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All packages downloaded from the registry are stored in a global cache at ~/.bun/install/cache. They are stored in subdirectories named like ${name}@${version}, so multiple versions of a package can be cached.

{% details summary="Configuring cache behavior" %}

[install.cache]
# the directory to use for the cache
dir = "~/.bun/install/cache"

# when true, don't load from the global cache.
# Bun may still write to node_modules/.cache
disable = false

# when true, always resolve the latest versions from the registry
disableManifest = false

{% /details %}

Minimizing re-downloads

Bun strives to avoid re-downloading packages multiple times. When installing a package, if the cache already contains a version in the range specified by package.json, Bun will use the cached package instead of downloading it again.

{% details summary="Installation details" %} If the semver version has pre-release suffix (1.0.0-beta.0) or a build suffix (1.0.0+20220101), it is replaced with a hash of that value instead, to reduce the chances of errors associated with long file paths.

When the node_modules folder exists, before installing, Bun checks that node_modules contains all expected packages with appropriate versions. If so bun install completes. Bun uses a custom JSON parser which stops parsing as soon as it finds "name" and "version".

If a package is missing or has a version incompatible with the package.json, Bun checks for a compatible module in the cache. If found, it is installed into node_modules. Otherwise, the package will be downloaded from the registry then installed. {% /details %}

Fast copying

Once a package is downloaded into the cache, Bun still needs to copy those files into node_modules. Bun uses the fastest syscalls available to perform this task. On Linux, it uses hardlinks; on macOS, it uses clonefile.

Saving disk space

Since Bun uses hardlinks to "copy" a module into a project's node_modules directory on Linux, the contents of the package only exist in a single location on disk, greatly reducing the amount of disk space dedicated to node_modules.

This benefit does not extend to macOS, which uses clonefile for performance reasons.

{% details summary="Installation strategies" %} This behavior is configurable with the --backend flag, which is respected by all of Bun's package management commands.

  • hardlink: Default on Linux.
  • clonefile Default on macOS.
  • clonefile_each_dir: Similar to clonefile, except it clones each file individually per directory. It is only available on macOS and tends to perform slower than clonefile.
  • copyfile: The fallback used when any of the above fail. It is the slowest option. On macOS, it uses fcopyfile(); on Linux it uses copy_file_range().
  • symlink: Currently used only file: (and eventually link:) dependencies. To prevent infinite loops, it skips symlinking the node_modules folder.

If you install with --backend=symlink, Node.js won't resolve node_modules of dependencies unless each dependency has its own node_modules folder or you pass --preserve-symlinks to node. See Node.js documentation on --preserve-symlinks.

$ bun install --backend symlink
$ node --preserve-symlinks ./foo.js

Bun's runtime does not currently expose an equivalent of --preserve-symlinks. {% /details %}