[Bug] Destructuring {...$props} on an element does not omit undefined attributes

[zaydek]

Bug
In SSR mode, <a target={true ? undefined : "..."} {...$$props}> renders <a target="undefined"> instead of <a>. This does not appear to happen when {...$$props} is not present.

Expected behavior
The expected behavior is that undefined attributes should be omitted. These are PRs that I found are similar in scope:

#1668
#4498
#1815

Repro
I created a repro to demonstrate my use case: https://github.com/zaydek/svelte-undefined-bug. Simply run yarn test or npm run test to see the same result as in the screenshot.

Furthermore, in this repl: https://svelte.dev/repl/aaedb6456d714a9f9ccfab9c84c8b391?version=3.32.1, when SSR mode is enabled, you can see that it generates:

const App = create_ssr_component(($$result, $$props, $$bindings, slots) => {
	return `<a${spread([
		{
			target: escape(true ? undefined : "_blank")
		},
		$$props
	])}>Hello, world!
</a>`;
});

I suspect escape is the culprit; it should be omitting the value instead of escaping it as a string literal.

[neurocmd]

This is a very huge bug. I came across the same problem. On server side I get <img src="some.jpg" srcset="undefined"> instead <img src="some.jpg"> and it breaks an image because srcset takes precedence over src. Therefore, users with javascript disabled and Google won`t see images. Are there any insurmountable obstacles to fix this bug?

[stale]

This issue has been automatically marked as stale because it has not had recent activity. It will be closed if no further activity occurs. Thank you for your contributions.

[stale]

This issue has been closed as it was previously marked as stale and saw no subsequent activity.