JavaScript (Fetch) backend

A JavaScript backend implemented using the Fetch API and backed via Future.

This is the default backend, available in the main jar for JS. To use, add the following dependency to your project:

"com.softwaremill.sttp.client3" %%% "core" % "3.0.0-RC13"

And create the backend instance:

val backend = FetchBackend()

Timeouts are handled via the new AbortController class. As this class only recently appeared in browsers you may need to add a polyfill.

As browsers do not allow access to redirect responses, if a request sets followRedirects to false then a redirect will cause the response to return an error.

Note that Fetch does not pass cookies by default. If your request needs cookies then you will need to pass a FetchOptions instance with credentials set to either RequestCredentials.same-origin or RequestCredentials.include depending on your requirements.


Running sttp in a node.js will require downloading modules that implement the various classes and functions used by sttp, usually available in browser. At minima, you will need replacement for fetch, AbortController and Headers. To achieve this, you can either use npm directly, or the scalajs-bundler sbt plugin if you use sbt :

npm install --save node-fetch
npm install --save abortcontroller-polyfill
npm install --save fetch-headers

You then need to load the modules into your runtime. This can be done in your main method as such :

val g =
g.fetch = g.require("node-fetch")
g.Headers = g.require("fetch-headers")


Streaming support is provided via FetchMonixBackend. Note that streaming support on Firefox is hidden behind a flag, see ReadableStream for more information.

To use, add the following dependency to your project:

"com.softwaremill.sttp.client3" %%% "monix" % "3.0.0-RC13"

An example of streaming a response:

import sttp.client3._
import sttp.client3.impl.monix._

import java.nio.ByteBuffer
import monix.eval.Task
import monix.reactive.Observable

val backend = FetchMonixBackend()

val response: Task[Response[Observable[ByteBuffer]]] =


Currently no browsers support passing a stream as the request body. As such, using the Fetch backend with a streaming request will result in it being converted into an in-memory array before being sent. Response bodies are returned as a “proper” stream.