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0.2.41 • Public • Published


A fast node package manager for better productivity and efficiency

NPM version Apache 2.0 License Build Status Coverage Status Dependency Status devDependency Status

fyn is a super fast node package manager with some unique features:

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

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.


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? If you just want to try a different approach to installing your node_modules, then it's worth a look.

fyn's flatten node_modules is the smallest 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.

If your development in Node.js are typically simple and involves only a single module or small applications, then fyn's advantage may not be apparent to you, but if your Node.js project is large and complex, then fyn may be helpful to you. Please read further to learn more.

Enhanced npm link

fyn has a fynlocal mode that's designed specifically to be a much better npm link. It treats packages on your local disk like they've been published. You can install and use them directly, and quickly test changes iteratively. 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.

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.

At the top level, it installs a chosen version of each package. All other versions are installed under the directory node_modules/__fv_/<version>/<package_name>.

When necessary, packages have their own node_modules with symlinks/junctions inside pointing to dependencies under __fv_.

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:

$ fyn stat chalk
> loaded lockfile ~/fyn
> done resolving dependencies 0.113secs
> chalk matched these installed versions chalk@2.4.1, chalk@1.1.3(fv)
> chalk@2.4.1 has these dependents eslint@4.19.1, inquirer@3.3.0, table@4.0.2, visual-exec@0.1.0, visual-logger@0.1.8, webpack-bundle-analyzer@2.13.1, xclap@0.2.24, ~package.json
> chalk@1.1.3 has these dependents babel-code-frame@6.26.0, electrode-server@1.5.1

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.


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.

  • The way fyn uses symlinks to resolve nested dependencies is also fully compatible with Node.js. The only caveat is Node.js module loader always resolve a package's path to its real path.

    For example, if A depends on B@1.0.0 that's not at the top level, then node_modules/A/node_modules/B is a symlink to node_modules/B/__fv_/1.0.0/B.

    Without preserve symlinks, B's path would be resolved to the real path node_modules/B/__fv_/1.0.0/B, instead of the symlink path node_modules/A/node_modules/B.

    If you want to keep the symlink path, then set the environment variable NODE_PRESERVE_SYMLINKS to 1. It doesn't affect normal operations either way unless you have code that explicitly depend on the path, which should be avoided. The subtle difference is that with preserve symlink, each symlink path of the same module will be loaded as its own instance by Node's module system.

  • 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.

Package Resolution and Layout

As a package manager, the top level node_modules installed by fyn is a flat list of all the modules your application needs. It's easier to view and smaller in size. Extra versions of a module will be installed under a directory __fv_, and linked through symlinks or flat-module.

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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