mutate optimisticData does not use updated optimistic state on consecutive calls

[winkcor]

Bug report

Description / Observed Behavior

  const { data, mutate: localMutate } = useSWRImmutable<{ quantity: number }>(
    "key",  () => ({ quantity: 0 }),  {
      fallbackData: { quantity: 0 },
    }
  );

below code work fine

    await mutate( async () => {
        await new Promise((r) => {
          setTimeout(r, 2000);
        });
        console.log("globalLocalState Finish");
        return { quantity: data!.quantity + 1 };
      },
      {
        revalidate: false,
        optimisticData: () => {
          console.log("globalLocalState prev:", data);
          return { quantity: data!.quantity + 1 };
        },
      }
    );

However, in the following code, prev refers to the previous state, not the optimistic state. So when running this twice in a row, prev.quantity remains 0 for both runs, and after the last fetch function resolves, it updates to 1.

As a result, if run multiple times in quick succession, the value only increases once:

    await mutate( "key", async (prev) => {
        await new Promise((r) => {
          setTimeout(r, 2000);
        });
        console.log("globalWithPrev Finish");
        return { quantity: prev!.quantity + 1 };
      },
      {
        revalidate: false,
        optimisticData: (prev) => {
          console.log("globalWithPrev prev", prev);
          return { quantity: prev!.quantity + 1 };
        },
      }
    );

This problem is the same in global mutate and mutate from hook.

Expected Behavior

When optimistic data increases the value, the next update should reflect the increased value, not the initial state.

Repro Steps / Code Example

Click on "with prev" multiple times optimistic update not work till fetch resolve:

CodeSandbox

Additional Context

SWR version: "swr": "^2.2.5", "^2.3.2"
Next.js version: "next": "14", "15"

[arjendevos]

I think it's intended to work like this. Because even with your global state, it doesn't work when you get an error. It will rollback to the state it had where the error occurred as you can see here: https://codesandbox.io/p/devbox/swr-render-error-forked-p5wxxn?workspaceId=ws_HrH9nNH3hTztr5r98WfYRD.

[winkcor]

Hi @arjendevos

You're right. In my example, I use optimistic updates for the "Add to Cart" button, and it causes issues when trying to increase the quantity multiple times. While I can revalidate when an error occurs, the problem is that after the first request resolves, the state updates to a lower quantity than expected when clicking multiple times.

If this behavior is intentional, it would be helpful to explicitly mention it in the documentation. It took me a while to figure out because I was also working with several other libraries and initially thought the issue was coming from them, which led to a lot of wasted time.

If this isn't planned to be change, please close this issue.