Twig is a mature, feature-rich template engine for PHP that compiles templates to optimized PHP code. It employs a lexer-parser-compiler pipeline to transform template source into an abstract syntax tree (AST), which is then compiled to executable PHP. The engine emphasizes security through automatic escaping and a sandboxing system, and extensibility via a robust extension system that allows adding custom functions, filters, tests, and token parsers. Twig supports template inheritance, macros, and expressive syntax with features like null-safe operators and arrow functions. The architecture is modular and follows several well-established patterns: a plugin-based expression parser, a decorator-pattern caching system, a visitor pattern for AST manipulation, and an attribute-driven extension registration mechanism (using PHP 8+ attributes). The codebase demonstrates careful attention to extensibility, security, and performance, with comprehensive test coverage and active maintenance. While the project manages significant backward compatibility (notably preparing for Twig 4.0), it maintains a clean separation of concerns and provides reusable components that could be adapted for other projects, such as its chainable cache, expression parser registry, and sandbox security policy.
Nunjucks is a full-featured JavaScript templating engine heavily inspired by Jinja2 (Python). It compiles templates into JavaScript functions for high-performance rendering, supporting template inheritance, macros, filters, tests, async control, and custom extensions. The library runs in both Node.js and browser environments, with a slim build for precompiled templates. It integrates with Express as a view engine and provides a CLI tool for precompiling templates offline. The architecture follows a classic compiler pipeline: lexer → parser → AST → compiler → JavaScript function. The public API exposes Environment, Template, Loader, and various configuration methods. Templates are loaded via loaders (FileSystemLoader, WebLoader, PrecompiledLoader) and rendered synchronously or asynchronously. The library includes a rich set of built-in filters (escape, default, sort, groupby, etc.) and tests (defined, odd, mapping, etc.), all extensible via addFilter/addTest. Key entry points include nunjucks/index.js (main module), bin/precompile (CLI), and browser/nunjucks.js (UMD build). External dependencies are minimal: a-sync-waterfall for async control flow, asap for microtask scheduling, and chokidar for file watching. The codebase is well-tested with Mocha and has CI via GitHub Actions. While the core engine is well-crafted and actively maintained, the repository carries technical debt from vendored documentation dependencies (Bootstrap 3, jQuery 2, Bower) and an outdated build toolchain (Babel beta, PhantomJS). The library itself is production-ready and widely used (8.9k stars).