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Hosting the Bundler


You can also host the bundler by yourself, all necessary files are in the sandpack folder.

If you want to host the bundler yourself, you will need to do a few things.

  • The bundler is part of the codesandbox-client codebase: https://github.com/codesandbox/codesandbox-client
  • Clone the codesandbox-client and install the dependencies in the root folder (yarn install).
  • yarn build:deps to build some of the packages lerna needs for internal links.
  • create your instance of sandpack with yarn build:sandpack.

This creates a www folder in the root of codesandbox-client. That www folder is the sandpack folder sandpack-client connects to on {version}-sandpack.codesandbox.io. Once you have this hosted on your end you can pass bundlerURL when calling:

new SandpackClient(iframe, sandboxInfo, {
  bundlerURL: "https://your-hosted-version",

or, if you use sandpack-react, you can bundlerURL in the options of the Sandpack preset.


There are few reasons for hosting the bundler like this, as opposed to having it exported as library code.


The bundler evaluates and transpiles all files in an iframe under a different subdomain. This is important, because it prevents attackers from tampering with cookies of the host domain when evaluating code.


We heavily make use of Web Workers for transpilations. Almost all our transpilation happens in web workers, and there is no easy way yet to bundle this in a library.

Bundle Size

Another reason to host the bundler externally is because of code splitting: we split all our transpilers away and load them on-demand. If a user doesn't use sass we won't load the transpiler. This wouldn't be possible if we would give one big JS file as the library.

Offline Support

We use Service Workers to download all transpilers in the background, so the next time a user visits your website they don't have to download the bundler anymore and it can be used offline. This is possible because we host the service worker externally.