
headers: {
'Access-Control-Expose-Headers': 'Content-Disposition'
}
})
I hope this is helpful for the next 5 people who open this in the next 10 years!

CORS error using fetch API - React create app

I'm consuming an API using fetch but i'm getting CORS error.
I tried multiples headers, but I'm not understading what's the problem.
I'm not the owner of the API, so I couldn't change it, but checking the response it's returning access-control-allow-origin.
Following is my request method:
export const execPOST = (url, body) => {
return fetch(url, {
method: "POST",
headers: {
"Access-Control-Allow-Origin": "*",
"Content-Type": "application/json"
},
body: JSON.stringify(body)
});
};
The response is:
Request URL: http://api
Request Method: OPTIONS
Status Code: 405 Method Not Allowed
Remote Address: ip
Referrer Policy: no-referrer-when-downgrade
Response Headers:
Access-Control-Allow-Origin: *
Isn't this response above enough to allow my request?
console error:
OPTIONS http://api net::ERR_ABORTED 405 (Method Not Allowed)
Access to fetch at 'http://api' from origin 'http://localhost:3000' has been blocked by CORS policy: Response to preflight request doesn't pass access control check: It does not have HTTP ok status.
I got this working (meanwhile I develop) using "https://cors-anywhere.herokuapp.com/", but I don't think that I should use this for production enviroment.
I found a lot of material about this problem, but nothing that worked besides implements a backend to make the request or use something else as a proxy to make the request and so on...

Update code as given below (use 'mode' with the value 'no-cors' ):
For more details follow the link => https://developer.mozilla.org/en-US/docs/Web/API/Fetch_API/Using_Fetch
export const execPOST = (url: string, body: any) => {
return fetch(url, {
mode: 'no-cors',
method: 'POST',
headers: {
'Content-Type': 'application/json'
},
body: JSON.stringify(body)
});
};

CORS headers are set by the API to protect users from malicious code making requests to sites on their behalf.
This means that you cannot enable or disable it from the client side as the Access-Control-Allow-Origin header is a server side only header.
If you don't have access to the API to change the headers then you won't be able to use the API from the client side.
In production you would have to create your own API that will handle the requests to the API you are trying to contact.

POST working with Postman but not via fetch in JavaScript

The post request to the Django Rest API framework works via Postman when the appropriate parameters are filled in the 'body' section. But the same does not work with the following JavaScript code:
var data = {emp_id:50,emp_name:'test',password:'pass123'};
fetch('http://127.0.0.1:8000/signup/',{
method:"POST",
body: JSON.stringify(data),
mode:"no-cors",
headers: {
"Content-Type": "application/json",
// "Content-Type": "application/x-www-form-urlencoded",
},
})
.then(response => response.json());
The following is the def that handles the POST request in the views.py of the REST-API:
#api_view(['GET', 'POST', ])
def signup(request):
serializer = employeeSerializer(data=request.data)
if serializer.is_valid():
serializer.save()
return Response(serializer.data, status=status.HTTP_201_CREATED)
else:
return Response(serializer.errors, status=status.HTTP_400_BAD_REQUEST)
I'm new to this, can anyone tell me why the JavaScript code won't work?
EDIT:
The error which the browser console shows is:
POST http://127.0.0.1:8000/signup/ 415 (Unsupported Media Type)

The issue is that by using no-cors mode you constrain yourself to using simple requests, which in turn cannot have content-type other than application/x-www-form-urlencoded, multipart/form-data, or text/plain. In fact, if you look at the headers sent by the browser with your request, you'll see that the content type changes from application/json to text/plain - hence the error.
To fix your issue: remove no-cors mode and add cors headers to responses in your django app. You can use django-cors-headers for that.
Also, you have no issues with postman because it does not care about same-origin policy.

Try change headers to
headers: {
'Accept': 'application/json',
'Content-Type': 'application/json'
}
The accept header is used by to determine what format to sent the data back to the client in the response, guess it might be needed

I found the solution here : https://learning.postman.com/docs/sending-requests/generate-code-snippets/#generating-code-snippets-in-postman
with postman you can see the code of headers sent on the request on many languages (Node Axios, javascript fetch ...), then just copy paste the headers and all the data sent by postman to your app

Why is Fetch API call not returning the response headers?

