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svelte-2.html

<div on:click="set({ count: count + 1 })">
  {count} {double}
</div>

<script>
  export default {
    data() {
      return { count: 0 };
    },
    
    computed: {
      double: ({ count }) => count * 2
    }
  };
</script>

svelte-3.html

<script>
  let count = 1;
  const double = () => count * 2;
</script>

<div on:click="count += 1">
  {count} {double()}
</div>

Other things I'm thinking about:

• namespace: 'svg' — perhaps this becomes <svelte:meta namespace="svg">?
• tag — perhaps this becomes <svelte:meta tag="my-element">? (We no longer need props, since this is defined by export let xxx)
• Firing events. The best thought I've had on this so far is

<script>
  import { createEventDispatcher } from 'svelte';

  const dispatch = createEventDispatcher();
</script>

<div on:click="dispatch({ type: 'foo', x: 1 })">...</div>

• Speaking of events: would be cool to align these with CustomEvent and get things like bubbling.
• Also speaking of events: does it now make sense to go with <button on:click={e => doThing(e)}>?
• Component bindings
• Component refs... do these still make sense?
• Interacting with a component from JS. What is the external API?
• The mechanism for responding to changes: At the moment, onstate fires for updated props or internal state (because there's no distinction between the two). This results in cyclicality (when you call this.set(...) inside onstate) which leads to confusion and bugginess. I'd propose that we make that completely impossible by only calling onstate (or more likely onchange) when props have changed. I can't think of anything that would become impossible as a result, except for things that we want to be impossible. Also, we'd get rid of the arguments to onstate — no longer necessary.

Preload! We need to figure out how to support preload — doesn't need to be the exact same mechanism, but something like it. (We could abandon it in favour of doubling down on Suspense, but it wouldn't quite cover the same area.

In fact SSR more generally is a big unknown. In this new world, the initial setup code (which is currently what we'd call oncreate) always runs, on both client and server. onchange and onrender (or whatever) would never run. ondestroy could either be a noop (in which case any mount code would have to be wrapped in if (process.browser) or something), or it could be called immediately following render.

It's possible that we'd lose the API distinction between client and server. We could also consider introducing async SSR rendering. Lot of possibilities, lot of stuff to figure out.

Oh, and all this needs to work with standalone components as well as normal ones