Undefined attributes in spread

[RedHatter]

When some attributes are undefined and set through spread, svelte sets the dom attribute to "undefined". This is caused by src/runtime/internal/dom.ts#L103-L104. The only way I can think to solve this is to include the list of boolean attributes in the runtime.

See repel.

The "undefined" attributes happen whether in a spread or not.

I think the more general issue is that most of us expect the framework to "eat" undefined values and not pollute the DOM. From a purist point of view, Svelte is doing the right thing by not implicitly doing anything so I suppose this would be more of an enhancement.

See: REPL

[RedHatter]

After #3013 most attributes ignore undefined. class is an exception.