component.set({ foo: undefined }) should 'unset' foo

[Rich-Harris]

In the following case...

<svelte:component this={Whatever} bind:foo/>

...it's very likely that when Whenever changes, you don't want the new component to have the value of foo from the old component imposed upon it. But if you try to clear the value...

this.set({ foo: undefined });
this.set({ Whatever: SomeNewComponent });

...then the SomeNewComponent instance is initialised with foo: undefined rather than being able to use its default value, which is what you want in that situation. The workaround is somewhat hacky:

delete this._state.foo;
this.set({ Whatever: SomeNewComponent });

I'd argue that the current behaviour is a bug, and that undefined values should be deleted from the state object automatically. Outside of the <svelte:component> case, I don't think it would result in any different outcomes.

[Rich-Harris]

We might not need this, because of #1490

[eyeseast]

Would it make sense to have a this.del('key') that explicitly removes a key from state? (Though in all honestly, I can't think of when I've actually needed this.)

[Rich-Harris]

In Svelte 3 we seem to have the opposite bug — the child value always overrides the parent value https://v3.svelte.technology/repl?version=3.0.0-alpha12&gist=181691a0863f4a40fd80f05ba81736b5