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rust_browser/docs/js_feature_matrix.md
Zachary D. Rowitsch 0c21feb090
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Implement JSON.stringify and JSON.parse with full spec compliance 
Add the JSON global object with both methods following the sentinel
NativeFunction + interpreter interception pattern. The implementation
includes a dedicated recursive descent JSON parser (strict spec: no
trailing commas, no single quotes, no hex/octal, surrogate pair support),
JSON.stringify with replacer function/array, space indentation, toJSON,
boxed primitive unwrapping, and circular reference detection via ptr_eq.

Safety: depth-limited to 512 levels for both parse and stringify to
prevent stack overflow from malicious input. Space string truncation
uses char boundaries to avoid panics on multi-byte UTF-8.

138 unit tests, 2 conformance tests (js262).

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>
2026-02-26 17:00:47 -05:00

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# JavaScript Feature Matrix
Status of JS language features in the custom interpreter (`js_parser` + `js_vm`).
## Legend
| Status | Meaning |
|--------|---------|
| Supported | Feature works and has conformance tests |
| Not Supported | Feature not yet implemented (tracked as known_fail) |
| Partial | Some sub-features work, others don't |
## Language Core
| Feature | Status | Conformance Tests | Known Failures | Notes |
|---------|--------|-------------------|----------------|-------|
| `var` declarations | Supported | 11 | 0 | Hoisting, function scope, block visibility |
| `let` declarations | Supported | (in var-decl tests) | 0 | Block scoping, TDZ |
| `const` declarations | Supported | (in var-decl tests) | 0 | Immutability enforced |
| Number literals | Supported | (in expr tests) | 0 | IEEE 754 f64 |
| String literals | Supported | (in expr tests) | 0 | UTF-8, escape sequences |
| Boolean literals | Supported | (in type tests) | 0 | |
| `null` / `undefined` | Supported | (in type tests) | 0 | |
| Arithmetic operators | Supported | 16 | 0 | `+`, `-`, `*`, `/`, `%` with coercion |
| Comparison operators | Supported | (in expr tests) | 0 | `==`, `===`, `!=`, `!==`, `<`, `>`, `<=`, `>=` |
| Unary operators | Supported | (in expr tests) | 0 | `typeof`, `delete`, `void`, `-`, `!` |
| Logical operators | Supported | 2 | 0 | `&&`, `\|\|` with short-circuit evaluation |
| Bitwise operators | Supported | 33 | 0 | `&`, `\|`, `^`, `~`, `<<`, `>>`, `>>>` |
| Ternary operator | Supported | 4 | 0 | `? :` |
| Exponentiation operator | Supported | 2 | 19 | `**`, `**=` (ES2016) |
| Nullish coalescing | Supported | 1 | 0 | `??` (ES2020) |
| Logical assignment | Supported | 6 | 0 | `&&=`, `\|\|=`, `??=` (ES2021) |
| Compound assignment | Supported | 1 | 0 | `+=`, `-=`, `*=`, `**=`, `&=`, `\|=`, `^=`, `<<=`, `>>=`, `>>>=` |
| Increment/decrement | Supported | 1 | 0 | `++`, `--` (prefix and postfix) |
| Computed member access | Supported | 1 | 0 | `obj[key]` |
| `instanceof` | Supported | 2 | 0 | Walks prototype chain |
| `in` operator | Supported | 1 | 0 | Walks prototype chain |
| `delete` operator | Supported | 1 | 0 | Member, computed, identifier |
| `void` operator | Supported | 1 | 0 | Returns `undefined` |
| ToPrimitive (valueOf/toString coercion) | Supported | (in test262 tests) | 0 | `[Symbol.toPrimitive]`, `valueOf()`, `toString()` hint-based coercion |
| Wrapper objects (`new Number/String/Boolean`) | Supported | (in test262 tests) | 0 | `new Number()`, `new String()`, `new Boolean()` with `valueOf()`/`toString()` |
| `class` declarations | Supported | 10 | 0 | constructor, methods, extends, super(), super.method(), static |
| Template literals | Supported | 24 | 0 | `` `${expr}` `` |
| `if`/`else` | Supported | 6 | 0 | Nested, truthiness rules |
| `for` loops | Supported | 6 | 0 | `for (init; test; update)`, break, continue, let scoping |
| `for...in` | Supported | 20 | 0 | Object key enumeration incl. prototype chain, arrays, break/continue, let/const scoping |
