Current Data vs Data

[abhieshekumar]

I have been trying to understand the difference between currentData and data -

1. Is it that data will have the freshest copy while current data holds on the data provided in last successful fetch.
2. Also, if the later half is true does it also imply the current data is undefined only if the query has never resolved successfully.
3. I was trying to log currentData here, but couldn't see it. Please let me know if I have missed anything?

[markerikson]

Is it that data will have the freshest copy while current data holds on the data provided in last successful fetch.

Yep, this is it!

I was trying to log currentData here, but couldn't see it.

Looks like the sandbox is using RTK 1.6, and we added that in either 1.7 or 1.8. Bump the RTK version and it should be there.

[phryneas]

@josias-r

You start with usePostsQuery({ page: 3 }) - data will be for page 3 and currentData will be for page 3.
You change to usePostsQuery({ page: 4 }) - while page 4 is not loaded yet, data will stay page 3 so you have something to display, while currentData gives you the data for page 4 - undefined, as nothing is fetched yet.
Data is loaded for page 4: data will be data for page 4 and currentData will be data for page 4.

[josias-r]

@phryneas Yup thank you! I keep checking this in the code and finding out the same result, but with your comment I can come back to this every time thread from now on haha.

I think it slowly gets into my head as current as a term indicates to be more fleeting. So current will be more temporary and reset to undefined when the query changes and it is still fetching

[phryneas]

currentData is always bound exactly to the argument that is passed in, while data is trying to always have some value that you can display.
Most apps will probably only use data, in combination with some loading indicator or graying UI items out.
Does that make sense?

[wkigenyi]

Yes it makes sense.

[jtiret]

I've observed another distinction between data and currentData and am curious whether this behavior is intentional or a bug. Previously, I relied on data rather than currentData and frequently used api.util.updateQueryData for optimistic updates. I've noticed that when a query is in progress (e.g., triggered by polling), updates made via api.util.updateQueryData are not reflected in data, but are visible in currentData (even when I haven't altered the hook argument at all). This discrepancy isn't apparent when queries are fast, but with queries taking 200-300ms and a short polling interval, optimistic updates happening within this timeframe fail to apply.