Nested computeds sometimes trigger parent computed

[slim ocean]

Hey, I've been messing around with nested computeds as an optimization strategy & have noticed some stuff I find weird. I'm wondering if anyone understands & could help me out by explaining what's going on 🙂

I'd expect the first computed "unstable" to behave the same as "stable"...where a value update only triggers the internal computed a-la SolidJS. But in the "unstable" case it also triggers the outer one on every value change.

So my question is why wrapping the inner computed in an object makes a difference? o.O

P.S. I've also asked this question on the Angular subreddit, I'll update both if we figure something out 🙂

function nestedTest(source: WritableSignal<number[]>) {
  const length = computed(() => source().length);

  const unstable = computed(() =>
    Array.from({ length: length() }).map((_, i) => computed(() => source()[i])),
  );

  effect(() => {
    console.log('unstable', unstable()); // triggers once (first time) & every time source changes, underlying signals also trigger
  });

  const stable = computed(() =>
    Array.from({ length: length() }).map((_, i) => ({
      value: computed(() => source()[i]),
    })),
  );

  effect(() => {
    console.log('stable', stable()); // triggers once (first time), but does not when source changes. underlying signals also trigger
  });

  return {
    trigger: () => {
      source.update((cur) => cur.map((v, i) => (i === 1 ? v + 1 : v))); // update the second element
    },
  };
}

[tardy gale]

Your "unstable" effect logs the value of unstable to the console. That value is an array, which will log every entry in the array. These entries are computed, which in dev mode log their value when logged to the console.
So, indirectly, your "unstable" effect reads every value in the array, too. "stable" does not, because the object in the array does not access the nested computed when logged to the console.

effect(() => {
  const u = unstable();
  console.log('unstable');
});

This will behave the same as "stable"