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.

Test262 Progress

| Test Files | Scenarios | Passing | Failing | Pass Rate | |------------|-----------|---------|---------|-----------| | 51,525 | 99,020 | 99,020 | 0 | 100.00% |

Covers language/, built-ins/, annexB/, and intl402/.

Staging tests (not included above): 2,762 / 2,808 (98.36%). Staging failures include flaky timeouts, libm precision gaps, and a spec-compliance conflict with AnnexB arguments hoisting.

Per the test262 specification (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.

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)
• test262/ — Official test suite (submodule from 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

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

Running test262

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

Options: -j <n> for parallelism (default: nproc/2), --timeout <s> (default: 120).

The runner supports multiple engines via --engine:

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.

Engine Comparison (test262)

Benchmark run on 2026-03-10 using scripts/run-test262.py -j 64 --timeout 120 on a 128-core machine. All three engines ran the same test262 checkout (commit at test262/) with the same harness, timeout, and scenario expansion.

| Engine | Version | Scenarios | Run | Skip | Pass | Fail | Rate | |--------|---------|-----------|-----|------|------|------|------| | JSSE | latest | 101,234 | 101,234 | 0 | 101,044 | 190 | 99.81% | | Boa | v0.21 | 91,986 | 91,986 | 0 | 83,260 | 8,726 | 90.51% | | Node | v25.8.0 | 91,986 | 91,187 | 799 | 79,201 | 11,986 | 86.86% |

Failure breakdown

| Category | Node v25.8.0 | Boa v0.21 | |----------|-------------|-----------| | Temporal | 8,980 | 0 | | Atomics/agent | 236 | 0 | | Dynamic import | 162 | 594 | | RegExp property escapes | — | 284 | | Class elements/destructuring | — | 960+ | | AssignmentTargetType | — | 608 |

Caveats

• Node: 75% of its failures are Temporal (not shipped in Node 25). Module tests (799 scenarios) are skipped — the adapter can't run ES modules via Node. Some Atomics agent tests time out