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    fyn is a node package manager that makes your disk a direct registry. It enables you to develop, publish, and test all your packages using local copies directly.

    Additionally, it has more unique features to improve productivity and efficiency:

    fyn demo

    Quick Start

    Interested in giving it a quick test? Just install and run it on your project:

    npm i -g fyn
    cd <your-project>

    Want to add a package on your local disk as a dependency to your project? Do this:

    fyn add ../another-package

    To see detailed stats about any package, use the stat command:

    fyn stat lodash
    • It can read and use some settings from your .npmrc.
    • It can use npm-shrinkwrap.json or package-lock.json files.

    Want to find out more? Please read on below:

    Table Of Contents



    • Focus on improving workflow and productivity.
    • Very comprehensive and proper handling of optionalDependencies.
    • A new devOptDependencies allows optional devDependencies.
    • Guaranteed single copy of a package => smaller node_modules.
    • The best at installing and linking local packages - better npm link.
    • Install local packages like they are published (fynlocal mode)
    • Works particularly well with lerna monorepos.
    • Shows detailed stats of your dependencies.
    • Efficient disk space usage with optional central storage.
    • Central storage mode is fast (and very fast on Linux) once cache is hot.
    • Install dependencies with a time stamp lock.


    • A super fast node package manager for installing modules.
    • Production quality with a lot of unit tests and verified on real applications.
    • 100% compatible with Node.js and its ecosystem.
    • A flat and simple dependency lock file that can be diffed and edited.
    • Always deterministic node_modules installation.
    • Compatible with npm by internally using the same modules as npm.
    • Maintains as much of npm's behaviors as possible.
    • Able to use npm's npm-shrinkwrap.json or package-lock.json.
    • built-in support for maintaining a mono-repo workspace.


    fyn is the result of a long pursuit to make developing and managing large and complex software in Node.js easier. To realize that, it ultimately ends up being a node package manager.

    It started out as small experiments for a single goal of better local package installing and linking, ie: better npm link, but has gradually grown to a fully functional node package manager for the [flat node_modules design]. It is fast, production quality, and maintains 100% compatibility.

    While it has all the bells and whistles to make it an extremely fast and efficient package manager, it's not just another npm.

    It comes with two unique features that are very useful when you are working on a large Node.js application that consists of many packages.


    So why would you want to use this?

    fyn's node_modules structure is the smallest possible in size because there are no multiple copies of the exact same package installed.

    It also has a special fynlocal mode that's a better npm link for handling local packages.

    It's workspace aware and fits perfectly with the mono-repo concept.

    Enhanced npm link

    fyn has a fynlocal mode that's designed specifically to be a much better npm link. It effectively makes your disk a npm registry by treating packages on your local disk like they've been published. You can install and use them directly, and quickly test changes iteratively. It fits perfectly with the mono-repo workspace concept. It would be very useful if you've ever done any of these:

    • Debug your application by inspecting code inside node_modules.
    • Live edit your package that's installed to node_modules, and then have to copy the changes out to commit.
    • Use lerna to maintain and develop multiple packages. fyn works particularly well with a lerna repo.
    • Or just have to juggle a lot of packages as part of your development.

    fynlocal mode

    What is this? Think npm link, but better. fyn subjects local packages to the same dependency resolution logic as those from the npm registry. Then you can test changes to any module locally as if they were published, which basically makes your disk a npm registry.

    To enable, use the path to your local modules as semver in your package.json, or you can use the fyn add command.

    For example:

    fyn add ../my-awesome-module

    That will install my-awesome-module into your node_modules. You can continue to develop and test my-awesome-module in its own directory and have the changes within existing files reflected in your app directly. Unlike npm link, your app resolves dependencies for my-awesome-module instead of relying on having them installed under my-awesome-module/node_modules.

    If you add/remove files/directories in your local package, then running fyn install would take only seconds to update.

    fyn will also save a file package-fyn.json with local dependencies in a section called fyn. You should not commit this file and .gitignore it. fyn will automatically check this file when installing, but you can turn off fynlocal mode with with the flag --no-fynlocal easily.

    Smaller node_modules

    As a package manager, fyn employs a different approach that installs only one copy of every required versions of a package in a flat node_modules structure. Hence the name fyn, which stands for Flatten Your Node_modules.

    It installs a copy of each version under the directory node_modules/.f/_/<package_name>/<version>. And it uses symlink to hoist a single version to node_modules for visibility. A package may have their own node_modules to resolve version differences.

    This approach has the benefit of guaranteeing a single copy of a package installed and therefore slightly smaller size node_modules.

    Easier Debugging node_modules

    With a guaranteed single copy of a package, it makes debugging easier when you have to reach into code under node_modules.

    node_modules installed by npm could potentially have multiple copies of an identical package. So even if you've identified the module under node_modules to investigate your issue, you may still need to figure which copy.

