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jsse

★ 71 Open GitHub ↗

An agent-coded JS engine in Rust. I didn't touch a single line of code here. Not one. This repo is a write-only data store. I didn't even create this repo by hand -- my agent did that.

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# jsse

An agent-coded JS engine in Rust. I didn't touch a single line of code here. Not one. This repo is a write-only data store. I didn't even create this repo by hand -- my agent did that.

**Goal: 100% test262 pass rate.**

Read more: [jsse - a JavaScript engine built by an agent](https://p.ocmatos.com/blog/jsse-a-javascript-engine-built-by-an-agent.html).

Per the test262 specification ([INTERPRETING.md](https://github.com/tc39/test262/blob/main/INTERPRETING.md)), test files without `noStrict`, `onlyStrict`, `module`, or `raw` flags must be run **twice**: once in default (sloppy) mode and once with `"use strict";` prepended. Our test runner implements this dual-mode execution. Current default run: **99,568 / 99,568 (100.00%)**.

*ES Modules now supported with dynamic `import()` and `import.meta`. Async tests run with Promise/async-await support.*

## Structure

- `spec/` — ECMAScript specification (submodule from [tc39/ecma262](https://github.com/tc39/ecma262))
- `test262/` — Official test suite (submodule from [tc39/test262](https://github.com/tc39/test262))
- `tests/` — Additional custom tests
- `scripts/` — Test runner and tooling

