Yarn: From Design to Implementation

YARN is a package manager for JavaScript that focuses on determinism, stability, clean UX, and being fully open source. It was created to address issues with consistency, security, and performance in the JavaScript ecosystem.

YARN was created because the developers felt that contributing to NPM wouldn't address all the issues they wanted to solve. Each project has its own priorities, and YARN aimed to make different trade-offs and follow a different exploration path.

The core focus areas of YARN are determinism, stability, clean UX, and being fully open source. In YARN 2 and beyond, additional focus areas include monorepos, dogfooding, partitioning, and evolutiveness.

YARN Classic refers to the versions of YARN before 2.0. It was designed to improve consistency, security, and performance in large codebases, particularly at Facebook, where it originated.

Key features in YARN Modern include monorepos as a core concept, dogfooding, better partitioning of code, and a modular pipeline for resolution, fetching, and linking of packages. It also supports various linkers for different installation strategies.

YARN supports offline installations through its offline mirror feature, which allows you to store tarballs of your project's dependencies in a controlled location, such as a Git repository or specific filesystem. This ensures that projects can be installed even if the NPM registry is down.

The constraint engine in YARN allows users to create custom rules for their projects. For example, it can enforce that no two workspaces in the same project depend on different versions of the same package. It uses a JS-based engine for easier rule creation.

YARN uses a file system library called yarn-pkg-slash-fslim to handle path compatibility issues across different operating systems, such as Windows and POSIX systems. This library centralizes the logic for path handling, ensuring consistent behavior.

YARN contributes to the Node.js ecosystem through initiatives like Core Pack, which aims to make it easier to use different package managers. YARN also participates in working groups to tackle large issues affecting multiple projects, such as defining loaders in Node.js.

YARN's modular pipeline allows for different resolvers, fetchers, and linkers to handle various aspects of package management. This modular approach provides flexibility and makes it easier to add new features or make improvements without affecting other parts of the system.