Yarn

Yarn (Berry) is a modern package manager. It introduced Plug'n'Play (PnP) to eliminate node_modules and support Zero Installs.

When to Use

• Modern JavaScript Package Management (Yarn v4+): Fast, reliable dependency management with Corepack support.
• Plug'n'Play (PnP) & Zero-Installs: Eliminating node_modules folders for instant workspace switching and CI runs.
• Enterprise Monorepos: Coordinated workspace resolution, constraints, and release workflows.
• Hardened Security & Integrity: Strict lockfile checksums and automated dependency audits.

Quick Start

corepack enable
yarn set version stable
yarn init -2

# Install
yarn add react

Core Concepts

Modern Yarn Configuration (.yarnrc.yml)

Configuring node-modules linker or Plug'n'Play:

# .yarnrc.yml
nodeLinker: node-modules # Use 'pnp' for Zero-Installs, 'node-modules' for standard compatibility
compressionLevel: mixed
enableGlobalCache: true

packageExtensions:
  "@types/react@*":
    peerDependencies:
      react: "*"

Workspace Commands in Monorepos

Executing commands across workspaces:

# Run build command in parallel across all workspaces
yarn workspaces foreach --parallel --verbose run build

# Run tests only on workspaces changed relative to main branch
yarn workspaces foreach --since=main run test

# Add dependency to a specific workspace
yarn workspace @my-org/web-app add swr

Upgrading Dependencies with Interactive CLI

Upgrading packages adhering to SemVer:

# Interactively review and upgrade outdated packages
yarn upgrade-interactive

# Re-validate lockfile integrity
yarn install --immutable

Common Patterns

Yarn Berry Monorepo Workspaces with TypeScript Loose Mode

Problem: Slow installs and node_modules bloat across multiple internal monorepo packages.

Solution: Configure Yarn Berry workspaces with .yarnrc.yml:

# .yarnrc.yml
nodeLinker: node-modules # Use node-modules linker for maximum ecosystem compatibility
yarnPath: .yarn/releases/yarn-4.5.0.cjs

packageExtensions:
  "react-scripts@*":
    dependencies:
      "typescript": "*"

Root package.json:

{
  "private": true,
  "workspaces": ["apps/*", "packages/*"]
}

Best Practices

Do:

• Target modern Yarn (v4+) enabled through Corepack (corepack enable yarn).
• Use yarn install --immutable in CI/CD pipelines to prevent unintended lockfile updates.
• Leverage yarn workspaces foreach with --since=main to test only modified packages in monorepos.
• Use packageExtensions in .yarnrc.yml to cleanly resolve missing peer dependency warnings from older third-party packages.

Don't:

• Use legacy Yarn 1.x (Classic); migrate to modern Yarn v4+ for enhanced performance and security.
• Edit yarn.lock manually; resolve conflicts via yarn install.
• Mix npm or pnpm lockfiles within a Yarn repository.

Troubleshooting

| Error | Cause | Solution | | :------------------------------------------------------- | :--------------------------------------------------------------- | :---------------------------------------------------------------------- | | Usage Error: Couldn't find the node_modules state file | Project running under PnP mode when tools expect node_modules. | Set nodeLinker: node-modules in .yarnrc.yml and run yarn install. | | YN0002: Missing peer dependency | Package requires peer dependency not declared in consumer. | Declare missing package in root .yarnrc.yml packageExtensions. | | Yarn command not found | Yarn Berry binary missing in .yarn/releases/. | Run corepack enable && corepack use yarn@stable. |

References

• Yarn Documentation