I'm building a SignUp form and have the following API call to POST the form:
const request = new Request('http://localhost:4300/auth/', {
method: 'POST',
headers: new Headers({
'Accept' : 'application/json',
'Content-Type' : 'application/json',
}),
body: JSON.stringify({ user: data })
});
fetch(request).then(response => {
console.log(response);
const auth = response.headers.get('Authorization');
console.log(auth)
});
The problem is response.headers.get('Authorization') is returning as null. Even though if I look at Chrome's Network XHR request I see the Response Headers being sent by the API server.
Why is React not providing me with response.headers via the request above?
Thanks

The value of the Access-Control-Expose-Headers response header for the response from http://localhost:4300/auth/ must include "Authorization" if you want your requesting frontend JavaScript code to be allowed to access the Authorization response header value.
If the response includes no value for the Access-Control-Expose-Headers header, the only response headers browsers will let you access from client-side JavaScript in your web app are Cache-Control,
Content-Language,
Content-Type,
Expires,
Last-Modified
and
Pragma.
See https://fetch.spec.whatwg.org/#cors-safelisted-response-header-name for the spec.

How to send authorization header with axios

How can I send an authentication header with a token via axios.js?
I have tried a few things without success, for example:
const header = `Authorization: Bearer ${token}`;
return axios.get(URLConstants.USER_URL, { headers: { header } });
Gives me this error:
XMLHttpRequest cannot load http://localhost:8000/accounts/user/. Request header field header is not allowed by Access-Control-Allow-Headers in preflight response.
I have managed to get it work by setting global default, but I'm guessing this is not the best idea for a single request:
axios.defaults.headers.common['Authorization'] = `Bearer ${token}`;
Update :
Cole's answer helped me find the problem. I am using django-cors-headers middleware which already handles authorization header by default.
But I was able to understand the error message and fixed an error in my axios request code, which should look like this
return axios.get(URLConstants.USER_URL, { headers: { Authorization: `Bearer ${data.token}` } });

On non-simple http requests your browser will send a "preflight" request (an OPTIONS method request) first in order to determine what the site in question considers safe information to send (see here for the cross-origin policy spec about this). One of the relevant headers that the host can set in a preflight response is Access-Control-Allow-Headers. If any of the headers you want to send were not listed in either the spec's list of whitelisted headers or the server's preflight response, then the browser will refuse to send your request.
In your case, you're trying to send an Authorization header, which is not considered one of the universally safe to send headers. The browser then sends a preflight request to ask the server whether it should send that header. The server is either sending an empty Access-Control-Allow-Headers header (which is considered to mean "don't allow any extra headers") or it's sending a header which doesn't include Authorization in its list of allowed headers. Because of this, the browser is not going to send your request and instead chooses to notify you by throwing an error.
Any Javascript workaround you find that lets you send this request anyways should be considered a bug as it is against the cross origin request policy your browser is trying to enforce for your own safety.
tl;dr - If you'd like to send Authorization headers, your server had better be configured to allow it. Set your server up so it responds to an OPTIONS request at that url with an Access-Control-Allow-Headers: Authorization header.

This has worked for me:
let webApiUrl = 'example.com/getStuff';
let tokenStr = 'xxyyzz';
axios.get(webApiUrl, { headers: {"Authorization" : `Bearer ${tokenStr}`} });

Rather than adding it to every request, you can just add it as a default config like so.
axios.defaults.headers.common['Authorization'] = `Bearer ${access_token}`

Try this :
axios.get(
url,
{headers: {
"name" : "value"
}
}
)
.then((response) => {
var response = response.data;
},
(error) => {
var status = error.response.status
}
);

You are nearly correct, just adjust your code this way
const headers = { Authorization: `Bearer ${token}` };
return axios.get(URLConstants.USER_URL, { headers });
notice where I place the backticks, I added ' ' after Bearer, you can omit if you'll be sure to handle at the server-side

Instead of calling axios.get function Use:
axios({ method: 'get', url: 'your URL', headers: { Authorization: `Bearer ${token}` } })

const response=await axios(url,
method:"GET"
{
headers: {
"Authorization" : `Bearer ${token}`
}
})

You can try this.
axios.get(
url,
{headers: {
"Access-Control-Allow-Origin" : "*",
"Content-type": "Application/json",
"Authorization": `Bearer ${your-token}`
}
}
)
.then((response) => {
var response = response.data;
},
(error) => {
var status = error.response.status
}
);

create a new axios instace for your request
const instance=axios.create({
baseURL:'www.google.com/'
headers:{
'Content-Type':'application/json',
'Acess-Control-Allow-Origin':'*',
'Authorization':`Bearer ${token}`,
'Accept': "application/json"
}
})
await instance.get(url,data)

Install the cors middleware. We were trying to solve it with our own code, but all attempts failed miserably.
This made it work:
cors = require('cors')
app.use(cors());
Original link

This is the Postman way of doing it:
headers: {
'Authorization': `Basic ${Buffer.from('username:password').toString('base64')}`
}

res.setHeader('Access-Control-Allow-Headers',
'Access-Control-Allow-Headers, Origin,OPTIONS,Accept,Authorization, X-Requested-With, Content-Type, Access-Control-Request-Method, Access-Control-Request-Headers');
Blockquote
: you have to add OPTIONS & Authorization to the setHeader()
this change has fixed my problem, just give a try!