| `for...of` | Partial | 19 | 0 | Arrays and strings only; no general iterator protocol yet |
| `while` loops | Supported | 3 | 0 | break, continue |
| `do...while` loops | Supported | 2 | 0 | Executes body at least once |
| `switch` | Supported | 1 | 0 | Basic matching, fall-through, break, default |
| Function declarations | Supported | 10 | 0 | Hoisting, recursion, closures |
| Function expressions | Supported | (in func tests) | 0 | Named and anonymous |
| Arrow functions | Supported | 3 | 0 | Expression/block body, lexical `this`, `new` rejection |
| `try`/`catch`/`finally` | Supported | 5 | 0 | All clause combinations, catch param scoping |
| `throw` | Supported | (in try-catch tests) | 0 | Throw + catch integration |
| Array literals + methods | Supported | 19 | 0 | `[1,2,3]`, `Array.isArray()`, `.push()`, `.pop()`, `.join()`, `.indexOf()`, `.lastIndexOf()`, `.includes()`, `.slice()`, `.concat()`, `.reverse()`, `.shift()`, `.unshift()`, `.map()`, `.forEach()`, `.filter()`, `.find()`, `.findIndex()`, `.some()`, `.every()`, `.reduce()`, `.reduceRight()`, `.sort()`, `.toString()` |
| Object literals | Supported | 9 | 0 | Creation, dot access, assignment, nesting, `Object.keys/values/entries`, `Object.create`, `Object.getPrototypeOf`, `Object.assign` |
| `this` keyword | Supported | 5 | 0 | Global scope, constructors, method calls |
| String methods | Supported | 3 | 0 | `.length`, `.charAt()`, `.indexOf()`, `.slice()`, etc. (ASCII-focused, `chars()` indexing) |
| `Function.name` / `.call()` | Supported | 1 | 0 | Direct member-call only (`fn.call(thisArg, ...)`) |
| `Object.prototype.toString` | Supported | 1 | 0 | `Object.prototype.toString.call(value)` type tagging |
| `hasOwnProperty` / `propertyIsEnumerable` | Supported | 2 | 0 | Own vs inherited, enumerable check; works on objects and functions |
| Non-enumerable properties | Supported | (internal) | 0 | Builtin prototype methods non-enumerable; `for...in` / `Object.keys()` / spread respect it |
| Function properties | Supported | (in test262 tests) | 0 | `fn.prop = val`, `fn.prop`, `fn.method()` |
| Spread / rest `...` | Supported | 6 | 0 | Arrays, strings, function args/params, object spread, `Symbol.iterator` iterables; object spread key order non-deterministic for non-numeric keys |
| Destructuring | Supported | 8 | 0 | Array/object patterns in `let`/`const`/`var`, defaults, holes, rest, nesting, renaming, function params, arrow params, assignment, for-of/in; shorthand properties `{x}` and array elisions `[1,,3]` |
| `typeof` | Supported | 5 | 0 | All value types |
| Strict mode (`"use strict"`) | Supported | 35 | 0 | Program/function/arrow/class; directive prologue detection, `has_escape` guard, reserved words, duplicate params, non-simple params, `delete` identifier, `this` binding, `.call()` coercion |
| `eval()` | Supported | 3 | 0 | Direct eval with scope access, indirect eval, strict mode isolation, parse errors as SyntaxError |
| `RegExp` | Supported | 41 | 0 | Regex literals, `RegExp` constructor, `test()`, `exec()`, `toString()`, flags (`g`, `i`, `m`, `s`, `u`, `y`), `lastIndex`, `String.prototype.match/search/replace/split` with regex; no function replacers, no `matchAll`/`replaceAll`, no `$<name>` named group substitution |
| `Symbol` | Supported | 25 | 0 | Unique identity, `.description`, `.toString()`, `Symbol.for()`/`Symbol.keyFor()`, symbol-keyed properties, `Object.getOwnPropertySymbols()`, well-known symbols (iterator, toPrimitive, hasInstance, toStringTag, species, isConcatSpreadable) |
| `WeakMap` | Supported | 11 | 0 | `new WeakMap()`, `.get()`/`.set()`/`.has()`/`.delete()`, object-key-only enforcement, `.set()` returns `this` for chaining |
| `Proxy` | Supported | 17 | 0 | `new Proxy(target, handler)`, get/set/has/deleteProperty/apply traps, `Proxy.revocable()`, function proxy targets |
| `JSON` | Supported | 2 | 0 | `JSON.stringify()` (replacer function/array, space, toJSON, circular detection, boxed primitives), `JSON.parse()` (reviver, strict JSON spec) |
