bun.sh/src/node-fallbacks/README.md

Browser polyfills for bun build --target=browser

When using bun build --target=browser, if you attempt to import a Node.js module, Bun will load a polyfill for that module in an attempt to let your code still work even though it's not running in Node.js or a server.

For example, if you import zlib, the node-fallbacks/zlib.js file will be loaded.

Not used by Bun's runtime

These files are not used by Bun's runtime. They are only used for the bun build --target=browser command.

If you're interested in contributing to Bun's Node.js compatibility, please see the src/js directory.