Live mode scaffolds each Svelte variant with a props script that annotated the
declaration:
/** @type {{ title: string; }} */
let { title } = $props();
A JSDoc `@type` written directly before a value is also JSDoc's cast syntax,
and esrap 2.3.3, the printer Svelte emits JS through, moves that annotation
onto the template's own declaration:
var /** @type {{ title: string; }} */ (h1) = root();
`var (h1) = ...` does not parse. The .svelte source is valid, the compile
succeeds, and the failure lands in the browser's dynamic import as "Unexpected
token '('": the variant never mounts and the session shows nothing. `@typedef`
carries the same shape without being a cast, so both builders emit that.
This is not test-only. Every Svelte variant we generate carried the construct,
so live mode was broken for any user whose install resolved esrap 2.3.3.
Svelte declares `esrap: ^2.2.12`, so a fresh install takes it; this repo's
lockfile pins 2.3.0, which is why unit tests stayed green while the fixture,
which installs into a temp dir, did not.
Two reasons the existing pre-publish guard could not have caught it, now
recorded next to it:
- `compileCheckVariants` compiles with `generate: false`, so there is no
emitted JS to inspect.
- `loadSvelteCompiler` resolves the compiler through createRequire, which
Svelte's export map routes to a prebuilt CJS build. A dev server imports
`src/compiler`, and only that path runs the app's installed printer. The
guard was checking a different compiler than the browser runs.
The new suite therefore imports the compiler as ESM and asserts the emitted
JavaScript parses, rather than pinning the comment style: a future printer that
mangles some other construct fails it too. The first draft used createRequire
and reported green against the exact input that breaks in a browser, which is
the mistake worth not repeating.
Verified against svelte 5.56.9 with esrap 2.3.3. Full live-e2e sweep green,
26 fixtures.
Written with AI assistance (Claude Code).
Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Paul BakausandClaude Opus 5
5961269cb5