Broader TS→C++ Translator Coverage

Status: development → future · Priority: high

Note: This is a future plan, not a commitment. The syntax and API shown here are proposals — they can be completely different when actually implemented.

Closing the gap between "JS you can write" and "JS that compiles to C++". morph check already audits unsupported syntax; this page catalogs exactly what's missing and what each piece unlocks.

Why it matters

The translator (crates/morpher/) is the heart of the "write JS, get native binary" promise. Every unsupported operator, builtin, or method is a workaround users hit. Coverage growth is also the top contribution area.

Currently supported (the baseline)

• Statements: if/while/for/do-while/switch/try-catch/throw, variable declarations, functions + arrow functions, classes/interfaces (inheritance, super, generics), return/break/continue
• Expressions: binary/unary/ternary/sequence/update/assignment, template literals, array literals with spread, object literals, new, member access, calls, await
• Builtins: fetch(), console.*, setTimeout/setInterval/clearTimeout, morphState/morphEffect, Error
• Value semantics: truthiness, ==/!=, typeof, string concatenation coercion, array push/pop/index, object has/keys, string toUpperCase/toLowerCase/trim/indexOf/substring/slice/replace/charAt/split/includes/startsWith/endsWith, optional chaining ?., nullish coalescing ??

Missing — operators (currently compiled to /* comment */ or rejected)

Construct	Today	Unlocks
** / **=	/* pow */ / error	exponent math
>>> (unsigned shift)	/* >>> */	bit-twiddling code
typeof	/* typeof */	runtime type checks
void / delete	/* void */ / /* delete */	JS idioms
&&= / ||= / ??=	/* ... */	shorthand assign
instanceof / in	error	type/containment checks
Optional chaining ?.	✅ Shipped (emit_chain in morpher)	safe deep access
Generator functions / yield	error	lazy sequences
for...in / for...of	error	iteration over keys/values
Destructuring (except morphState pattern)	error	multi-return unpacking
Spread in call args / object spread	error	functional style

Missing — globals (morph check rejects: mx-js-global)

document, window, localStorage, alert/prompt/confirm, requestAnimationFrame, Math, JSON, Date, RegExp, Promise, Proxy, Map/Set/WeakMap, parseInt, event constructors, addEventListener.

Interesting candidates (the ones a native UI framework plausibly implements):

Global	What it would map to
Math	native <cmath> — trivial, high value
JSON	the existing JSON DOM parser in dev runtime
Date	std::chrono
parseInt / parseFloat	small wrappers
Map / Set	hash containers
requestAnimationFrame	co_await next_frame() (already exists!)
localStorage	a config-file-backed store

Missing — methods (mx-js-method)

State values implement only {length, push, pop, get, toString}. Strings implement {length, empty, toUpperCase, toLowerCase, substring, slice, charAt, indexOf, replace, trim, toString}.

• Arrays: map, filter, forEach, reduce, includes, indexOf, find, join, concat, sort
• Strings: startsWith/endsWith/includes/split ✅ shipped; still missing: padStart, padEnd, repeat, match

Array map/filter/forEach/includes are the highest-value additions — JSX list rendering already does .map in templates, but the general method form is unbuilt.

Note on morph check

All of the above are already audited with precise diagnostics and suggestions (mx-js-op, mx-js-syntax, mx-js-global, mx-js-member, mx-js-method), so progress on coverage is measurable: each item disappears from morph check output when done.

Current state

Piece	State
Array spread + slice + .length semantics	✅ Shipped (latest cycle)
Optional chaining ?. + ?? + String.split/includes/startsWith/endsWith	✅ Shipped
Operator / builtin / method coverage above	❌ Missing (each item either comments itself out or errors)
morph check diagnostics for every missing item	✅ Shipped

Suggested order

1. Math + parseInt/parseFloat (trivial, high usage)
2. Array.map/filter/forEach/includes (JSX list rendering already does .map in templates — the pattern is proven)
3. String.split/includes/startsWith/endsWith ✅ Shipped
4. Optional chaining ?. (biggest ergonomics win) ✅ Shipped
5. Map/Set, Date, JSON, requestAnimationFrame