How to retrieve oldValue as an object from onSet effect?

[stefanstaynimble]

I am using effects_UNSTABLE on my atom to make PATCH requests to my API with updates made the atom. The response time of the API call is dependent on the payload sent to the server.

In order to reduce the payload, I want to be able to get the difference from the new value and old value using the onSet function. Problem is, oldValue returns a DefaultValue instance, so I cannot determine the differences from the original atom state and the new state.

Example:

export const userState = atom({
	key: "user",
	default: userRequestState,
	effects_UNSTABLE: [
		({ onSet, trigger }) => {
			onSet((newValue, oldValue) => {
					if (trigger === "set") return;
					axios.patch("/me", objectDiff(newValue, oldValue));
				});
			};
		},
	],
});

Any ideas how I can achieve this?