Getting "TypeError: Failed to fetch" when the request hasn't actually failed

I'm using fetch API within my React app. The application was deployed on a server and was working perfectly. I tested it multiple times. But, suddenly the application stopped working and I've no clue why. The issue is when I send a get request, I'm receiving a valid response from the server but also the fetch API is catching an exception and showing TypeError: Failed to fetch. I didn't even make any changes to the code and it's the issue with all of the React components.

I'm getting a valid response:

But also getting this error at the same time:

fetch(url)
.then(res => res.json())
.then(data => {
  // do something with data
})
.catch(rejected => {
    console.log(rejected);
});

When I remove credentials: "include", it works on localhost, but not on the server.

I tried every solution given on StackOverflow and GitHub, but it's just not working out for me.

This could be an issue with the response you are receiving from the backend. If it was working fine on the server then the problem could be within the response headers.

Check the value of Access-Control-Allow-Origin in the response headers. Usually fetch API will throw fail to fetch even after receiving a response when the response headers' Access-Control-Allow-Origin and the origin of request won't match.

I understand this question might have a React-specific cause, but it shows up first in search results for "Typeerror: Failed to fetch" and I wanted to lay out all possible causes here.

The Fetch spec lists times when you throw a TypeError from the Fetch API: https://fetch.spec.whatwg.org/#fetch-api

Relevant passages as of January 2021 are below. These are excerpts from the text.

4.6 HTTP-network fetch > To perform an HTTP-network fetch using request with an optional credentials flag, run these steps:
> ...
> 16. Run these steps in parallel:
> ...
> 2. If aborted, then:
> ...
> 3. Otherwise, if stream is readable, error stream with a TypeError.

> To append a name/value name/value pair to a Headers object (headers), run these steps: > 1. Normalize value. > 2. If name is not a name or value is not a value, then throw a TypeError. > 3. If headers’s guard is "immutable", then throw a TypeError.

Filling Headers object headers with a given object object: > To fill a Headers object headers with a given object object, run these steps: > 1. If object is a sequence, then for each header in object: > 1. If header does not contain exactly two items, then throw a TypeError.

Method steps sometimes throw TypeError: > The delete(name) method steps are: > 1. If name is not a name, then throw a TypeError. > 2. If this’s guard is "immutable", then throw a TypeError.

> The get(name) method steps are: > 1. If name is not a name, then throw a TypeError. > 2. Return the result of getting name from this’s header list.

> The has(name) method steps are: > 1. If name is not a name, then throw a TypeError.

> The set(name, value) method steps are: > 1. Normalize value. > 2. If name is not a name or value is not a value, then throw a TypeError. > 3. If this’s guard is "immutable", then throw a TypeError.

> To extract a body and a Content-Type value from object, with an optional boolean keepalive (default false), run these steps:
> ...
> 5. Switch on object:
> ...
> ReadableStream
> If keepalive is true, then throw a TypeError.
> If object is disturbed or locked, then throw a TypeError.

In the section "Body mixin" if you are using FormData there are several ways to throw a TypeError. I haven't listed them here because it would make this answer very long. Relevant passages: https://fetch.spec.whatwg.org/#body-mixin

In the section "Request Class" the new Request(input, init) constructor is a minefield of potential TypeErrors: > The new Request(input, init) constructor steps are:
> ...
> 6. If input is a string, then:
> ...
> 2. If parsedURL is a failure, then throw a TypeError.
> 3. IF parsedURL includes credentials, then throw a TypeError.
> ...
> 11. If init["window"] exists and is non-null, then throw a TypeError.
> ...
> 15. If init["referrer" exists, then:
> ...
> 1. Let referrer be init["referrer"].
> 2. If referrer is the empty string, then set request’s referrer to "no-referrer".
> 3. Otherwise:
> 1. Let parsedReferrer be the result of parsing referrer with baseURL.
> 2. If parsedReferrer is failure, then throw a TypeError.
> ...
> 18. If mode is "navigate", then throw a TypeError.
> ...
> 23. If request's cache mode is "only-if-cached" and request's mode is not "same-origin" then throw a TypeError.
> ...
> 27. If init["method"] exists, then:
> ...
> 2. If method is not a method or method is a forbidden method, then throw a TypeError.
> ...
> 32. If this’s request’s mode is "no-cors", then:
> 1. If this’s request’s method is not a CORS-safelisted method, then throw a TypeError.
> ...
> 35. If either init["body"] exists and is non-null or inputBody is non-null, and request’s method is GET or HEAD, then throw a TypeError.
> ...
> 38. If body is non-null and body's source is null, then:
> 1. If this’s request’s mode is neither "same-origin" nor "cors", then throw a TypeError.
> ...
> 39. If inputBody is body and input is disturbed or locked, then throw a TypeError.

