rust_browser/docs/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, -, !
Logical operators	Supported	2	0	&&, || with short-circuit evaluation
Bitwise operators	Not Supported	0	1	&, |, ^, ~, <<, >>
Ternary operator	Not Supported	0	1	? :
Compound assignment	Supported	1	0	+=, -=, *=
Increment/decrement	Supported	1	0	++, -- (prefix and postfix)
Computed member access	Supported	1	0	obj[key]
instanceof	Not Supported	0	1
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	Not Supported	0	1
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	Supported	6	0	[1,2,3], .push(), .pop(), .join(), .indexOf(), .slice(), .concat()
Object literals	Supported	4	0	Creation, dot access, assignment, nesting
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
Function properties	Supported	(in test262 tests)	0	fn.prop = val, fn.prop, fn.method()
typeof	Supported	5	0	All value types

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

Test Counts Summary

Category	Pass	Known Fail	Total
JS262 conformance	121	5	127
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)	395	—	395
Unit (js_vm value)	57	—	57
Unit (js_vm other)	50	—	50
Unit (js_parser)	214	—	214
Unit (js_parser tokenizer)	39	—	39
Unit (js_parser token)	13	—	13
Unit (js crate)	28	—	28
Unit (web_api)	111	—	111
Total	1136	5	1142

Test262 Coverage (Phase A)

50 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	3	2	5
block-scope	12	8	20
comments	5	0	5
directive-prologue	5	0	5
expressions	10	5	15
Total	35	15	50

See docs/test262_roadmap.md for the integration plan and docs/js_conformance_report.md for known-fail details.