[WIP] Remove an undefined attribute instead of setting it to "undefined"

[kaisermann]

Related to #1434

[kaisermann]

Does it make sense to use == null instead of === undefined to catch null attributes as well?

[Rich-Harris]

Nice, thanks!

Hmm... good question. I think so? I suppose it's hard to imagine a situation where you'd want the string literal "null" instead.

I noticed the test has skip-ssr — would be great if we can get the same behaviour in both targets 😀

[Rich-Harris]

btw to update the js tests, you can do node test/js/update.js rather than updating them manually, once you're happy that the generated code is correct

[kaisermann]

@Rich-Harris Oh, nice to know 🤣 I've just updated them manually...

I didn't have time to mess around with svelte's ssr side but I will try to make it work!

[jacwright]

@kaisermann what about property and data attributes? E.g. id attributes such as in https://svelte.technology/repl?version=2.12.0&gist=e3ec1b2f5f00eaf3a2afe8f79ce571d4

I actually did a full branch that prevented undefined from showing up in attributes and text but then ran across #659 (and didn't see your PR either until now). Not sure if we want it, if it goes too far and we want to keep pieces of it, or what.

https://github.com/sveltejs/svelte/tree/mustache-undefined

It does solve the property/data-set issue by adding methods for checking.

[Rich-Harris]

Closing this in favour of #1815 (which is this + SSR support)