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All packages downloaded from the registry are stored in a global cache at ~/.jstime/install/cache. They are stored in subdirectories named like ${name}@${version}, so multiple versions of a package can be cached. 
[install.cache]
# the directory to use for the cache
dir = "~/.jstime/install/cache"

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

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

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Minimizing re-downloads

JSTime 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, JSTime will use the cached package instead of downloading it again.
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, JSTime checks that node_modules contains all expected packages with appropriate versions. If so jspm install completes. JSTime 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, JSTime 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.

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Fast copying

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

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Saving disk space

Since JSTime 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.
This behavior is configurable with the --backend flag, which is respected by all of JSTime’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.
$ jspm install --backend symlink
$ node --preserve-symlinks ./foo.js
JSTime’s runtime does not currently expose an equivalent of --preserve-symlinks.