Unexpected circularity error when variable is initialized from a class member

🔎 Search Terms

loss of type, for loop, for-loop, class field

🕗 Version & Regression Information

• This changed between versions ______ and _______
• This changed in commit or PR _______
• This is the behavior in every version I tried, and I reviewed the FAQ for entries about _________
• I was unable to test this on prior versions because _______

⏯ Playground Link

https://www.typescriptlang.org/play/?#code/JYOwLgpgTgZghgYwgAgIIAd0BtgLmYAexGQG8AoZK5YAEwC5kBnMKUAcwG5zLq9J2hKAE9GFahJoNmrDtwkBfckvIIscJkzSYc-IiC3jq6AK4AjXcgBioWhmy58+gBQBHE9FEy2IdgEpGe10nYmQAH2QQEywsMl5JZCgIMBMoEiiY+WolFTUNLQAVCBY4iQBlCCwIBEg7HUcCYiZGCrAAHhYfdgA+ZABeSIgAd2RW5z8sqiCG-WbtBz0m8Mjo2IGMrEnkdmTRyura6cWDACFhAGF8CEERccYABTgoAjgsNoAlaqFaDtlfABo9u1OhxumDlhtSglkAB6GHIADqQgA1k9CCYQLRkABGRgAW0IADcUGAABbALQWQgIZE0Ax0FAwITILCEQjoeKSOGIlFojFYgBMLTgxOQAAMyRSAHRHEIGMXIMCEZBwZAIJpgRWkiAkExMEmk-CciTAGDIZySpgy+rHLR9e0rGJ+KHQ6hJFJpR2bY3ZHiu9UGTVJJjRMAPJ4vN6fdVQH4ggFA35dMG9AakBTcH1UJlQc0BkpwG1ygCSABFkIQzZapRUqjUIHUFnKmM6jK6qPnNYWm40SANq7Le1abJjBy5u8Fe2WJlnJKbzROZqF7etVq3Z-7iAQohAtq6VO2+Bq1VcbsIy-0VUXe1L+NchMIpXQ99DO1ee7NL8HQwBtO9nssAF1kAAfhAwYRlaJNQXGF8EkXW0pTgWhaGcBC5SfWgZ0PKhuXOQg8TxHUwBhDF1UI4itUZQgYkIIYOBZUASWVfUUGATUzCSOBkVIkBOIgbiNzdYpf3-B8gMvdChzg5AD2hd1UhIb8sDALYciAA

💻 Code

interface Application {
    id: string;

    category: {
        id: string;
    }
}

class Applications {
    public FindApplication(query: string): Application | null {
        return null;
    }
}

class Test {
    SelectedApplications: Set<string> = new Set();
    Applications: Applications | null = null;
    get SelectedApplicationsByCategory(): Partial<Record<string, Set<string>>> | null {
        // Workaround 1: move this block inside for loop
        // Workaround 2: Save this.Applications to a const then use that
        if (this.Applications === null) {
            return null;
        }

        const result: Partial<Record<string, Set<string>>> = {};

        for (const applicationID of this.SelectedApplications) {
            const application = this.Applications.FindApplication(applicationID);
            if (application === null) {
                continue;
            }

            const categoryID = application.category.id;
            const applications = result[categoryID] ?? new Set<string>();
            applications.add(application.id);
            // Comment/uncomment the following line to see it break/unbreak
            result[categoryID] = applications;
        }

        return result;
    }
}

🙁 Actual behavior

application has type: any. FindApplication returns Application | null, but that gets lost somewhere.

🙂 Expected behavior

type for application should be Application | null.

Additional information about the issue

See comments in code for workarounds and to see another line break type inference.

Start with the linked TypeScript Playground reproduction and compare the two documented workarounds in the getter. Trace why the assignment to result[categoryID] changes the inferred type of application; done means application remains Application | null without either workaround.