@tlhunter



Node.js Package Management

Thomas Hunter II


Adapted from Distributed Systems with Node.js:
bit.ly/34SHToF

Topics

  1. Basics
  2. Dependency Tree
  3. Module Resolution
  4. Best Practices

Basics

How do dependencies work?

Let's Define some Terms

  • A Module is a require()-able file containing code
  • A Package is an archive of modules, supporting files
  • A Dependency is a Package that an app depends on
  • A Package makes it easy to distribute modules
  • Packages are hosted on npmjs.com
  • Packages should be beneficial to different apps
  • Most applications have >95% LoC in dependencies:
$ npx @intrinsic/loc

Dependency Installation

  • Dependencies are listed in package.json
  • There are several CLI tools to manage packages
  • Latest Node.js ships with corepack wrapper
  • Applications can have deps and dev deps
  • Dependencies are extracted to node_modules/
  • Install scripts, compilation, non-determinism
$ npm init # creates package.json, node_modules/
$ npm install foo # installs node_modules/foo

Packages are Versioned

  • Packages follow SemVer (Semantic Versioning)
  • Definition of MAJOR.MINOR.PATCH changes:
How SemVer Major Minor Patch correlates to Breaking Feature Bugfix

Dependency Tree

Where do those dependencies go?

Logical Dependency Tree

  • Dependencies have Subdependencies
  • Aka Transitive Dependencies
app@1.0.0
├── qs@6.7.0
├── pg@8.3.0
└─┬ express@4.17.1
  ├── qs@6.7.0
  └── content-type@1.0.4

Physical Dependency Tree, Naive

  • Filesystem resembles logical tree
  • Notice how the qs module is duplicated
  • Circular references = infinite filesystem usage
app/
app/node_modules/qs
app/node_modules/pg
app/node_modules/express
app/node_modules/express/node_modules
app/node_modules/express/node_modules/qs
app/node_modules/express/node_modules/content-type

Physical Dependency Tree, Deduplicated

  • Common modules are “hoisted” up
  • The duplicated qs module is removed
  • Cannot remove all duplicate packages from disk
app/
app/node_modules/qs
app/node_modules/pg
app/node_modules/express
app/node_modules/express/node_modules
app/node_modules/express/node_modules/content-type

Determining Physical Tree is Complex

  • Situation #1: Compatible versions are deduped
    • foo depends on qs@~5.1.2
    • bar depends on qs@~5.1.4
    • qs@5.1.9 is installed and hoisted
  • Situation #2: Incompatible versions aren't deduped
    • foo depends on qs@~5.1.2
    • bar depends on qs@~6.2.3
    • qs@6.2.9 is installed and hoisted
    • qs@5.1.9 is installed at foo/node_modules/qs

Another Dependency Tree Approach

  • The pnpm package manager uses symlinks
  • Creates a node_modules/.pnpm directory
  • node_modules/foo is a symlink to:
    • .pnpm/foo@1.2.3/node_modules/foo
  • foo's dependencies are symlink'd here:
    • .pnpm/foo@1.2.3/node_modules/*
  • This approach results in minimal disk usage

Install Determinism

  • Each $ npm install can grab different deps
  • Due to package ranges and package publishes
  • The npm CLI supports a package-lock.json file
  • This file enforces an exact dependency hierarchy
  • After a while $ npm outdated results appear
  • No lock can provide 100% deterministic filesystem

Module Resolution

How does require() work?

Overview of require()

  • Paths beginning with . or / look for modules
  • Global packages aren't readily require-able
  • Looks for packages in node_modules/ up to root
  • Determines the absolute path to module for caching
  • Requires a “Module Resolution Algorithm”

Module Resolution Examples

Module resolution from /srv/server.js

require('url')Core url module
require('./module.js')/srv/module.js
require('left-pad')/srv/node_modules/left-pad/,
/node_modules/left-pad/
require('foo.js')/srv/node_modules/foo.js/,
/node_modules/foo.js/
require('./foo')/srv/foo.js, /srv/foo.json, /srv/foo.node,
/srv/foo/index.js, /srv/foo/index.json...

Require Cache

  • Requiring the same file twice hits the cache
  • The cache is available at require.cache
Contents of the require.cache object

Best Practices

What's the best way to use this stuff?

Deployment and Workflow

  • Use $ npm outdated to list outdated packages
  • Use a tool like $ npx depcheck to find orphans
  • Don't check node_modules/ into repository
  • Never deploy dev dependencies to production

Requiring Packages

  • Specify file extensions when using require()
    • This eliminates ambiguity, broken refactors
    • Also helps with code completion
  • Never require() a transitive dependency
    • It will work today and break tomorrow
  • instanceof breaks for packages at different paths
    • Duck typing or Symbol.hasInstance can help

Security Concerns

  • Use the fewest number of packages possible
  • Favor modules with few or zero dependencies
  • Consult $ npm ls after installing new packages
  • Malicious Modules, typo squatting are a real threat

Fin