core-js is a modular JavaScript standard library and polyfill system, not an end-user application. In production it supplies spec-compliant implementations of ECMAScript features, TC39 proposals, and selected web platform APIs for environments that lack native support. Its primary runtime behavior is side-effectful import: consumers load entry points such as `core-js`, `core-js/actual`, `core-js/full`, `core-js/es`, or granular per-feature modules, and the library either patches built-in constructors/prototypes or, in the `core-js-pure` variant, exports standalone implementations without global pollution. The architecture is a monorepo of packages: `core-js` (global polyfills), `core-js-pure` (pollution-free), `core-js-bundle` (prebuilt bundle), `core-js-compat` (compatibility data and module-selection engine), and `core-js-builder` (custom bundle generator). The implementation is split into thousands of tiny modules under `packages/core-js/modules/`, backed by a shared `internals/` layer that provides low-level spec operations, feature detection, environment/bug detection, iterator construction, collection internals, and export plumbing. Polyfill activation is decided by a `$({ target, stat, proto, forced }, ...)` export contract plus per-module `FORCED` flags derived from runtime probes and engine-version checks. Entry-point tiers (`es`, `stable`, `actual`, `full`, `stage`, `proposals`) map to spec maturity levels. Its primary data flow is configuration-driven: a consumer chooses an import path or, through Babel/swc integration, a set of target environments; `core-js-compat` converts targets into the minimum required module set; and `core-js-builder` can produce a custom bundle. The library itself depends on very few runtime dependencies; Babel, webpack, QUnit, Karma, and a matrix of JS engines are used for building, bundling, and testing. The codebase is unusually rigorous about spec conformance, including engine-specific bug workarounds and dual global/pure test suites. It is effectively the reference polyfill engine behind Babel's `useBuiltIns` and is highly study-worthy for anyone implementing low-level JavaScript compatibility layers.
ESLint is a mature, widely-used static analysis tool for JavaScript and TypeScript. It operates as both a CLI tool and a programmatic library, parsing source code into an AST and applying a configurable set of lint rules to detect and fix code quality, style, and correctness issues. The architecture is modular: a core linter (`lib/linter/linter.js`) orchestrates parsing (via espree or custom parsers), rule execution, and fix application; a configuration system (`lib/config/`) resolves flat config files; and a CLI engine (`lib/cli.js`) handles command-line invocation, file globbing, and result formatting. Key entry points include `bin/eslint.js` (CLI), `lib/api.js` (public API exposing `ESLint`, `Linter`, `RuleTester`, `SourceCode`), and `lib/universal.js` (browser-compatible Linter). Data flows: source files are read, parsed into ASTs, traversed by rule visitors, and lint messages are collected, filtered, and formatted. External dependencies include espree (parser), eslint-scope (scope analysis), eslint-visitor-keys, and various community utilities. The codebase is self-hosting, using its own rules and config for development, and includes extensive test suites, fuzzing tools, and ecosystem regression testing.
Jint is a high-performance JavaScript interpreter for .NET, implementing the full ECMAScript specification (ES2025+) with extensive Web API support (Fetch, Streams, Crypto, Encoding, Workers, etc.) and a headless browser automation layer. The core engine provides a rich embedding API with CLR interop, module loading, execution constraints, and debugging via the Chrome DevTools Protocol. The architecture is modular: the engine itself is a library (Jint), with separate packages for DevTools debugging (Jint.DevTools), browser automation (Jint.Browser), and source generators for efficient interop (Jint.SourceGenerators). The codebase is exceptionally well-tested with thousands of unit tests, Web Platform Tests (WPT), and Test262 conformance tests, plus a comprehensive benchmark suite. It is used in production by projects like RavenDB, EventStore, and OrchardCore as an embedded scripting engine.
Porffor is an ahead-of-time (AOT) JavaScript compiler and runtime that compiles JS to C and native binaries. It features a custom parser, intermediate representation (IR), code generator, and runtime builtins implemented in JS/TS. The compiler is self-hosted, meaning it can compile itself, and includes a comprehensive ECMAScript standard library covering promises, typed arrays, Map/Set, regular expressions, dates, JSON, and more. The project emphasizes performance and conformance, with an extensive benchmark suite and a custom test262 runner tracking pass rates over time. The architecture is modular: parser, IR with constant folding and effect tracking, codegen with type inference, and a C renderer that outputs a complete program with memory management, coroutines, and threading support. Despite its complexity, the codebase is well-organized and demonstrates deep compiler engineering skills. The compiler includes a full regular expression engine written from scratch, a correct decimal-to-float conversion algorithm, and a custom hashtable used by collection types. The parser supports modern ECMAScript and TypeScript. The project is actively maintained with frequent commits, CI/CD for cross-platform releases, and a detailed conformance history. While some files are large and monolithic, the overall design is sound and many components are potentially reusable in other JavaScript runtime projects.
