npm link: What the first `npm link` really does and how it can also be used

Published 2018-11-28, 13:39

There are loads of articles about npm link, how it works (npm link in library dir, then npm link library in project that should use that local library as a dependency) and what it can be used for (linking to a local folder for a dependency instead of having a npm downloaded version in node_modules).

But most skip over or only scrape what the first of the two steps really does – and how that can be used besides the default use case.

TLDR: npm link not only creates a symlink to the library in the global node_modules (that can be used in the second step of npm link ...), but also makes the library main script or binary accessible via a global npm module, similar to what happens when you npm install -g something.

The details:

When you run npm link as the first step of the two-step linking process, you create a „symlink in the global folder„.

Let’s follow the individual sub steps:

λ npm list -g --depth=0 
C:\Program Files\nodejs
+-- npm@6.4.1

We start in a specific folder (C:\Projects\Cordova in my example) and check our globally installed packages: We have nothing besides npm installed on this machine.

λ cd cordova-paramedic 
λ npm link
removed 1 package in 3.649s
C:\Program Files\nodejs\cordova-paramedic -> C:\Program Files\nodejs\node_modules\cordova-paramedic\main.js
C:\Program Files\nodejs\node_modules\cordova-paramedic -> C:\Projects\Cordova\cordova-paramedic

Now we switch the directory of our library that we want to link, cordova-paramedic here. We run npm link which creates two symlinks:

  1. C:\Program Files\nodejs\cordova-paramedic -> C:\Program Files\nodejs\node_modules\cordova-paramedic\main.js
  2. C:\Program Files\nodejs\node_modules\cordova-paramedic -> C:\Projects\Cordova\cordova-paramedic

The first one is a symlink for the „binary“, the main script, of our library.
The second is a symlink for the whole library into the global node_modules.
(C:\Program Files\nodejs\ is our global node/npm folder on this machine)

That first symlink is not mentioned in most of the articles about npm link on the web as the example libraries being linked don’t include a main script in their package.json.

λ npm list -g --depth=0
C:\Program Files\nodejs
+-- cordova-paramedic@0.6.0-dev -> C:\Projects\Cordova\cordova-paramedic
`-- npm@6.4.1

When we check the globally installed modules again, we see that our library was added to that list.

So not only can we now use npm link cordova-paramedic, the second command in the npm link two step process, to add this library to any project from our local checkout – but we can also use it as a global command cordova-paramedic!

λ cordova-paramedic
Error missing args.

So using npm link you can not only link local development versions of libraries to use in another project, but you can also create a global command that uses a local version of your command code. This is very useful when developing such a library.

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