> The clone() method steps are:
> 1. If this is disturbed or locked, then throw a TypeError.

In the Response class: > The new Response(body, init) constructor steps are:
> ...
> 2. If init["statusText"] does not match the reason-phrase token production, then throw a TypeError.
> ...
> 8. If body is non-null, then:
> 1. If init["status"] is a null body status, then throw a TypeError.
> ...

> The static redirect(url, status) method steps are:
> ...
> 2. If parsedURL is failure, then throw a TypeError.

> The clone() method steps are:
> 1. If this is disturbed or locked, then throw a TypeError.

In section "The Fetch method" > The fetch(input, init) method steps are:
> ...
> 9. Run the following in parallel:
> To process response for response, run these substeps:
> ...
> 3. If response is a network error, then reject p with a TypeError and terminate these substeps.

In addition to these potential problems, there are some browser-specific behaviors which can throw a TypeError. For instance, if you set keepalive to true and have a payload > 64 KB you'll get a TypeError on Chrome, but the same request can work in Firefox. These behaviors aren't documented in the spec, but you can find information about them by Googling for limitations for each option you're setting in fetch.

I know it's a relative old post but, I would like to share what worked for me: I've simply input "http://" before "localhost" in the url. Hope it helps somebody.

If your are invoking fetch on a localhost server, use non-SSL unless you have a valid certificate for localhost. fetch will fail on an invalid or self signed certificate especially on localhost.

Note that there is an unrelated issue in your code but that could bite you later: you should return res.json() or you will not catch any error occurring in JSON parsing or your own function processing data.

Back to your error: You cannot have a TypeError: failed to fetch with a successful request. You probably have another request (check your "network" panel to see all of them) that breaks and causes this error to be logged. Also, maybe check "Preserve log" to be sure the panel is not cleared by any indelicate redirection. Sometimes I happen to have a persistent "console" panel, and a cleared "network" panel that leads me to have error in console which is actually unrelated to the visible requests. You should check that.

Or you (but that would be vicious) actually have a hardcoded console.log('TypeError: failed to fetch') in your final .catch ;) and the error is in reality in your .then() but it's hard to believe.

In my case I got "TypeError" when using online JS tools like jsfiddle or stackblitz and the problem was that my url was http instead of https.

Behind the scenes the XHR client sends an HTTPOPTIONS request, called pre-flight, to look for certain security permission headers, called CORS Allow headers. If the required headers are not found (as in you might not have the necessary permissions), TypeError is the result since it has not actually tried to send your POST/GET request. You can observe this behavior in the browser console: it looks like the browser makes two requests to the same location, first having the HTTP Method: OPTIONS.

I was getting this issue since I have not added CORS in my flask app. I need to add it, then it worked, here are the lines:

...
from flask_cors import CORS

def create_app(test_config=None ):

    app = Flask(__name__)

    app.config.from_object('config')  # Import things from config

    CORS(app)

    # CORS Headers 
    @app.after_request
    def after_request(response):
        response.headers.add('Access-Control-Allow-Headers', 'Content-Type,Authorization,true')
        response.headers.add('Access-Control-Allow-Methods', 'GET,PUT,POST,DELETE,OPTIONS')
        return response

    ...

I had this same error when using Firebase Cloud Functions and was able to fix it using the CORS npm package like this:

const functions = require("firebase-functions");
const cors = require('cors')({origin: true});

exports.exampleCORSFunction = functions.https.onRequest((req, res) => {
    cors(req, res, () => {
       //do your CORS stuff
       return res.send({response: "success"});
    });
});

Wrap your cloud function in the CORS module and it will return header values the allow for cross origin.