What does the second parameter in set do exactly?

[fa-901]

In this example in the devtools doc, the second parameter is set to undefined. When I hover over it, it shows replace

Just curious about what exactly the purpose of replace is

[dai-shi]

If replace is falsy and the state is object, set will "shallow merge" the previous object and the new object.

zustand/src/vanilla.ts

Lines 76 to 78 in 6737115

	(replace ?? (typeof nextState !== 'object' || nextState === null))
	? (nextState as TState)
	: Object.assign({}, state, nextState)

[fa-901]

Thanks!