This repository is the `eslint-plugin-react` package, a widely-used ESLint plugin that provides over 100 lint rules for React and JSX codebases. It is not a standalone application but a library consumed by ESLint to enforce best practices, prevent common errors, and maintain code style in React projects. The plugin supports both legacy `.eslintrc` and modern flat config (`eslint.config.js`) formats, and works with multiple parsers including babel-eslint and TypeScript-ESLint. The architecture is modular: each rule is a self-contained module in `lib/rules/`, sharing common utilities from `lib/util/` for component detection, AST traversal, version checking, and error reporting. The codebase is well-tested with a comprehensive test suite that runs across parser versions and ESLint versions. It is actively maintained with frequent releases and community contributions.
Nova is a custom ECMAScript (JavaScript) engine implemented in Rust, designed as a modular, embeddable runtime. It features a full bytecode compiler and virtual machine, a custom mark-compact garbage collector with typed arena allocation, and a shape-based (hidden class) object model for property access optimization. The engine closely follows the ECMAScript specification, implementing built-in objects (Promise, Array, Map, Set, Proxy, Temporal, etc.), module loading, and concurrent agents. Its architecture emphasizes data-oriented design and safe GC interaction through Rust-specific patterns like `GcScope`/`NoGcScope` and the `Bindable` trait. The project includes a standalone CLI (`nova_cli`) for running scripts and a test262 harness, currently passing approximately 80% of the official test suite. While actively developed, the engine has known limitations: many features are incomplete or behind feature flags (e.g., RegExp lacks full compliance, Temporal has placeholders, and several built-ins have `todo!()` stubs). The codebase exhibits high craftsmanship in its GC safety system, bytecode compiler, and object shape mechanics, but relies heavily on unsafe Rust and manual lifetime management. Nova is primarily a research-grade implementation aimed at embedding and exploration, not yet production-ready for general use.
This repository provides ECMAScript compatibility tables for JavaScript engines, browsers, and compilers. It ingests test definitions (embedded as commented JavaScript code) and expected results per engine version, then builds static HTML pages via Node.js scripts. The data covers ES5 through ES2025+ proposals, Intl, and non-standard features, with thousands of subtests mapped to hundreds of engine versions. The codebase is primarily a data pipeline with a build system (using cheerio for HTML generation) and optional test runners that execute tests against real engines (e.g., Duktape, GraalVM) to validate or update results. The output is the well-known kangax.github.io/compat-table static site. While the data is meticulously maintained, the repository is not a reusable library or application; it is a specialized data project with moderate reproducibility due to reliance on external engine binaries and outdated dependencies like jQuery 1.12.
This repository is an educational cheatsheet/documentation project that provides code examples for ECMAScript features from ES2015 through ES2025. It is not a production application but rather a reference collection of JavaScript feature demonstrations organized by ECMAScript version. The code examples are designed to be run with Babel Node and serve as learning materials for developers exploring new JavaScript features. The architecture is a flat file structure with examples organized in versioned directories (es2015/, es2016/, etc.). Each .js file contains self-contained examples demonstrating specific language features like arrow functions, classes, promises, async/await, and newer features like Temporal API and Iterator Helpers. The repository uses Babel for transpilation support and includes a GitHub Actions workflow for generating documentation artifacts. This is primarily a documentation/educational resource rather than a reusable code library. The examples are intentionally simple and lack production-grade patterns like error handling, testing, or abstraction layers. They serve their purpose as learning materials but should not be directly imported into production codebases.
This repository contains the TC39 proposal for adding enum declarations to ECMAScript. The proposal aims to introduce a syntax and semantics for enums that are compatible with TypeScript's enums, with some differences such as the removal of auto-initializers and reverse mapping. The repository includes the specification written in Ecmarkup, a grammar file extending the ECMAScript grammar, and a build system to generate the HTML specification. The architecture is centered around generating the specification document. The source files (in src/) are processed by gulp and ecmarkup to produce the final HTML specification in the docs/ directory. The build system uses standard gulp tasks for cleaning, building, watching, and serving the documentation. While the specification itself is well-crafted and follows the Ecmarkup conventions, the build system relies on outdated versions of gulp and related plugins, indicating some technical drift. The repository does not contain any broadly reusable software components outside of the context of generating this specific specification.
This repository is a curated collection of front-end interview questions organized by topic (HTML, CSS, JavaScript, React, etc.). Each question links to a YouTube video with an answer. It is a static documentation resource, not a software application. There is no executable code, build system, or API. The content is well-structured but offers no reusable software components. The repository serves as a study guide for front-end developers preparing for technical interviews.