    With fyn's flat node_modules design, there is only one copy of any version so it's easier for you to set your breakpoint.

    Using fyn

    Installing fyn

    Please install fyn to your Node.js setup globally.

    npm install -g fyn

    Installing Your Dependencies

    Change into the directory for your project with the package.json file, and run:

    • Which is a shorthand for fyn install since install is the default command.

    Depending on the size of your dependencies and your network speed, this could take anywhere from a few seconds to a few minutes.

    Running npm scripts

    As a convenience, fyn implements npm run by utilizing the same modules from npm. You can run your npm scripts in package.json. An alias command fun is available also:

    • test - fyn test or fun test
    • any script - fyn run <script-name> or fun <script-name>
    • list scripts - fyn run -l or fun -l

    The stat command

    If you have a lockfile, then fyn takes sub seconds to regenerate the entire dependency tree even on very large applications. This makes it very fast to probe what's installed.

    It has a stat command that's very fast and can let you know all copies of a package installed and all others that depend on it.

    For example:

    > loaded lockfile
    > done resolving dependencies 0.157secs
    lodash matched these installed versions lodash@4.17.20
    => lodash@4.17.20 has 15 dependents: @babel/core@7.12.13 @babel/helper-module-transforms@7.12.13 @babel/traverse@7.12.13 @babel/types@7.12.13 @eslint/eslintrc@0.3.0 @xarc/module-dev@3.2.1 async@2.6.3 electrode-confippet@1.6.0 electrode-server@3.3.0 eslint@7.19.0 eslint-plugin-jsdoc@30.7.13 table@6.0.7 webpack-bundle-analyzer@3.9.0 ~package.json
    => lodash@4.17.20 has 258 dependency paths, showing the 18 most significant ones below:
      > @babel/preset-env@7.12.13 > @babel/plugin-transform-modules-amd@7.12.13 > @babel/helper-module-transforms@7.12.13 > lodash@4.17.20
      > @babel/preset-env@7.12.13 > @babel/plugin-transform-modules-commonjs@7.12.13 > @babel/helper-module-transforms@7.12.13 > lodash@4.17.20
      > @babel/preset-env@7.12.13 > @babel/plugin-transform-modules-systemjs@7.12.13 > @babel/helper-module-transforms@7.12.13 > lodash@4.17.20
      > @babel/preset-env@7.12.13 > @babel/plugin-transform-modules-umd@7.12.13 > @babel/helper-module-transforms@7.12.13 > lodash@4.17.20
      > @babel/preset-env@7.12.13 > @babel/types@7.12.13 > lodash@4.17.20
      > @xarc/module-dev@3.2.1 > lodash@4.17.20
      > babel-eslint@10.1.0 > @babel/traverse@7.12.13 > lodash@4.17.20
      > babel-eslint@10.1.0 > @babel/types@7.12.13 > lodash@4.17.20
      > electrode-server@3.3.0 > async-eventemitter@0.2.4 > async@2.6.3 > lodash@4.17.20
      > electrode-server@3.3.0 > electrode-confippet@1.6.0 > lodash@4.17.20
      > electrode-server@3.3.0 > lodash@4.17.20
      > eslint-plugin-jsdoc@30.7.13 > lodash@4.17.20
      > eslint@7.19.0 > @eslint/eslintrc@0.3.0 > lodash@4.17.20
      > eslint@7.19.0 > lodash@4.17.20
      > eslint@7.19.0 > table@6.0.7 > lodash@4.17.20
      > lodash@4.17.20
      > nyc@15.1.0 > istanbul-lib-instrument@4.0.3 > @babel/core@7.12.13 > lodash@4.17.20
      > webpack-bundle-analyzer@3.9.0 > lodash@4.17.20
    > stat completed for lodash

    Locking Dependencies by Time

    Ever want to install your dependencies only consider packages published up to a certain date in the past? fyn's got you covered with the --lock-time option.

    • First rename or remove fyn-lock.yaml file.
    • Then run install like this:
    rm fyn-lock.yaml
    fyn install --lock-time "12/01/2018"


    fyn install --lock-time "dec 01, 2018"

    And fyn will only consider packages published up to Dec 01, 2018 when installing.

    Refreshing Optional Dependencies

    If you have any optional dependencies, then they will not be re-evaluated if you have a lock file.

    You can re-evaluate optional dependencies with --refresh-optionals option:

    fyn install --refresh-optionals

    Using with Lerna

    lerna actually implements its own internal npm link like feature to support a monorepo with packages that depend on each other.

    fyn works particularly well with a lerna monorepo, but since it offers an enhanced npm link, it replaces lerna's bootstrap feature.

    To bootstrap a lerna repo with fyn's enhanced npm link, please use the module fynpo.

    fyn also has a central storage option that would saves you a lot of disk space when working with lerna repos.

