Does not trigger with an initial value

[c-dante]

Because of the var currentValue = getValue(getState(), objectPath) on initial call to the watch constructor, the change handler that you use to subscribe won't get called until the value has a first change.

I don't want to duplicate my side effect logic by having an initial state, then a watch which performs the same logic, so I end up doing this:

const updateFn = (current, prev) => {
  // ...logic
};
const w = watch(() => someSelector(store.getState()));
const unsub = store.subscribe(w(updateFn));
updateFn(someSelector(store.getState()));

It's strange that the current value is captured at the time the watch function is created, but the callback change function isn't run until the next time the state changes.

[xapple]

It's been over a year, and no answer. Is this library abandoned ?

[RyanZim]

If updateFn was called initially, what would be the value of prev?

[c-dante]

If updateFn was called initially, what would be the value of prev?

• Not passed, can use ...args to capture
• Undefined, ambiguous if you depend on that value
• A third param to indicate the first call
• Can add a second arg to w(...) for the initial call, then it's w(updateFn, initialValue) (which can be updateFn if your business logic understands)

[RyanZim]

Not passed, can use ...args to capture

Yeah, that isn't great, because you then get objectPath as the second parameter

Undefined, ambiguous if you depend on that value

Right, selectors returning undefined is common enough.

A third param to indicate the first call

I'm not sure I understand this proposal?

Can add a second arg to w(...) for the initial call, then it's w(updateFn, initialValue)

That's one possibility. Is it crazy to make an initial call only if initialValue is provided? Not sure if I like that implicit behavior change.

which can be updateFn if your business logic understands

Sorry, I'm not understanding this bit