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Lerna Hoisting

When an overall project is divided into more than one NPM package, this organizational improvement generally comes with a cost: the various packages often have many duplicate dependencies in their package.json files, and as a result hundreds or thousands of duplicated files in various node_modules directories. By making it easier to manage a project comprised of many NPM packages, Lerna can inadvertently exacerbate this problem.

Fortunately, Lerna also offers a feature to improve the situation - Lerna can reduce the time and space requirements for numerous copies of packages in development and build environments, by "hoisting" shared dependencies up to the topmost, Lerna-project-level node_modules directory instead.

--hoist is intended to be transparent in use, a runtime optimization that ideally does not require any other modifications to a project. When the --hoist flag is used:

  • Common dependencies will be installed only to the top-level node_modules, and omitted from individual package node_modules.
  • Mostly-common dependencies are still hoisted, but outlier packages with different versions will get a normal, local node_modules installation of the necessary dependencies.
    • In this instance, lerna bootstrap will always use npm install with the --global-style flag, regardless of client configuration.
  • Binaries from those common packages are symlinked to individual package node_modules/.bin directories, so that package.json scripts continue to work unmodified.
  • Well-behaved Node-based software should continue to work unmodified.

Module resolution

The Node module resolution algorithm is recursive: When looking for package A, it looks in a local node_modules/A directory, then in ../node_modules/A, ../../node_modules/A, ../../../node_modules/A, etc.

Tooling that follows this specification can transparently find dependencies which have been hoisted.

Unfortunately, some tooling does not follow the module resolution spec closely, and instead assumes or requires that dependencies are present specificially in the local node_modules directory. To work around this, it is possible to symlink packages from their hoisted top-level location, to individial package node_modules directory. Lerna does not yet do this automatically, and it is recommended instead to work with tool maintainers to migrate to more compatible patterns.