Error TS2532: Object is possibly 'undefined' after checking if it is defined

Here is the code for a helper class that creates a hash:

export default class PageUtil {
    private size: number;
    private step: PageUtilStep;
    private cursor: unknown[] | undefined;

    public constructor(size: number, step: PageUtilStep, cursor?: unknown[]) {
        this.size = size;
        this.step = step;
        this.cursor = cursor;
    }

    public createHash(): string {
        const json = JSON.stringify([this.size, this.step, this.cursor]);

        return createHash("sha1").update(json).digest("hex");
    }
}

type PageUtilStep = "backward" | "forward";

Here is the code where a get the typescript error on the return line:

export default class TattooLoader {
    private artistCache: Record<string, DataLoader<number, TattooEntity[]>>;

    public constructor() {
        this.artistCache = {};
    }

    public async fillArtist(artist: number, pageUtil: PageUtil): Promise<TattooEntity[]> {
        const hash = pageUtil.createHash();

        if (!this.artistCache[hash]) {
            this.artistCache[hash] = new DataLoader(async (artists) => this.batchArtists(artists, pageUtil));
        }

        return this.artistCache[hash].load(artist);
    }

    private async batchArtists(artists: readonly number[], pageUtil: PageUtil): Promise<TattooEntity[][]> {
        ...
    }
}

I don't understand because a get this error after to test if this.artistCache[hash] is undefined and create it if so.

If I change the code to be more specific to test if it's undefined I get the same error:

    public async fillArtist(artist: number, pageUtil: PageUtil): Promise<TattooEntity[]> {
        const hash = pageUtil.createHash();

        if (this.artistCache[hash]) {
            return this.artistCache[hash].load(artist);
        }

        this.artistCache[hash] = new DataLoader(async (artists) => this.batchArtists(artists, pageUtil));

        return this.artistCache[hash].load(artist);
    }

Indexed access doesn't narrow the undefined.

You'll have to create an intermediate variable :

const loader = this.artistCache[hash]

if(loader) {
  ...
}