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Railpack Docs

Bun

Railpack supports Bun as a JavaScript runtime and package manager.

Detection

Section titled “Detection”

Railpack selects Bun through JavaScript package manager detection. Bun is selected when the packageManager field declares it, or when no higher-priority package manager is configured and an engines.bun field, bun.lock, or bun.lockb file exists.

Bun is also installed when a package script or configured start command invokes it.

Versions

Section titled “Versions”

The Bun version can be configured using:

  • The RAILPACK_BUN_VERSION environment variable
  • The packageManager field in package.json, such as [email protected]
  • The engines.bun field in package.json
  • A .bun-version, mise.toml, or .tool-versions file

The version defaults to latest when none is configured.

Node.js Compatibility

Section titled “Node.js Compatibility”

Some Bun projects also require Node.js for Corepack, scripts, frameworks, or native module compilation. Railpack installs Node.js automatically when it is needed.

Bun applications otherwise use the same framework, monorepo, and SPA detection described in the Node.js documentation.