Core reactive effect system: tracks dependencies via Proxy get/set traps, schedules effects with configurable scheduler (sync vs microtask). Demonstrates non-obvious dependency graph management with cleanup on re-run. Published standalone as @vue/reactivity, extractable to any JS project.

Framework-agnostic renderer implementing vnode patching: same-level diffing with keyed children, patch flags for dynamic vs static nodes, and a platform-agnostic host API. The core reconciliation algorithm is isolated from DOM specifics, making it portable to custom renderers (native, canvas, etc.).

Template compilation pipeline: parse → transform → generate. Uses a plugin-based transform system (similar to Babel) with directive/expression visitors, hoisting, and static-node caching. The compiler is platform-agnostic (compiler-dom adds HTML-specific handling), and generated code references runtime helpers via the helperNameMap symbol table.

Priority job scheduler: queues render effects and flushes them in microtask batches, with pre/post flush callbacks and job deduplication. A clean, generic async queue pattern that could be extracted to any application needing batched updates.

Transition component implementing an enter/leave state machine: detects CSS transitions vs JS hooks, coordinates leave/enter timing, and exposes a declarative API for animation. Self-contained and portable — the same pattern applies to any component library needing mount/unmount animations.

Defines unique symbol constants for every runtime helper the compiler can emit, mapped via helperNameMap to actual function names. This decouples compiler output from hardcoded strings — generated code references symbols resolved at build time. A clever pattern for maintaining a stable contract between compile-time and runtime.