PreprocessError

What this error means and what can trigger it

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PreprocessError is the common base for synchronous failures raised while SER lowers component script or markup. It extends RuntimeError with a filename, which lets Vite and other tooling retain source context without having to parse the diagnostic text.

Most failures use a concrete subclass. The base class itself has one production trigger, albeit an unusual one: a class field initializer contains a top-level yield*. For example, class Store { value = yield* LoadValue() } places effect work inside object construction rather than component-level script work.

The detector stops at nested function boundaries. A yield* inside an arrow, method, accessor, or other nested function is not treated as top-level work belonging to the field initializer.

Exactly where it triggers

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const bad_member = find_class_member_with_yield_star(stmt);

if (!bad_member) {
	return;
}

throw new PreprocessError(
	[
		`[ASYNC_EFFECT_IN_CLASS_MEMBER]: ${filename}: yield* cannot be used inside class members.`,
		`Class fields and methods are not component top-level reactive work. Move the Effect work into a script effect statement before assigning it to the class instance.`,
		"",
		"Problematic member:",
		slice(content, bad_member),
	].join("\n"),
	filename,
);

The compiler raises this synchronously during the Vite/SER transform, before the component module can evaluate and before any effect runtime exists.

For reference, the base class is declared at modules/svelte-effect-runtime/src/errors.ts L55 @ C14.

Why this is the case

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SER lowers component-level statements into dispatcher-owned work. A class field follows JavaScript object-construction timing and may run once per instance, long after or long before the component-level statement SER would have generated. Hoisting the expression would change evaluation order and ownership; leaving it in place would put generator syntax into a synchronous initializer.

The compiler rejects the field instead of guessing whether the result should belong to the component, the class instance, or every future instance.