el/spec/codegen-js.md

El JavaScript Backend (codegen-js)

Status: scaffolded. Hello-world compiles and runs. ~50% language coverage. Core runtime (~30 builtins) implemented. CGI / DHARMA / LLM / Engram intentionally stubbed.

Authoritative files

File	Role
el-compiler/src/codegen-js.el	El → JS code generator (mirrors codegen.el)
el-compiler/runtime/el_runtime.js	Browser/Node runtime that compiled programs link against
el-compiler/src/compiler.el	Adds compile_js() and --target=js CLI dispatch
spec/codegen-js.md	This document

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1. Why a JS backend exists

El compiles to C today. C is the right substrate for the agent runtime, the DHARMA daemon, and Engram. But three first-class consumers of El need to run in a browser, where C is not an option:

1. el-ui/runtime/ — the activation-based frontend framework written in JS. The long-term plan is to author components and the runtime itself in El and compile them down to JS.
2. cgi-studio — the web app for cultivating CGIs. Today it is hand-written JS. Once the JS backend is mature, the studio's UI logic can be authored in El and share types/identifier names with the CGI it cultivates.
3. Marketplace plugin UIs — third parties writing browser-side El that runs untrusted in a sandbox. They need a JS target.

A secondary motivation: El-on-Node. CLI tooling, build scripts, and tests benefit from a tight el → js → node cycle without a cc step.

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2. Type representation strategy

The C backend pretends every value is int64_t. That is a deliberate runtime trick to avoid dynamic dispatch in generated C. JavaScript already has tagged dynamic values, so the JS backend is simpler: every El value is a native JS value, and the tag of el_val_t collapses into the JS type system.

El type	C representation	JS representation
Int	int64_t (direct)	number (with Number.isSafeInteger caveat — see §6)
Float	int64_t bit-cast of double via el_from_float	number (no bit-cast — JS number IS a double)
Bool	int64_t, 0 = false, nonzero = true	boolean
String	(int64_t)(uintptr_t)cstring	string
Void	C void	undefined
[T] (List)	el_val_t pointer to refcounted struct	Array<any>
Map<K,V>	el_val_t pointer to refcounted struct	plain object {[key]: any}
EL_NULL (0)	(el_val_t)0	null
Any	el_val_t	any (no compile-time check)

Key consequences:

• + on two strings is JS + (string concat) — no el_str_concat() runtime call needed for the common case. The runtime DOES export el_str_concat for the cases where codegen does not know the types.
• == on strings is === — not str_eq(). Same disambiguation logic as the C backend (look at left/right kind, fall back to str_eq for identifiers without int annotation).
• Map access m["foo"] compiles to JS m["foo"] (no el_get_field). For Field access (m.foo) we emit m["foo"] so it works on plain objects regardless of prototype shape.
• List access arr[i] is JS arr[i]. No bounds checking — same as C (which segfaults on bad index). Could add el_list_get wrapper later for safe access.
• EL_NULL becomes JS null, not undefined. The runtime checks for === null consistently. This avoids the JS undefined/null fork and matches El's single null value.

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3. Builtin runtime layer (el_runtime.js)

Same function names as el_runtime.c wherever possible, so codegen-js can emit the same call sites. The runtime is a single ES module that exposes every builtin as a named export AND attaches them to a globalThis.__el namespace (so generated code can do either import * as el from './el_runtime.js' or assume globals).

The codegen-js generated output uses the global-namespace style: every emitted file starts with import './el_runtime.js' (which side-effects the globals) so call sites stay flat — println(x) not el.println(x). This matches the C backend's flat call surface and keeps the generated code grep-compatible across targets.

Implemented today (~30 builtins)

Category	Functions
I/O	println, print
String	el_str_concat, str_concat, str_eq, str_starts_with, str_ends_with, str_len, int_to_str, str_to_int, str_slice, str_contains, str_replace, str_to_upper, str_to_lower, str_trim, str_index_of, str_split, str_char_at, str_char_code, str_lower, str_upper
Math	el_abs, el_max, el_min
List	el_list_new, el_list_len, el_list_get, el_list_append, el_list_empty, el_list_clone, list_push, list_join, list_range
Map	el_map_new, el_get_field, el_map_get, el_map_set
HTTP	http_get, http_post, http_post_json (via fetch(), returns Promise<string> — see §5 async caveat)
FS	fs_read, fs_write, fs_list (Node-only, throw in browser)
JSON	json_parse, json_stringify, json_get, json_get_string, json_get_int
Time	time_now, time_now_utc, sleep_secs (Node), sleep_ms
Bool	bool_to_str
Process	exit_program (Node process.exit, throw in browser)
Refcount	el_retain, el_release (no-ops — JS has GC)
ARC method-call shortforms	append, len, get, map_get, map_set
Native VM aliases	native_list_get, native_list_len, native_list_append, native_list_empty, native_list_clone, native_string_chars, native_int_to_str
args	args() returns process.argv.slice(2) in Node, [] in browser
state_*	In-memory Map keyed by string
env	process.env[k] in Node, throws in browser

Stubbed (throw at runtime)

Every function in this list compiles successfully but throws Error("not supported in JS target — needs server-side delegation: <name>") when called. This is a runtime error, not a compile error, so it doesn't block compilation of code that has dead-code paths through these functions.

