Author app cores in the TypeScript subset

An app core is the logic tier of a Native SDK app: Model (the app state), Msg (a discriminated union of everything that can happen), update(model, msg) (the one pure transition function), and the pure helpers they call. You write it as a TypeScript module rooted at src/core.ts - splitting into more modules under src/ when it grows (see "Splitting a core into modules") - and the @native-sdk/core transpiler compiles the whole import graph to Zig at build time. No JS engine ships in the binary — the program either passes the subset checker and compiles to native, or you get a teaching error naming the rule, the fix, and the reason. The same file is executable TypeScript: it typechecks with stock tsc and runs unmodified under node, so you can poke behavior with plain node scripts before the native build.

A whole TS app is three files of truth and zero Zig: src/core.ts (this guide; plus any modules it imports under src/), src/app.native (the markup view over the core's emitted model), and app.zon (windows, identity, permissions). native init scaffolds exactly that; the build detects src/core.ts in the tree (never a flag or config — a tree with both src/core.ts and src/main.zig is a teaching error) and generates the wiring outside the app. The loop:

native dev --core   # the fastest loop: run the core under node's virtual host —
                    # dispatch Msgs as JSON lines ({"kind":"add"}, {"$bytes":"…"}
                    # for bytes payloads, {"advance":1000} to run virtual timers),
                    # watch the model + effect transcript. Logic only, no renderer.
native dev          # build and run the real app (markup hot reload)
native check        # subset-check core.ts + validate markup + app.zon
native build        # ReleaseFast binary; native test runs the app's tests

The complete reference app in this idiom is examples/soundboard-ts in the SDK repo: the soundboard music library as three files and zero Zig — const catalog tables, REAL audio through the Cmd.audioPlay stream, scrub-to-seek on a markup slider, a motion-gated Sub.timer playback clock, the full text-edit engine on a search field, controlled scroll, registered cover assets, the width-adaptive grid through the frame channel, clipboard, and context menus, with an end-to-end suite driving the shipping markup.

The contract