Why SWR always assert the data may be undefined?

[cw1997]

The sample code at here:

const {data, error, isLoading} = useSWR(['passport/session/get'], () => get_session())
if (!isLoading && error == null) {
  console.log(data.user_id)
}

In my opinion, I think if isLoading is false that means fetcher is executed finished, and if error is null, that means the executed result is successful, so data is must not undefined.

But I see the TypeScript report the data may be undefined in this case, is there any way to optimize the logic of typing narrow? Actually, I think in this condition, SWR should assert the data must not be undefined, is this an issue that can be improved?

Thank you very much!

[GoldenChrysus]

Isn't this just a TypeScript LSP concern rather than an SWR one? You haven't actually confirmed that data isn't null or undefined in that block, nor guaranteed to SWR that data will always have a value, so the LSP is warning you. The LSP isn't aware of the runtime relationship between isLoading, data, and error. Without a guarantee, data itself definitely must allow for empty values considering there will be no data before the request to get the data.

data being guaranteed to have data means you already have data to provide (fallbackData or use suspense: true) per the SWR typing:

interface SWRResponse<Data = any, Error = any, Config = any> {
    /**
     * The returned data of the fetcher function.
     */
    data: BlockingData<Data, Config> extends true ? Data : Data | undefined;
    ...
}

type BlockingData<Data = any, Options = SWROptions<Data>> = Options extends undefined ? false : Options extends {
    suspense: true;
} ? true : Options extends {
    fallbackData: Data;
} ? true : false;

Therefore a simple example like this guarantees that data has a value and is no longer undefined or null in your LSP.

interface MyConfig extends SWRConfiguration {
    fallbackData: string,
}

const { data } = useSWR<string, string, MyConfig>('hello', () => 'b', {
    fallbackData: 'a',
});

However, you have not provided SWR with any initial data or a suspense guarantee, so from SWR's perspective there is every chance you will not receive any data from your fetcher and thus have no data to serve.

Otherwise, a simple check that data is actually present will do.