[jarrednorrisdev]

Fixes #18932.

The problem

With experimental.async, an event handler whose value comes from a call after a top-level await is never attached in a production build:

<script>
	const make = (name) => () => console.log(name);

	await Promise.resolve();

	const onclick = make('click');
</script>

<button {onclick}>click</button>

The declaration after the await is assigned inside $.run, but the event attribute passes the variable straight to $.delegated, while it's still undefined:

var onclick;
var $$promises = $.run([async () => await Promise.resolve(), () => {
	onclick = make('click');
}]);
// ...
$.delegated('click', button, onclick);

In a plain Vite app with @sveltejs/vite-plugin-svelte, the button logs click in vite dev, but after vite build + vite preview it does nothing and nothing throws. Non-delegated events ($.event) and <svelte:window> handlers behave the same way. An attribute or component prop that reads the same kind of value works, since it already waits on $$promises[1].

The cause

build_event_handler returns a local identifier as the handler as-is outside dev mode, on the assumption that the variable is already assigned when the listener is attached. A binding with a blocker isn't assigned at that point. Dev builds didn't show the bug because there the handler is always wrapped in $.apply(() => onclick, ...), which reads it when the event fires.

The fix

Skip that shortcut when the binding has a blocker. The handler then goes through the existing wrapper that imports already use, function (...$$args) { onclick?.apply(this, $$args); }, which reads the variable when the event fires. Handlers without a blocker compile exactly as before, so components without top-level await don't change.

I considered wrapping the $.event/$.delegated call in $.run_after_blockers, like #17198 did for attachments, actions, transitions, animations and bindings. Those need the value before they can run. An event listener doesn't: it can be attached at once and read the handler on dispatch, which is what dev mode already does. That keeps the change to one condition and leaves the attach order unchanged.

Tests

runtime-runes/samples/async-event-handler-blockers covers a delegated event (onclick), a non-delegated one (onmouseenter) and a <svelte:window> handler (onresize), each assigned from a call after a top-level await. Samples compile without dev by default, so the sample exercises the production path. It fails without the change ([] instead of ['click', 'mouseenter', 'resize'] in both the dom and hydrate variants) and passes with it.

Test plan

• pnpm vitest run packages/svelte/tests/runtime-runes/test.ts -t async-event-handler-blockers: 2 passed (2 failed on main)
• pnpm test: 7,808 passed, 55 skipped, all 34 test files passing
• pnpm lint (ESLint and Prettier) and pnpm check pass
• The repro above, in a plain Vite app with the patched compiler: the click, submit and mouseenter handlers fire in both vite dev and vite build + vite preview. Before the change, none fired in preview.

Before submitting the PR, please make sure you do the following

• It's really useful if your PR references an issue where it is discussed ahead of time. In many cases, features are absent for a reason. For large changes, please create an RFC: https://github.com/sveltejs/rfcs
• Prefix your PR title with feat:, fix:, chore:, or docs:.
• This message body should clearly illustrate what problems it solves.
• Ideally, include a test that fails without this PR but passes with it.
• If this PR changes code within packages/svelte/src, add a changeset (npx changeset).

Tests and linting

• Run the tests with pnpm test and lint the project with pnpm lint

🤖 Generated with Claude Code

[pkg-svelte-dev]

Install the latest version of svelte from 51df986:

pnpm add https://pkg.svelte.dev/svelte/c/51df98629508b3024f5f08f6dc0a0026bcb71ea1

Open in pkg.svelte.dev: https://pkg.svelte.dev/repos/svelte/pr/18933

Note

This PR is from a fork. A maintainer must approve approve each commit before it can be built and installed.

[changeset-bot]

🦋 Changeset detected

Latest commit: 51df986

The changes in this PR will be included in the next version bump.

This PR includes changesets to release 1 package

Name	Type
svelte	Patch

Not sure what this means? Click here to learn what changesets are.

Click here if you're a maintainer who wants to add another changeset to this PR

[dummdidumm]

thank you!