fix(query-core): Change to prevent undefined from overriding Mutation default side effects.

[mitchell-up]

Problem

undefined means a variable that has not been assigned. Therefore, when the onError property in useMutation receives undefined, the side effects of the default options are retained and are expected to be invoked, as in the below example. However, It overrides default onError to undefined.

// You can pass `onError` callback optionally.
// If you intend to retain default `onError` callback,
// you will invoke this hook without any parameters.
const useLogin = (onError?) => {
 return useMutation({
    mutationFn: login,
    onError: onError,
    meta: {
      showToast: true,
    },
  })
}

// But if the useLogin is called without its parameters,
// default `onError` options will not be triggered.
new QueryClient({
  defaultOptions: {
    // ...
    mutations: {
      onError(error) {
        if (isHttpError(error)) {
          defaultErrorHandlers(error)
        }
      },
    },
  },
}),

Suggestion

To clarify the intention of not using side effects of default options, assign null to the side effects. Because null means absence of any value, you can predict that the side effects will not be invoked.

const useLogin = (onError?) => {
 return useMutation({
    mutationFn: login,
    onError: onError,
    meta: {
      showToast: true,
    },
  })
}

// This `mutate` call will not trigger any `onError` side effects, including the default `onError`.
const { mutate } = useLogin(null)

// This `mutate` call will not trigger the default `onError`, but `errorHandler` will be called.
const { mutate } = useLogin(errorHandler)

// These `mutate` calls will trigger the default `onError`.
const { mutate } = useLogin()
const { mutate } = useLogin(undefined)

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