## Web APIs (DOM / Events / Scheduling)
| Feature | Status | Conformance Tests | Known Failures | Notes |
|---------|--------|-------------------|----------------|-------|
| `document.getElementById()` | Supported | 6 | 0 | Returns null for missing |
| `document.createElement()` | Supported | (in dom tests) | 0 | |
| `element.appendChild()` | Supported | (in dom tests) | 0 | Returns appended child |
| `element.removeChild()` | Supported | (in dom tests) | 0 | Error if not a child |
| `element.textContent` | Supported | (in dom tests) | 0 | Get and set |
| `element.addEventListener()` | Supported | 6 | 0 | Click events, bubbling |
| `element.removeEventListener()` | Supported | (in event tests) | 0 | |
| `event.preventDefault()` | Supported | (in event tests) | 0 | |
| `event.stopPropagation()` | Supported | (in event tests) | 0 | |
| `event.type` | Supported | (in event tests) | 0 | |
| `setTimeout()` | Supported | 7 | 0 | Deterministic virtual clock |
| `clearTimeout()` | Supported | (in sched tests) | 0 | |
| `queueMicrotask()` | Supported | (in sched tests) | 0 | |
| `Promise` constructor | Supported | 5 | 0 | `new Promise(fn)` |
| `Promise.resolve()` | Supported | (in promise tests) | 0 | |
| `Promise.reject()` | Supported | (in promise tests) | 0 | |
| `.then()` / `.catch()` | Supported | (in promise tests) | 0 | Chaining, error recovery |
| `console.log()` | Supported | (in unit tests) | 0 | Capture sink for testing |
## Error Handling
| Error Type | Status | Notes |
|------------|--------|-------|
| `ReferenceError` | Supported | Undeclared variables, TDZ |
| `TypeError` | Supported | Null property access, non-function calls, const assignment |
| `ParseError` | Supported | Syntax errors |
| `RuntimeError` | Supported | Statement limit exceeded |
| Source locations | Supported | Line:col in error messages |
### Error Constructors (29 unit tests, 3 js262 tests)
| Constructor | Status | Notes |
|------------|--------|-------|
| `Error` | Supported | Constructor, toString, instanceof, call-without-new |
| `TypeError` | Supported | Prototype chain → Error.prototype |
| `ReferenceError` | Supported | Prototype chain → Error.prototype |
| `SyntaxError` | Supported | Prototype chain → Error.prototype |
| `RangeError` | Supported | Prototype chain → Error.prototype |
| `EvalError` | Supported | Prototype chain → Error.prototype |
| `URIError` | Supported | Prototype chain → Error.prototype |
| Internal → Error objects | Supported | Caught runtime errors are proper Error objects with instanceof |
## Test Counts Summary
| Category | Pass | Known Fail | Total |
|----------|------|------------|-------|
| JS262 conformance | 569 | 165 | 735 |
| Integration (js_tests) | 45 | — | 45 |
| Integration (js_dom_tests) | 25 | — | 25 |
| Integration (js_events) | 14 | — | 14 |
| Integration (js_scheduling) | 24 | — | 24 |
| Unit (js_vm interpreter) | 631 | — | 631 |
| Unit (js_vm value) | 97 | — | 97 |
| Unit (js_vm other) | 61 | — | 61 |
| Unit (js_parser) | 324 | — | 324 |
| Unit (js_parser tokenizer) | 88 | — | 88 |
| Unit (js_parser token) | 18 | — | 18 |
| Unit (js crate) | 28 | — | 28 |
| Unit (web_api) | 111 | — | 111 |
| **Total** | **1658** | **16** | **1674** |
## Test262 Coverage (Phase B)
550 real Test262 tests vendored from `tc39/test262` and run via `mode = "test262"` in the JS262 harness.
These are **in addition** to the hand-written conformance tests counted above.
| Feature Area | Pass | Known Fail | Total |
|--------------|------|------------|-------|
| asi | 20 | 5 | 25 |
| block-scope | 30 | 9 | 39 |
| comments | 15 | 0 | 15 |
| computed-property-names | 0 | 18 | 18 |
| destructuring | 11 | 3 | 14 |
| directive-prologue | 14 | 11 | 25 |
| expressions | 173 | 241 | 414 |
| **Total** | **263** | **287** | **550** |
**Pass rate: 70%** (up from 47.8%)
See `docs/test262_roadmap.md` for the integration plan and `docs/js_conformance_report.md` for known-fail details.
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