    You can use fynpo's local command to update and commit your monorepo's packages' package.json, and you can run fyn to install and update their dependencies without having to do it through bootstrap.

    For example:

    fynpo local
    cd packages/my-awesome-package

    Configuring fyn

    fyn options can be listed in help:

    fyn --help

    fyn loads config from CWD/.fynrc, CWD/.npmrc, ~/.fynrc, and ~/.npmrc in this specified order, from highest to lowest priority.

    From .npmrc, only fields registry, @<scope>:registry,email, and _auth are read.

    .fynrc file can be an ini or YAML format. For the YAML format, the first line must be ---.

    Below is an YAML example, with all the options set to their default values:

    registry: https://registry.npmjs.org
    "@scope:registry": https://registry.custom.com
    offline: false
    forceCache: false
    lockOnly: false
    progress: normal
    logLevel: info
    production: false
    centralStore: false

    Or as an ini:


    Command Line Option to RC Mapping

    Any command line option can be converted to an option in the RC file by changing the name to camelCase form.

    If there's no RC file or command line override, then these defaults are used:

    • registry - https://registry.npmjs.org
    • progress - normal
    • logLevel - info

    Other RC Options

    Scope registry

    Scope registry can be specified in the RC files, the same as .npmrc.

    For example, in Yaml format:

    "@scope:registry": https://registry.custom.com

    In ini format:


    Central Storage

    Inspired by pnpm, fyn supports storing a single copy of all packages at a central location, and use hardlinks to install them into your node_modules.

    The main advantage of this is to save disk space and slightly faster install if the storage is primed.

    However, this feature is not enabled by default due to the following drawbacks:

    1. Creating hardlinks actually could take a lot more than trivial time.

      • What this means is the first time you install with fyn, when nothing is cached in the storage, central store mode will actually take noticeably more time, but subsequent installs could be faster.

      • In particular, very bad on MacOS (High Sierra). For example, using hardlinks to replicate the module nyc actually takes longer than untaring the tgz file. It improves somewhat with concurrency, but still significant.

      • On Linux with ext4 hardlinking appears to be more than 10 times more efficient than MacOS.

    2. You can't do your debugging and development by modifying code that's installed into node_modules directly.

      • Reason being that any change you make will affect the central copy, and therefore any other node_modules that's linked to it.

      • If you do this, then even after you blow away your node_modules and reinstall it, your "debugging" changes will be there again.

      • I imagine that this is actually a fairly big drawback for a lot of people.

      • However, the primary design goal of fyn is to make your module development easier with its local linking install feature. You should use that to develop and debug multiple modules locally.

    3. Similar to 2, but if any package has postinstall script that modifies its own files, then those modifications would affect all installations.

      • There should not be a lot of packages like this, but if you happen to use one, it's unlikely a central storage would work.

    In general if disk space is not an issue for you, then it's better to avoid this and the issues that will likely creep up on you when you least expect it.

    If you do have a use of this feature despite the drawbacks, then you can enable it with the --central-store CLI option.

    The recommendation is to add the following to .fynrc because then you don't have to remember to specify the option in the CLI every time.


    You can also set the env variable FYN_CENTRAL_DIR to 1 to enable it. If you set it to point to a directory then it will be used as the central store directory.

    And to work around the issues, fyn does the following:

    • issue 2: fyn has a --copy option that allows you to force any package to install with copying instead of hardlinking.
    • issue 3: fyn will not hard link packages from central store if they have preinstall, install, or postinstall npm scripts.

    Other Info


    • fyn's top level node_modules is 100% compatible with Node.js and 3rd party tools and modules. No special updates or changes needed.

    • fyn uses npm's pacote to do data retrieval. That means its package data handling is the same as npm and it can use npm's cache directly.

    • fyn will take npm's npm-shrinkwrap.json or package-lock.json if its own fyn-lock.yaml file doesn't exist, but will save fyn-lock.yaml after.

    • fyn has an asynchronous and concurrent dependency resolution engine that is 100% compatible with node's nesting design, and properly handles optionalDependencies.

    Thank you npm

    Node Package Manager is a very large and complex piece of software. Developing fyn was 10 times easier because of the generous open source software from the community, especially the individual packages that are part of npm.

    Other than benefiting from the massive package ecosystem and all the documents from npm, these are the concrete packages from npm that fyn is using directly.

    • node-tar - for untaring tgz files.
    • semver - for handling Semver versions.
    • pacote - for retrieving npm package data.
    • ini - for handling ini config files.
    • npm-packlist - for filtering files according to npm ignore rules.
    • npm-lifecycle - for npm_config env and offering run as a convenience.
    • npmlog - for offering the run command as a convenience.
    • And all the other packages they depend on.


    Copyright (c) 2015-present, WalmartLabs

    Licensed under the Apache License, Version 2.0.


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