## Supported Features

- CLI with file execution, `--eval`/`-e` inline evaluation, and REPL mode
- `--version` and `--help` flags
- Exit codes: 0 (success), 1 (runtime error), 2 (syntax error)
- Lexer: all ES2024 tokens, keywords, numeric/string/template literals, Unicode identifiers
- Parser: recursive descent, all statements, expressions, destructuring, arrow functions, classes, private fields, strict mode
- Interpreter: tree-walking execution with environment chain scoping
  - Variable declarations (`var`, `let`, `const` with TDZ, `using`, `await using`)
  - Explicit Resource Management (`using`/`await using` declarations, `DisposableStack`, `AsyncDisposableStack`, `SuppressedError`, `Symbol.dispose`, `Symbol.asyncDispose`)
  - Control flow (`if`, `while`, `do-while`, `for`, `for-in`, `for-of`, `for-await-of`, `switch`, `try/catch/finally`)
  - Functions (declarations, expressions, arrows, closures)
  - Classes (declarations, expressions, inheritance, `super`, static methods/properties, private fields, private methods, private accessors, `#x in obj` brand checks)
  - Operators (arithmetic, comparison, bitwise, logical, assignment, update, typeof, void)
  - Objects and arrays (literals, member access, computed properties)
  - Template literals (including tagged templates)
  - `String.raw`
  - `new` operator with prototype chain setup
  - `new.target` meta-property
  - `this` binding (method calls, constructors, arrow lexical scoping)
  - Property descriptors (data properties with writable/enumerable/configurable)
  - Prototype chain inheritance (Object.prototype on all objects)
  - Getter/setter support (object literals, classes, `Object.defineProperty`)
  - `Object.defineProperty`, `Object.getOwnPropertyDescriptor`, `Object.getOwnPropertyDescriptors`, `Object.defineProperties`, `Object.keys`, `Object.freeze`, `Object.getPrototypeOf`, `Object.setPrototypeOf`, `Object.create`, `Object.entries`, `Object.values`, `Object.assign`, `Object.is`, `Object.getOwnPropertyNames`, `Object.getOwnPropertySymbols`, `Object.groupBy`, `Object.preventExtensions`, `Object.isExtensible`, `Object.isFrozen`, `Object.isSealed`, `Object.seal`, `Object.hasOwn`, `Object.fromEntries`
  - `Function.prototype.call`, `Function.prototype.apply`, `Function.prototype.bind`
  - `Object.prototype.hasOwnProperty`, `Object.prototype.toString`, `Object.prototype.valueOf`, `Object.prototype.propertyIsEnumerable`, `Object.prototype.isPrototypeOf`, `Object.prototype.__defineGetter__`, `Object.prototype.__defineSetter__`, `Object.prototype.__lookupGetter__`, `Object.prototype.__lookupSetter__`
  - Number/Boolean primitive method calls (`toString`, `valueOf`, `toFixed`)
  - `instanceof` and `in` operators
  - ToPrimitive with valueOf/toString coercion for objects
  - Wrapper objects (`new Boolean`, `new Number`, `new String`) with primitive value
  - `eval()` support
  - `Function`, `GeneratorFunction`, `AsyncFunction`, `AsyncGeneratorFunction` constructors (dynamic function creation)
  - `Symbol` with well-known symbols (iterator, hasInstance, toPrimitive, etc.)
  - `delete` operator for object properties
  - Iterator protocol (`Symbol.iterator`, lazy `ArrayIterator`, `StringIterator`)
  - `Array.prototype.values()`, `.keys()`, `.entries()`, `[@@iterator]()` returning lazy iterators
  - `String.prototype[@@iterator]()` with Unicode code point iteration
  - Spread elements in arrays and function calls (iterator-protocol aware)
  - Rest parameters in function declarations
  - Destructuring (array and object patterns, iterator-protocol aware)
  - RegExp literals and `RegExp` constructor with `test`, `exec`, `toString`
  - Array prototype methods (`push`, `pop`, `shift`, `unshift`, `indexOf`, `lastIndexOf`, `includes`, `join`, `toString`, `concat`, `slice`, `splice`, `reverse`, `fill`, `forEach`, `map`, `filter`, `reduce`, `reduceRight`, `some`, `every`, `find`, `findIndex`, `findLast`, `findLastIndex`, `sort`, `flat`, `flatMap`, `at`)
  - `Array.isArray`, `Array.from`, `Array.of`
  - String prototype methods (`charAt`, `charCodeAt`, `indexOf`, `lastIndexOf`, `includes`, `startsWith`, `endsWith`, `slice`, `substring`, `toLowerCase`, `toUpperCase`, `trim`, `trimStart`, `trimEnd`, `repeat`, `padStart`, `padEnd`, `concat`, `split`, `replace`, `replaceAll`, `at`, `search`, `match`)
  - Proper Error objects (`TypeError`, `ReferenceError`, `SyntaxError`, `RangeError`) with prototype chains and `instanceof` support
  - `JSON.stringify` (replacer function/array, space/indent, toJSON, circular detection, BigInt TypeError, wrapper unwrapping), `JSON.parse` (reviver), `JSON.rawJSON`, `JSON.isRawJSON`
  - `String.fromCharCode`
  - `Map` built-in (constructor, `get`, `set`, `has`, `delete`, `clear`, `size`, `entries`, `keys`, `values`, `forEach`, `@@iterator`, `Map.groupBy`)
  - `Set` built-in (constructor, `add`, `has`, `delete`, `clear`, `size`, `entries`, `keys`, `values`, `forEach`, `@@iterator`, ES2025 set methods: `union`, `intersection`, `difference`, `symmetricDifference`, `isSubsetOf`, `isSupersetOf`, `isDisjointFrom`)
  - Generator functions (`function*`, `yield`, `yield*`) with replay-based execution
  - Generator prototype (`next`, `return`, `throw`, `Symbol.iterator`, `Symbol.toStringTag`)
  - ES Modules (`import`, `export`, `import()`, `import.meta`, top-level await)
  - `ArrayBuffer` (constructor, `byteLength`, `slice`, `isView`)
  - TypedArrays (`Int8Array`, `Uint8Array`, `Uint8ClampedArray`, `Int16Array`, `Uint16Array`, `Int32Array`, `Uint32Array`, `Float32Array`, `Float64Array`, `BigInt64Array`, `BigUint64Array`) with indexed access, prototype methods (`at`, `set`, `subarray`, `slice`, `copyWithin`, `fill`, `indexOf`, `lastIndexOf`, `includes`, `find`, `findIndex`, `findLast`, `findLastIndex`, `forEach`, `map`, `filter`, `reduce`, `reduceRight`, `every`, `some`, `reverse`, `sort`, `join`, `toString`, `toReversed`, `toSorted`, `entries`, `keys`, `values`, `from`, `of`)
  - `DataView` (constructor, all get/set methods for Int8 through BigUint64, little/big endian)
  - `Temporal` (all types: `Instant`, `ZonedDateTime`, `PlainDateTime`, `PlainDate`, `PlainTime`, `PlainYearMonth`, `PlainMonthDay`, `Duration`, `Temporal.Now`; full arithmetic, comparison, rounding, formatting; IANA timezone support via ICU4X; 12 calendar systems; 100% test262 + intl402/Temporal compliance)
  - `Intl.DateTimeFormat` (partial — dateStyle/timeStyle, calendar-aware Temporal formatting)
  - `Intl.NumberFormat` (partial)
  - `ShadowRealm` (constructor, `evaluate`, `importValue`; full cross-realm value wrapping with WrappedFunction exotic objects)
  - `globalThis`
  - Built-ins: `console.log`, `Error`, `Test262Error`, `$DONOTEVALUATE$`

