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Bun itself is MIT-licensed.

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JavaScriptCore

Bun statically links JavaScriptCore (and WebKit), which is LGPL-2 licensed. WebCore files from WebKit are also licensed under LGPL-2. Per LGPL-2:
(1) If you statically link against an LGPL’d library, you must also provide your application in an object (not necessarily source) format, so that a user has the opportunity to modify the library and relink the application.
Bun’s patched version of WebKit lives at https://github.com/oven-sh/webkit. To relink Bun with changes:
  • git clone https://github.com/oven-sh/WebKit vendor/WebKit
  • bun sync-webkit-source (checks out the version pinned in WEBKIT_VERSION in scripts/build/deps/webkit.ts)
  • bun run build:local
bun run build:local compiles JavaScriptCore, compiles Bun’s .cpp bindings for JavaScriptCore (the object files that use JavaScriptCore), and outputs a new bun binary with your changes.

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Linked libraries

Bun statically links these libraries:

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Polyfills

For compatibility, Bun embeds the following packages into its binary and injects them if imported.

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Additional credits

  • Bun’s JS transpiler, CSS lexer, and Node.js module resolver source code is a port of @evanw’s esbuild project.
  • Credit to @kipply for the name “Bun”.