rust_browser/docs/old/js_feature_matrix.md

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

ES Modules (ES2015+)

Feature	Status	Conformance Tests	Known Failures	Notes
import declarations	Supported	31 (parser) + 6 (VM)	0	Named, default, namespace, combined, side-effect
export declarations	Supported	(in module tests)	0	Named, default, declaration, re-export, aggregate
export default	Supported	(in module tests)	0	Expressions, named/anonymous functions and classes
export * from	Supported	(in module tests)	0	Star re-export (excludes default)
import() dynamic import	Partial	(in module tests)	0	Parsed and compiled; runtime returns undefined (Promise not yet wired)
Module registry (singleton)	Supported	5 (unit)	0	Modules evaluated once, cached by URL
Circular dependencies	Supported	(in registry)	0	Linking/Evaluating sentinel prevents deadlock
<script type="module">	Supported	2 (pipeline)	0	Inline and external, deferred timing
Module strict mode	Supported	(implicit)	0	Modules always strict per §10.2.1
import.meta	Not Supported	—	—	Clear error message; deferred
Top-level await	Not Supported	—	—	Deferred
Import maps	Not Supported	—	—	Bare specifiers error

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.