4 reasons to avoid using `npm link`

Apr 18, 2022 · 10min

The dangers of `npm link` and why you should use `npx link` instead

TL; DR

Instead of using npm link, use npm install or npx link to symlink a local package as a dependency:

Terminal
$ npx link <package-path>

npx link is a tool I developed as a safer and more predictable alternative to npm link.

Avoid using npm link because of the following footguns:

  1. Error-prone with multiple Node.js versions
  2. No fail-case and unexpected fallback to npm registry
  3. Unexpected binary installation
  4. Unexpected link removal

npm link is a command-line tool for symlinking a local package as a dependency during development. It is commonly used for testing packages before publishing them.

Read more about it in the official documentation.

Usage

Given the following packages:

  • my-library: an npm package that you want to test in another package as a dependency.

    The name property in my-library/package.json should be my-library.

  • my-application: the package/project you want to test in

Here’s how you would link them:

  1. Registration (Global installation)

    Run npm link in my-library to install it globally, making it possible to link my-library to any local project. Note: this is the same thing as running npm install --global.

Terminal 1
$ cd ./my-library
$ npm link
  1. Installation

    Run npm link my-library in my-application to link it:

Terminal 2
$ cd ./my-application
$ npm link my-library

Shortcut

npm link <package-path> is a shortcut to automate the two steps by simply passing in the package path.

Using the example above:

Terminal
$ cd ./my-application
$ npm link ../my-library

The shortcut approach is much easier to use and is less error-prone because it’s a single command that requires an explicit path to the package to link.

1. Multiple Node.js versions

If your environment has multiple Node.js versions using a manager like nvm, both npm link commands must be run using the same version.

As explained above, the first step of npm link is installing the package globally. Since each version of Node.js has its own global package registry, lookups will fail if different versions are used.

You can check if the global package registry is scoped to the Node.js version with the following command. If the Node.js version is in the path, the global package registry is scoped:

Terminal
$ npm root -g
~/.nvm/versions/node/v14.16.1/lib/node_modules

When working on multiple packages in separate terminal sessions, it’s very easy to overlook the Node.js version. The version discrepancy can be especially hard to notice since npm link doesn’t error when it’s unable to find the local package to link, which is discussed in the next section.

Pro tip: Add the recommended shell integration to automatically use the appropriate Node.js version when entering a directory with a .nvmrc file.

2. Non-existent fail-case

Try running npm link a in a package.

It will succeed despite never registering package a to be linkable before:

Terminal
$ npm link a
~/my-package/node_modules/a -> ~/.nvm/versions/node/v14.16.1/lib/node_modules/a

This is because when npm link can’t find package a as a global package, it installs it globally from the npm registry and creates a symlink to it.

It only fails when the package is also not found on the remote registry:

Terminal
$ npm link non-existent-package
npm ERR! code E404
npm ERR! 404 Not Found - GET https://registry.npmjs.org/non-existent-package - Not found
npm ERR! 404 
npm ERR! 404  'non-existent-package@*' is not in this registry.
npm ERR! 404 You should bug the author to publish it (or use the name yourself!)
npm ERR! 404 
npm ERR! 404 Note that you can also install from a
npm ERR! 404 tarball, folder, http url, or git url.

To tell if the link actually succeeded, you can check if the output has two arrows (->). (Notice how the false-positive above only has one arrow.) Two arrows means it created a symlink to the global package, which then points to the local package:

Terminal
$ npm link my-linked-package
~/my-package/node_modules/my-linked-package -> ~/.nvm/versions/node/v14.16.1/lib/node_modules/my-linked-package -> ~/my-linked-package

This check only works in npm v6. Unfortunately, starting in npm v7, the symlink paths are no longer logged. Looking at the output, it’s impossible to determine if linking the local package succeeded, or if an unintended package was accidentally installed and linked:

Terminal
$ npm link a

up to date, audited 3 packages in 671ms

found 0 vulnerabilities

To confirm the package was successfully linked, you can use realpath to verify the symlink path:

Terminal
$ realpath node_modules/package-name
~/my-linked-package

The lack of a proper fail case makes using npm link a confusing and frail process. Especially when compounded with having multiple Node.js versions.

3. Unexpected binary installation

The first step of npm link installs the package globally. This happens in the shortcut as well, because it just automates the two steps.

Global package installation (npm install --global ...) is a type of package installation used to make binaries available as a system-wide CLI command. So, if your package has a bin field, npm linking it will make it available as a CLI command.

Considering npm link is a tool for testing a package in development, global binary installation can be an unexpected and undesired side-effect. The implications of this unexpected behavior can be quite serious given packages can declare binaries with arbitrary names.

