Inconsistent autorun behavior between first run and subsequent runs.

[jereklas]

Intended outcome:

I would have expected autorun to behave the same on its first run or its nth run, but it appears to do something different on the first run? For the following code I would have expected the autorun to loop until the observable value it's tracking stops changing.

test("autorun", () => {
  const o = mobx.observable.box(0);
  const seen: number[] = [];

  mobx.autorun(() => {
    seen.push(o.get());
    if (o.get() < 5) o.set(o.get() + 1);
  });
  expect(seen).toStrictEqual([0, 1, 2, 3, 4, 5]); // this assertion fails as "seen" is [0]
});

Actual outcome:
The following test case is the actual behavior.

test("autorun", () => {
  const o = mobx.observable.box(0);
  const seen: number[] = [];

  mobx.autorun(() => {
    seen.push(o.get());
    if (o.get() < 5) o.set(o.get() + 1);
  });
  // autorun doesn't continue to run even though a tracked observable value has changed
  expect(seen).toStrictEqual([0]); 

  // triggering the autorun again then causes it to loop (but it has "lost" the value 1)
  o.set(o.get() + 1);
  expect(seen).toStrictEqual([0, 2, 3, 4, 5]);
});

Versions
6.11.0

[mweststrate]

This is working as intended; subscription happens after the body has completed. So you're subscribing only for the next change at the moment o is already 1, which is why you miss the first change. In other words, the first o.set doesn't trigger the autorun, because the autorun is not yet an observer of it.

[jereklas]

If the subscribe is happening after the body has completed, then using a computed in this scenario is inconsistent from an observable.

test("mobx", () => {
  const a = mobx.observable.box(0);
  const b = mobx.computed(() => a.get());
  const seen: number[] = [];

  mobx.autorun(() => {
    seen.push(b.get());
    a.set(1);
  });

  // this fails it's actually [0,1]
  expect(seen).toEqual([0]); 
});

I would expect it to behave the same as the observable case.