Removing unnecessary calls to mutateStreamData in useChat React hook

[tonyfarney]

This fixes #2950

[lgrammel]

Hey @tonyfarney meant to come back to this - the issue is legit, but I need to verify this fix actually works in different settings, e.g.. when something is already non-empty or when there are non-changes for non-empty data.

[tonyfarney]

Hey @tonyfarney meant to come back to this - the issue is legit, but I need to verify this fix actually works in different settings, e.g.. when something is already non-empty or when there are non-changes for non-empty data.

Looking the code, the only situation data is not an empty array when passed to onUpdate is in process-data-protocol-response.ts:

update([...previousMessages, ...merged], [...data]); // make a copy of the data array

I think it's possible the situation of calling the update callback for the exact same data. But for avoiding to mutate stream data in such situation, would be necessary to compare the arrays or maybe just to check the array length.