A fetch API polyfill for React Native with text streaming support.



npm version ci

A fetch API polyfill for React Native with text streaming support

This is a fork of GitHub's fetch polyfill, the fetch implementation React Native currently provides. This project features an alternative fetch implementation directy built on top of React Native's Networking API instead of XMLHttpRequest for performance gains. At the same time, it aims to fill in some gaps of the WHATWG specification for fetch, namely the support for text streaming.

In practice, this implementation is a drop-in replacement to GitHub's polyfill as it closely follows its implementation. Do not use this implementation if your application does not require to stream text.


GitHub's fetch polyfill, originally designed with the intention to be used in web browsers without support for the fetch standard, most notably does not support the consumption of a response body as a stream.

However, as React Native does not yet provide direct access to the underlying byte stream for responses, we either have to fallback to XMLHttpRequest or React Native's networking API for iOS and Android. Currently, only strings can be transfered through the bridge, thus binary data has to be base64-encoded (source) and while React Native's XHR provides progress events to receive incremental data, it concatenates the response string as data comes in. Although very inefficient, the response can be sliced up, each chunk encoded into its UTF-8 representation with TextEncoder and finally enqueued to the stream.

Instead of relying on XMLHttpRequest, which degrades performance, we remove it out of the equation and have fetch interact with React Native's Networking API directly instead. To make Response.body work, ReadableStream's controller was integrated with native progress events. It's important to stress that progress events are only fired when the native response type is set to text (https://github.com/facebook/react-native/blob/v0.63.4/Libraries/Network/RCTNetworking.mm#L544-L547), therefore limiting streaming to text-only transfers. If you wish to consume binary data, either blob or base64 response types have to be used. In this case, the downside is that the final response body is read as a whole and enqueued to the stream's controller as a single chunk. There is no way to read a partial response of a binary transfer.

For more context, read the following:



React Native v0.62.0+ is the minimum version supported where the Networking API has been made public.

This implementation depends on the following web APIs which are not currently available in React Native:

It should be possible remove the dependency on TextEncoder and TextDecoder, but not on ReadableStream. Either way, beware the bundle size of your application will inevitable increase.

To polyfill the above APIs, use react-native-polyfill-globals.


$ npm install react-native-fetch-api --save


The APIs provided by GitHub's implementation in React Native have to be replaced by those provided by this implementation. To do so, check and install react-native-polyfill-globals and follow the instructions therein.


No need to import anything after the setup is done. All APIs will be available globally.


  .then(response => response.json())
  .then(json => console.log(json))

Check fetch's official documentation to learn more about the concepts and extended usage.

Enable text streaming

A non-standard option was added to fetch to enable incremental events in React Native's networking layer.

fetch('https://jsonplaceholder.typicode.com/todos/1', { reactNative: { textStreaming: true } })
  .then(response => response.body)
  .then(stream => ...)

Aborting requests

It's possible to abort an on-going request and React Native already supports AbortController, so there is no need for a polyfill.

const controller = new AbortController();

fetch('https://jsonplaceholder.typicode.com/todos/1', { signal: controller.signal })
  .then(response => response.json())
  .then(json => console.log(json))

Learn more about aborting fetch at https://developers.google.com/web/updates/2017/09/abortable-fetch.


There is no concept of Cross-Origin Resource Sharing (CORS) in native apps. React Native only accepts a boolean value for the credentials option. As such, to send cookies you can either use same-origin and include.

The Set-Cookie response header returned from the server is a forbidden header name and therefore can't be programmatically read with response.headers.get(). Instead, the platform's native networking stack automatically manages cookies for you.

If you run into issues with cookie-based authentication, read the following:

Alternatively, you may consider using the react-native-cookies.

Request caching directive

The only values supported for the cache option are no-cache and no-store and Both achieve exactly the same result. All other values are ignored. Following GitHub's implementation, a cache-busting mechanism is provided by using the query parameter _ which holds the number of milliseconds elapsed since the Epoch when either no-cache or no-store are specified.

Redirect modes directive

The fetch specification defines these values for the redirect option: follow (the default), error, and manual. React Native does not accept such option but it does transparently follow a redirect response given the Location header for 30x status codes.


To run the test suite, you must use react-native-test-runner CLI. Run the run-tests.js wrapper script to spin up a local HTTP server to execute the networking tests against.


$ ./run-tests.js --platform ios --simulator '<simulator>' test/index.js 

Where <simulator> can be a combination of a device type and iOS version, e.g. iPhone 11 (14.1), or a device UUID. Check which simulators are available in your system by running the following command:

$ xcrun xctrace list devices


$ ./run-tests.js --platform android --emulator '<emulator>' test/index.js 

Where <emulator> is the name of the Android Virtual Device (AVD), e.g. Pixel_API_28_AOSP. Check which emulators are available in your system by running the following command:

$ emulator -list-avds
  • fix: refactor BlobResponse and ArrayBufferResponse

    fix: refactor BlobResponse and ArrayBufferResponse

    closes #11

    simplifies implementation of both classes by extending Request and only supplying constructor and clone. Added some tests that are copy of existing tests with a clone call shoved in there. Haven't updated package.json or changelog yet

    opened by slightlytyler 8
  • Looking for collaborators

    Looking for collaborators

    @qalqi @cpojer @mislav @MattiasBuelens @ambarc @mythz If any of you are interested in contributing to this react-native-community/fetch effort, please let me know!

