Summary of Implemented Features
The engine has undergone a major expansion, evolving from a minimal evaluator into a highly capable JavaScript runtime with broad ECMAScript coverage. The work includes parser improvements, runtime features, standard library additions, async functionality, typed AST support, and extensive conformance testing.
Language Syntax and Parser Enhancements
A large portion of JavaScript syntax is now supported, including:
- Ternary expressions, arrow functions, template literals, tagged templates, getters and setters, destructuring for arrays and objects, labeled statements, sequence expressions, and comma expressions.
- Support for hex, octal, and binary literals, scientific notation, numeric object keys, trailing commas in calls, and improved handling of string escape sequences and line continuations.
- Implementation of operators such as spread and rest, typeof semantics, delete, void, unary plus, instanceof, exponentiation, logical assignment operators, and correct semantics for undefined and non-callable values.
- Parser fixes for many edge cases, including for-in loops, empty statements, return with comma expressions, contextual keywords like async, await, yield, get, and set, and better error messages and source context.
Standard Library and Built-In Object Support
A wide range of JavaScript built-ins have been implemented or expanded:
- Array, String, Date, Number, Math, JSON, Object, Map, Set, WeakMap, WeakSet.
- Support for ArrayBuffer, Typed Arrays, DataView, BigInt, Symbol (including asyncIterator), and error types.
- Addition of global functions, console object, eval, and many prototype methods for built-ins.
- Full implementation of object property descriptors, Object.freeze, Object.seal, Object.create, and correct prototype chain behavior.
Async, Generators, and Event Loop Infrastructure
The runtime now includes a full async system:
- Promise support with an event loop and microtask queue.
- Async/await implemented through CPS transformation, with extensive fixes for control flow, try/catch, break/continue, and loop behavior.
- Generator and iterator support, including async iterators and for await loops.
- setTimeout and setInterval implemented within the event loop.
- Top level await now supported through synchronous microtask draining.
- Debugging tools such as __debug() with call stack introspection.
Runtime Semantics Improvements
Numerous deep semantic areas have been implemented or corrected:
- Comprehensive type coercion rules for toString, toNumber, and comparison operators.
- Automatic Semicolon Insertion (ASI).
- Correct behavior for sparse arrays, array length, null and undefined conversions, and function return semantics.
- Accurate handling of constructor returns, this binding, and hoisting rules.
- Implementation of eval semantics, realm behavior, and global instantiation logic.
Module System
A full ES module system has been added:
- import and export parsing and evaluation.
- dynamic import().
- namespace re-exports, export * as, module resolution order fixes.
Typed AST and Evaluator Rewrite
The engine has transitioned to a typed AST architecture:
- A typed AST translator and evaluator with constant folding and CPS rewriting.
- Typed support for destructuring, classes, super, generators, async constructs, tagged templates, modules, property access, and operator handling.
- Automatic fallback to the legacy evaluator when needed.
- Progressive alignment between typed and legacy pipeline through parity tests.
Conformance and Benchmarking
The project now includes extensive test suites:
- Test262 integration for ECMAScript conformance.
- SunSpider benchmarks fully unskipped with multiple compatibility fixes.
- NPM compatibility tests verifying pure JavaScript execution.
- Diagnostics added across parser, runtime, async system, and iterator behavior.
Documentation, Tooling, and CI
- Complete documentation overhaul with structured README, Mermaid diagrams, feature lists, and investigations.
- S-expression introspection tools and transformation tracing.
- GitHub Actions CI/CD pipeline.
- Documentation of missing and completed features and ongoing WIP items.