[dbrattli]

Summary

Adds a third backend — src/js — alongside Python (ASGI) and BEAM (Cowboy), answering issue #13 ("Does this run on Fable JS as well?"). Enabled by Fable 5.5.0's task { } → Promise<T> mapping (PR #4694), which lets the Task-based shared core compile to JavaScript.

Design

Each backend owns its HttpContext, wrapping its platform's HTTP primitives natively — the shared contract between backends is the HttpContext member surface that Core/Routing/Negotiation call, not any server protocol:

• Python wraps ASGI scope/receive/send; BEAM wraps a Cowboy Req; JS wraps Node http IncomingMessage/ServerResponse.
• src/js is self-contained — zero dependency on the other backends. Deleting the JS target = delete src/js / app/js / test/js, nothing else. (An earlier revision reused the Python ASGI context via a link; that was reverted per review — ASGI is Python's server substrate, not a cross-platform abstraction, and it coupled JS to Python.)
• New files mirror the per-target layout: PlatformHelpers.fs, Json.fs (typed Fable.Core.JS.JSON, no emits), HttpContext.fs (Node-native), Server.fs, WebHost.fs.

Host model & Express

Sits on Node's built-in http — the peer of uvicorn / Cowboy — not Express. The Giraffe pipeline is exposed as a Connect (req, res, next) middleware, so the Express/Connect ecosystem (cors, helmet, morgan, passport, OpenTelemetry…) is reusable without an Express dependency:

app.use(cors())
app.use(helmet())
app.use(builder.ToMiddleware())   // Giraffe as terminal handler; unmatched → next()

This is the JS equivalent of how real Giraffe reuses ASP.NET middleware and the Python target wraps ASGI middleware.

Verification

• End-to-end on Node over real HTTP: GET /ping→pong, GET /json→{"Name":"Dag","Age":53}, POST /echo→body, GET /log→DI+console logging, GET /missing→404, HEAD /ping→200 + Content-Length, no body.
• just test-js — Node smoke test (test/js/smoke.mjs), 4 cases, exercises the Connect middleware including next() fall-through.
• Existing suite unaffected: native F# + 50 pytest cases still green.

Notes / gotchas

• HttpHandler = HttpFunc -> HttpFunc is Fable's "function returning a function" uncurrying ambiguity; the handler is applied fully per request (handler earlyReturn ctx) — the shape Fable uncurries consistently. Corollary: build the middleware inside F# and export that, never round-trip a handler through hand-written JS.
• Bumps fable tool and fable-library (PyPI) to 5.5.0 (Python & BEAM unaffected).
• New Justfile targets: build-js, app-js, test-js. Output is ESM; recipes inject {"type":"module"}.

Follow-up

A tech-debt list was compiled while widening the architecture (shared cross-target tests, JSON union-serialization parity, host-abstraction convergence, extracting UseStructlog so the ASGI WebHostBuilder core is shareable, StaticFiles/Remoting on JS) — to be tackled in a follow-up PR.

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