• All dharma_* (membership in DHARMA network requires the daemon)
• All engram_* (needs the embedded SQLite + activation engine — could be reimplemented in JS later)
• All llm_* (CORS + API key handling — must go through a server-side proxy)
• http_serve (browsers don't host servers; Node could, but that's a separate runtime mode)
• el_cgi_init (CGI identity is a server-side concept)
• Crypto: sha256_*, hmac_sha256_*, base64* (deferred — can use crypto.subtle later)

Browser-side specific behavior

When running in a browser:

• println / print map to console.log (no stdout in browsers)
• http_get / http_post use fetch() (CORS applies)
• fs_* throws (browsers have no fs)
• args() returns []
• env(k) throws (or could read from a global config object — TBD)

When running in Node:

• println / print map to console.log and process.stdout.write
• fs_* use node:fs/promises (sync versions for the simple cases)
• args() returns process.argv.slice(2)
• env(k) returns process.env[k] ?? null

The runtime auto-detects via typeof window === 'undefined'.

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4. Tradeoffs vs the C backend

Concern	C backend	JS backend
Static types	El's Int becomes int64_t, real arithmetic	El's Int becomes number — loses precision past 2^53
Linking model	Static link against el_runtime.c + libcurl + libpthread	ES module import of el_runtime.js
Dynamic dispatch	dlsym for http_set_handler / llm_register_tool (requires -rdynamic)	JS function value lookup via globalThis[name] — no compiler flag
Tool registry	dlsym walks symbol table; tool fns must be top-level C symbols	Tool fns live as exports of the generated module; trivially callable
Memory model	Refcounted lists/maps with el_retain/el_release to avoid leaks	JS GC handles all of it; el_retain/el_release are no-ops
+ overload	Has to dispatch in codegen between el_str_concat and integer + because at C level both are int64_t	JS + is already overloaded: "a" + "b" → "ab", 1 + 2 → 3. Codegen still preserves the existing dispatch for safety, but the runtime fallback is correct
Concurrency	pthread-backed http_serve	Single-threaded event loop; http_serve not supported in this target
HTTP client	libcurl, blocking, returns body string	fetch() is async — see §5
CGI identity	el_cgi_init runs at start of main()	Not supported; UI code is not a CGI principal
DHARMA / LLM	Native, blocking, libcurl-backed	Not supported — all such calls throw and the program is expected to delegate to a server-side El daemon via plain HTTP
Compile speed	El → C → cc → binary (cc is the slow step)	El → JS → done. Faster iteration
Output size	Static binary ~2MB	Source .js + ~10kb runtime

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5. The async problem (the big deferred decision)

fetch() is async. The C backend's http_get(url) is synchronous and returns the body string directly. El source was written assuming sync. Three options:

1. Pretend it's sync from El's POV; use synchronous XHR (browser) or child_process.execSync('curl …') (Node). Bad: synchronous XHR is deprecated and frozen on the main thread; execSync is a hack.
2. Make every http_* builtin in the JS runtime return a Promise, and rewrite codegen-js to insert await everywhere. This requires turning every El function that transitively calls a network builtin into an async fn in JS. Doable, but invasive — the El AST does not currently mark async-ness.
3. Compile El's call sites with implicit await; compile-time taint tracking marks every fn that transitively calls a network builtin as async. Generated JS uses async function and await. This is the right answer long-term.

Decision for this scaffold: option 3, but only the runtime side is implemented. http_get in el_runtime.js returns a Promise<string>. codegen-js.el does NOT yet emit async/await. Calling http_get from compiled El will return a Promise that the El program will treat as a string (which produces "[object Promise]"). This is documented and accepted for the scaffold; the compile-time taint pass is a follow-up.

For now, programs that don't touch HTTP work correctly. That covers el-ui/runtime (which only manipulates the DOM and a graph), most of cgi-studio's pure UI components, and all hello-world style programs.

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6. Number precision

JS number is IEEE 754 double — only 53 bits of integer precision. El Int is int64_t and the runtime sometimes uses the full 64 bits (e.g. time_now_utc returns nanoseconds-since-epoch, which exceeds 2^53 in practice).

Decision for this scaffold: accept the precision loss. Document it. UI code does not use 64-bit timestamps. If/when a use case demands it, time_now_utc can return a BigInt and we can introduce a BigInt sub-mode. That's a follow-up.

