Utilities to Implement

[NullVoxPopuli]

• Object
• Array
• WeakMap
• WeakSet
• Map
• Set
• ArrayMap (Array.prototype.map, optimized)
• SignalAsyncData / load (wrap a promise)
• asyncFunction (wrap and invoke a function)
• @signal (wrapper around new Signal.State)
• localCopy / @localCopy
• keepLatest / @keepLatest
• dedupe / @dedupe
• @cached (computed, but on a getter)
• localStorage / @localStorage
  • other storages as well?
• fetch
• Relays - https://www.pzuraq.com/blog/on-signal-relays

Blocked on framework-specific semantics and timing (cleanup)

• throttle
• debounce
• Image
• effect
  • leaky microtaskqueue effect
• waitUntil
• resource

[justinfagnani]

It would be nice to have a utility similar to MobX's reaction() function.

reaction() is very useful when you need to track the pervious value of a signal. You give it a tracked data function and an untracked effect function that's run when the value changes.

Something a bit like this:

export const reaction = <T>(
  data: () => T,
  effect: (value: T, previousValue: T) => void,
  equality = Object.is,
) => {
  const computed = new Signal.Computed(data);
  let previousValue = computed.get();
  let isReacting = false;
  const watcher = new Signal.subtle.Watcher(async () => {
    if (isReacting === true) {
      return;
    }
    isReacting = true;
    await 0;
    isReacting = false;
    const value = computed.get();
    if (!equality(previousValue, value)) {
      effect(value, previousValue);
      previousValue = value;
    }
  });
  watcher.watch(computed);
};

[NullVoxPopuli]

oo nice! that does seem useful -- I know quite a few folks that would like such a utility for caching or memoizing complex calculations (tho I don't know an exact use case, but I know it's desired!).

Would you be willing to submit a PR adding this utility along with a bunch of tests?
(so we can ensure behavior is retained as the polyfill (and spec) evolves?)

[JosXa]

Reminds me of Vue's watch function that allows to declare multiple signals ("refs") to track:
https://vuejs.org/api/reactivity-core.html#watch