In this example package, an arbitrary binary name random-command is specified in the package.json file:

{
    "name": "my-package",
    "bin": {
        "random-command": "bin.js"
    }
}

Running npm link installs binary random-command:

Terminal
$ random-command
zsh: command not found: random-command

$ cd my-package && npm link
added 1 package, and audited 3 packages in 548ms

found 0 vulnerabilities

$ random-command
Suddenly works!

Global install can also override existing binaries depending on your PATH configuration—the variable of paths the shell uses to lookup commands from. If you’re using nvm, your configuration is likely susceptible to this.

In this example, I override the binary cat, a standard Unix utility:

Terminal
$ type cat
cat is /bin/cat

$ cd my-package && npm link
added 1 package, and audited 3 packages in 230ms

found 0 vulnerabilities

$ hash cash
$ type cat
cat is ~/.nvm/versions/node/v16.14.0/bin/cat

In regards to software installation, these risks are prevalent in every software manager and aren’t considered too dangerous from a security perspective.

However, npm link is not a package installer. It’s supposed to be a simple tool to setup symlinks for development. It’s worth pausing to reflect on how unexpected this behavior is, and what mistakes it could lead to.

By the way, if you ran npm link a in the prior section, a binary a has been installed to your system. You would think npm unlink a will uninstall it, but it only removes the local link and not the globally installed binaries.

Uninstall a global package and its binaries with:

Terminal
$ npm uninstall --global a

When linking multiple packages, previously linked packages are removed. This behavior is a regression introduced in npm v7.

In this example, pkg-a is linked and confirmed to be in node_modules. However, after linking a second package pkg-b, pkg-a is no longer in node_modules:

Terminal
$ npm link ../pkg-a
added 1 package, and audited 5 packages in 684ms
found 0 vulnerabilities

$ ls node_modules 
pkg-a

$ npm link ../pkg-b
added 1 package, removed 1 package, and audited 5 packages in 703ms
found 0 vulnerabilities

$ ls node_modules  
pkg-b

Removing previous links can be unexpected and confusing when working with multiple packages. Often times, after linking the second package, we’d continue to run code expecting the links to persist.

To link multiple packages, you must pass in all package paths into one command:

Terminal
$ npm link ../pkg-a ../pkg-b
added 1 package, and audited 6 packages in 645ms
found 0 vulnerabilities
                                                                           
$ ls node_modules 
pkg-a pkg-b

While this works, it’s not a great developer experience. In development, we don’t always know ahead of time all the packages that need to be linked. Or keep track of the previously linked packages.

This confusing behavior compounds to the poor usability and predictability of npm link.

Potential for accidents

As with any popular package registry, npm has a diverse collection with no standard for quality.

npm removes malicious packages, but risks mentioned above are not limited to attacks. When it’s unclear whether the right package was installed, there is always potential for accidents.

Many packages on npm are designed to make changes to the file-system, such as rimraf or a code linter. In an accident, the consequences of running file-system altering code can be detrimental.

Installing the wrong package is possible with npm install as well, but the risks are higher with npm link when the footguns above come together:

  • Package names can collide. It’s possible to link a local package with a name that’s on the npm registry. This can happen when developing and testing a new or private package before realizing the name is already taken.

  • No local resolution error. If the package being linked can’t be locally resolved, it will get resolved from the npm registry. If a package with the same name is found, an unexpected package can get globally installed.

  • Binaries are installed. If the wrong package is installed, it’s unintuitive that binaries get installed and to realize it needs to be uninstalled globally. This leaves unexpected binaries to be left installed and accidentally invoked.

Use npm install instead

A better alternative to npm link is npm install using a package path:

Terminal
$ npm install --no-save <package-path>

This creates a symlink to the package without installing it globally. This behavior is probably closer to what most people expect from npm link. The --no-save flag is to prevent the package path from getting saved in package.json.

However, this command still comes with a drawback. Like npm link, running npm install multiple times will remove previous links. To link multiple packages, pass in the package paths as arguments:

Terminal
$ npm install --no-save <package-path-a> <package-path-b> ...

An even better alternative to npm link is npx link, a tiny tool I developed to tackle the problems addressed in this post.

Using the command is simple:

Terminal
$ npx link <package-path>

npx link doesn’t globally install the linked package or its binaries. It doesn’t remove previous links. And it works across different versions of Node.js because it makes direct symlinks. It also has a clear fail-state when it can’t resolve the package path.

If you want to use binaries from the package, they will only be installed locally and will only be executable with npx or via package scripts.

As an added benefit for the community, package linking will still work for those who accidentally type in npx link instead of npm link!


Thanks for reading! Hope you'll stick around.
— Hiroki Osame
Open source Engineer. Living in NYC. Working at Square.
Follow @privatenumbr
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