Add page on running Scratch locally

content/docs/develop/testing-on-a-pre-release-version-of-scratch.md

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+---
+title: Testing on a Pre-Release Version of Scratch
+---
+
+This page explains the process of setting up a version of the Scratch editor or website not yet available on scratch.mit.edu to test Scratch Addons with.
+
+## Editor
+
+### Pre-built Branch
+
+The `develop` branch of the Scratch editor is available at https://scratchfoundation.github.io/scratch-editor/scratch-gui. Pre-release versions of Scratch Addons are configured to run on scratchfoundation.github.io by default. Other domains will require patches in Scratch Addons and Scratch Lab is not supported.
+
+### From source
+
+Start by cloning `scratch-editor` and specifying `--branch <branch-name>` if needed. `--depth=1` may be added to speed up the download as the Git history is very large.
+
+```sh
+git clone https://github.com/scratchfoundation/scratch-editor.git
+```
+
+Then in the freshly cloned repository run `npm ci` to install the dependencies and `npm run build` to build it.
+
+To start the editor run `npm run start` in `scratch-editor/packages/scratch-gui` and open `localhost:8601` in a browser.
+
+## Website
+
+These instructions are for running a local instance of the Scratch 3 pages (`scratch-www`). Scratch 2 pages (`scratchr2`) such as user profiles or the forums are not supported because the code is not open source.
+
+### Building
+
+```sh
+git clone https://github.com/scratchfoundation/scratch-www.git
+cd scratch-www
+npm install
+npm run translate
+npm run build
+```
+
+### Running
+
+Install and run [cors-anywhere](https://github.com/Rob--W/cors-anywhere) which will provide a proxy on port 8080.
+
+```sh
+npm install -D cors-anywhere
+node ./node_modules/cors-anywhere/server.js
+```
+
+Then in another terminal point the hosts to it and start `scratch-www`:
+
+```sh
+export API_HOST=http://localhost:8080/https://api.scratch.mit.edu
+export BACKPACK_HOST=http://localhost:8080/https://backpack.scratch.mit.edu
+npm run start
+```
+
+Finally open `localhost:8333` in a browser.
+
+### Fixing /session
+
+{{< admonition warning >}}
+Avoid including a token unless it is actually necessary, for example to test the backpack.
+{{</ admonition >}}
+
+Generate a data URI from a valid `/session` response. On macOS or Linux assuming it has already been downloaded as a file this can be done with:
+
+```sh
+echo "data:application/json;base64,$(base64 response.json)"
+```
+
+Then setup a redirect:
+
+1. Install the [Redirector](https://einaregilsson.com/redirector/) browser extension.
+2. Create a new redirect
+3. Set the include pattern to `http://localhost:8333/session/` where port 8333 is `scratch-www`
+4. Paste the data URI in the "redirect to:" field
+5. Expand the advanced options check "XMLHttpRequests (Ajax)"