$ show preactjs/preact
Preact is a fast, lightweight React-compatible library for building user interfaces in JavaScript/JSX. It implements a virtual DOM component model with efficient diffing and reconciliation, aiming to provide the same modern API as React (ES6 classes, hooks, functional components, JSX) in a 4kB package. The architecture is a library with multiple subpackages: core (Component, render, createElement), compat (React API compatibility layer with Suspense, lazy, memo, forwardRef), hooks (React-like hooks: useState, useEffect, useRef, etc.), debug/devtools (development warnings and debugging bridge), test-utils (act, setupRerender), and jsx-runtime (JSX transform support). Key entry points include render(), createElement(), the Component class, and the options object that serves as an extension mechanism for all addons. Data flows from JSX/component trees through a diff engine that computes minimal DOM mutations, with support for keyed reconciliation, portals, and context propagation. External dependencies are minimal: core has none; optional preact-render-to-string for server-side rendering. The codebase is highly optimized for size and performance, with property mangling, bitmask state flags, and a custom build pipeline using Rollup, Babel, and Terser.
The LLM identified 5 files worth highlighting for reuse:
Implements the core child diffing algorithm using skew-based matching and longest-increasing-subsequence for minimal DOM moves. This O(n log n) reconciliation engine is the heart of Preact's efficient updates and can be extracted for use in other virtual DOM or tree-diffing systems.
Defines the options object that serves as the central extension point for all addons (hooks, debug, devtools, test-utils). It exposes hooks like _diff, _render, diffed, _commit, unmount, _root, _catchError, _hook, debounceRendering, and requestAnimationFrame, enabling clean separation of concerns without modifying core code.
A self-contained implementation of React-compatible hooks (useState, useReducer, useEffect, useLayoutEffect, useRef, useMemo, useCallback, useContext, useId, useErrorBoundary, useImperativeHandle, useDebugValue) using the options hook system. It manages per-component hook state, effect scheduling with afterPaint flushing, and cleanup on unmount. Designed as a separate package, it can be reused in any Preact-based project.
Provides act(), setupRerender(), and teardown() for testing Preact components synchronously. act() flushes all pending effects and renders, supports nesting, async callbacks, and error recovery. It temporarily overrides options.debounceRendering and requestAnimationFrame to make rendering deterministic in tests. This is a reusable pattern for testing any asynchronous UI library.
Implements React-compatible Suspense and lazy components. Suspense detaches suspended subtrees into a hidden parent and restores them on resolution, with support for nested suspense, portals, and error boundaries. The lazy component defers rendering until data is available. This is a complex, self-contained module that demonstrates advanced DOM manipulation and lifecycle management.