    For context on why this fork exists, see this PR.

    Also, if anyone reading this is interested, feel free to comment here. Thanks!

    opened by pcowgill 5
  • StreamBlobResponse should not return a promise

    StreamBlobResponse should not return a promise

    I'm working on integrating this polyfill to enable text streaming in my app and have encountered a problem that goes like this:

    fetch('...').then(response => {
        console.log(response.headers); // map object
        const clone = response.clone();
        console.log(clone.headers); // undefined, and headers isn't optional
        clone.then(cloneResponse => {
            console.log(cloneResponse.headers); // map object

    It seems like StreamBlobResponse constructor should not return a promise https://github.com/react-native-community/fetch/blob/master/src/StreamBlobResponse.js#L11-L22

    I experimented with removing the returns and that does seem to fix it. The implementation treats the respective class properties as optional so everything continues to work. Was this an oversight or is there a deeper reason that it's implemented like this which breaks the Response object?

    opened by slightlytyler 3
  • Implement response.body as a stream

    Implement response.body as a stream

    Moving this issue https://github.com/github/fetch/issues/746 here

    opened by pcowgill 2
  • Add CI

    Add CI

    opened by acostalima 0
  • Fix iOS job in GitHub workflow

    Fix iOS job in GitHub workflow

    Investigate why the iOS job is now failing with the following error:

    Invalid runtime: com.apple.CoreSimulator.SimRuntime.iOS-14-2

    The runtime in question was available before, so GitHub might have changed something in the latest macOS virtual environment.

    See: https://github.com/actions/virtual-environments/blob/main/images/macos/macos-11.0-Readme.md

    opened by acostalima 0
  • Fix iOS workflow

    Fix iOS workflow


    • Tests on iOS platform now run in React Native 0.62.0 temporarily, i.e., until the iOS build on 0.63.4+ does not fail in the latest macOS.
    • Added cache for npm, Android build and iOS build.
    • Added badges to README.
    • Updated react-native-test-runner to 5.0.0.

    Fixes #9

    opened by acostalima 0
  • Get rid of the dependency on TextEncoder and TextDecoder

    Get rid of the dependency on TextEncoder and TextDecoder

    It should be possible to rewrite the code to not rely on both TextEncoder and TextDecoder, just like GitHub as done:

    • Decode: https://github.com/github/fetch/blob/a8aa427de0ed808ff26c0e3eb2e59c122c44488a/test/test.js#L79
    • Encode: https://github.com/github/fetch/blob/a8aa427de0ed808ff26c0e3eb2e59c122c44488a/test/test.js#L69

    By removing the dependency on both APIs, the app's bundle size can be further reduced. Considering that React Native's environment does not provide these APIs, there would be no need to polyfill them anymore. However, we should, by default, check whether TextEncoder and TextDecoder are available and only then fallback.

    enhancement help wanted 
    opened by acostalima 0
  • Use DOMException to throw errors

    Use DOMException to throw errors

    I believe React Native's environment does not provide DOMException, so we need to use a polyfill ~~or develop our own~~. Check domexpection. If that's the case, for consistency's sake, the polyfill should be delivered via react-native-polyfill-globals. Check GitHub's implementation for inspiration.

    DOMException should be thrown when:

    • fetch is aborted

    See: https://developers.google.com/web/updates/2017/09/abortable-fetch

    enhancement help wanted 
    opened by acostalima 0
  • Technical planning and discussion

    Technical planning and discussion

    @cpojer I'm reaching out to follow up on a previous discussion about coming up with a custom Fetch API implementation for React Native at https://github.com/react-native-community/fetch with support for streaming (https://github.com/facebook/react-native/issues/27741). We, MOXY and Protocol Labs, are planning to start this effort shortly and we'd like to know your thoughts on the matter.

    In short, what we currently have in mind is as follows:

    • Rewrite whatwg-fetch in modern JS.
    • Remove XHR out of the equation and implement fetch directly on top of RN's JavaScript Network API exactly for the same reasons outlined at https://github.com/react-native-community/discussions-and-proposals/issues/99.
    • Assume ReadableStream is available in RN's environment which can be polyfilled with web-streams-polyfill. If ReadableStream is expected to only be used in the context of Fetch, we can probably bundle them together.
    • Have tests in place running in an actual RN app on GitHub's CI.

    Even if Facebook has no plans to add support for streaming to Fetch for the time being, app developers can easily swap out whatwg-fetch for @react-native-community/fetch manually. Even better would be React Native to do this automatically via some sort of extension or plugin API.

    Are there any guidelines which React Native Community packages should conform to? e.g. linter and linting config, CI process config, etc.

    CC @satazor @hugomrdias

    opened by acostalima 13
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