## Building & Running

```bash
cargo build --release
./target/release/jsse <file.js>
./target/release/jsse -e "1 + 1"
./target/release/jsse              # starts REPL
```

## Development checks

Install the pinned repository hooks once per checkout:

```bash
uv tool install pre-commit
pre-commit install
```

Run every hook against the full repository when changing hook configuration or
tooling:

```bash
pre-commit run --all-files
```

## Running test262

```bash
cargo build --release
uv run python scripts/run-test262.py
```

Options: `-j <n>` for parallelism (default: nproc/2), `--timeout <s>` (default: 120).
The runner exits non-zero for baseline regressions. Use `--fail-on-failures`
when a selected test set must be 100% passing, as CI does for smoke/sample runs.

The runner supports multiple engines via `--engine`:

```bash
uv run python scripts/run-test262.py --engine jsse      # default
uv run python scripts/run-test262.py --engine node --binary /path/to/node
uv run python scripts/run-test262.py --engine boa --binary /path/to/boa
```

Per-test timing data is written to `/tmp/timing-{engine}.json` after each run.

## Running JetStream 3

```bash
gh repo clone WebKit/JetStream /tmp/JetStream -- --depth 1
cargo build --release
uv run python scripts/run-jetstream.py --test OfflineAssembler --iterations 1 --timeout 120
```

The runner covers pure-JS JetStream 3 workloads and skips Wasm/Worker-dependent tests.

## Running the Node-compat library tests

Beyond test262, real-world npm libraries are run as engine stress tests: each
library's own upstream test suite is bundled with esbuild and executed on
`target/release/jsse`, cross-checked against Node as a reference oracle.

```bash
cargo build --release
./scripts/run-library-tests.sh <lib>          # e.g. decimal.js, acorn, zod, moment
./scripts/run-library-tests.sh <lib> --clean  # force a fresh clone/rebuild
./scripts/run-library-tests.sh <lib> --node   # run on Node only (reference)
```

`<lib>` is any config name under `scripts/libs/`: `acorn`, `ajv`, `big.js`,
`bignumber.js`, `decimal.js`, `highlight.js`, `js-md5`, `js-sha256`, `lodash`,
`luxon`, `moment`, `prismjs`, `qs`, `uglify-js`, `zod`. None of this is baked
into jsse's globals or run in CI — it's a manual, additive-only harness that
never affects test262. See [`scripts/README.md`](scripts/README.md) for the
full recipe, the current per-library status table, and how to add a library.

jsse

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