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7. What's NOT supported in JS target initially

This is the canonical list. Programs that use any of these compile (no #error-style fail-fast like the C backend's capability check) but throw at runtime or behave as documented.

Feature	Status	Notes
cgi {} block	Compiled to a no-op + warning comment	CGI identity is server-side. UI code is not a CGI.
service {} block	Compiled to a no-op + warning comment	Same.
All dharma_*	Stub throws	Programs needing DHARMA must call a server-side daemon over HTTP
All engram_*	Stub throws	Could be ported to in-browser (IndexedDB-backed) later
All llm_*	Stub throws	Browser cannot hold API keys; route through server
llm_register_tool	Stub throws	Same
http_serve	Stub throws	Browsers cannot serve. Node-mode could, deferred
http_set_handler	Stub throws	Same
match expressions	Compiled (basic)	LitInt/LitStr/LitBool/Wildcard/Binding all work via if/else chain. Tagged-union match deferred
type (struct) defs	Skipped at codegen	Treated as documentation; structs are plain JS objects. t["field"] works
enum defs	Skipped at codegen	Same — enum values are bare strings or ints
? postfix (nil-prop)	No-op	Same as C backend's current state
try postfix	Stripped to inner	Same as C backend
Capability enforcement	Not enforced	The C backend uses #error directives; the JS backend lets the runtime stubs throw. Future: emit throw new Error('capability violation') at compile time
VBD role check	Not enforced	Same
Float bit-cast	Not needed	JS number is already a double
Crypto primitives	Stub throws	Easy to add via crypto.subtle later
state_*	In-memory only	No persistence; resets on page reload
args()	Node-only	Browser returns []
fs_*	Node-only	Browser throws

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8. CLI dispatch — --target=js

The compiler entry point compiler.el adds a compile_js(source: String) -> String alongside the existing compile(). The CLI behavior:

elc <source.el> <output>          # default — emit C
elc --target=c <source.el> <out>  # explicit — emit C
elc --target=js <source.el> <out> # emit JS

elc --target=js source.el         # write JS to stdout (no out path)

The argv parser scans for a --target=<lang> token; remaining positional args are <source> and optional <out>. The dispatch logic stays in El: a compile_dispatch(target, source) -> String switch.

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9. The path to compiling el-ui/runtime through this backend

This is the real-world test. el-ui/runtime/src/ is currently 5 hand-written .js files. The path to authoring them in El:

1. Phase 1 — Hello-world (this scaffold). Done.
2. Phase 2 — language coverage. Get codegen-js to ~95% parity with codegen.el for non-network features. Specifically: match, struct/enum field access, ?-propagation, full for-over-list, complete unary/binary operators, lexical closures (the C backend doesn't have these but we'll need them for el-ui's component model).
3. Phase 3 — DOM bridge. Add dom_* builtins to el_runtime.js: dom_create_element, dom_set_text, dom_append_child, dom_query, dom_listen, etc. These are Node-as-El builtins for the browser; the C backend will add a stub set that errors. Source-shareable El UI code becomes possible.
4. Phase 4 — Component class lowering. El doesn't have classes; el-ui's Component is a JS class. Decide: extend El with a component keyword that compiles to JS class + C struct? Or have el-ui authors define components as fn render_<name>(state) -> String and provide a small bootstrap. The latter is the lower-impact path.
5. Phase 5 — Async taint pass. Implement compile-time async tracking so http_get and friends produce await fetch() correctly. Required before authoring code that fetches data.
6. Phase 6 — Port el-ui/runtime/. Translate the 5 JS files to El, compile to JS, swap in. Run el-ui's existing tests. Iterate.
7. Phase 7 — Port cgi-studio UI. Larger surface area; same pattern.
8. Phase 8 — Marketplace plugins. Open the door for third-party UI El.

The blocking item between phase 1 and phase 2 is incremental — every El construct used by el-ui's source needs codegen-js coverage. Phase 5 (async) is the architectural decision that needs explicit user buy-in, because it changes the language's effective semantics on the JS target.

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10. Test

echo 'fn main() -> Void { println("hello from el-js") }' > /tmp/hello.el
elc --target=js /tmp/hello.el > /tmp/hello.js
node /tmp/hello.js
# → hello from el-js

This should pass after the bootstrap rebuild. See §11.

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11. Bootstrap status

Adding --target=js to compile() requires regenerating the shipped elc binary at dist/platform/elc. The rebuild path is:

1. Existing elc binary compiles updated elc-combined.el (which now includes codegen-js.el and the --target=js dispatch) → elc.c.
2. cc compiles elc.c → new elc binary.
3. New elc binary supports --target=js.

The scaffold checks all four scaffold files in. The bootstrap rebuild happens as a follow-up step, gated on review of this design doc.