Parse. Don't validate. - https://lexi-lambda.github.io/blog/2019/11/05/parse-don-t-validate/

main.ts

// HINT: Definitely use ts-result for this.
type Result<A, B> = { success: true, data: A } | { success: false, error: B };

function Ok<A>(x: A): { success: true; data: A; } {
    return { success: true, data: x };
}

function Err<B>(e: B): { success: false; error: B; } {
    return { success: false, error: e };
}

function parseString(x: any): string {
    if (typeof x !== "string") {
        throw new Error(`Expected a string and got ${JSON.stringify(x)} (${typeof x})`);
    }
    return x;
}

type Person = { fname: string, lname: string; };

function get<A>(url: string, payload: A): Promise<A> {
    return new Promise((resolve) => setTimeout(() => resolve(payload), 1000 + Math.random() * 2500));
}

// HINT: Maybe use Zod for this.
function parsePerson(x: any): Result<Person, string> {
    if (typeof x !== "object") {
        return Err(`Expected object got ${JSON.stringify(x)} (${typeof x})`);
    }
    try {
        const supported = ["fname", "lname"];
        if (Object.keys(x).every((k) => supported.includes(k))) {
            const fname = parseString(x["fname"]);
            const lname = parseString(x["lname"]);
            return Ok({ fname, lname });
        } else {
            return Err("Unexpected keys in object");
        }
    } catch (e) {
        if (e instanceof Error) {
            return Err(e.message);
        } else {
            return Err("Unsupported error");
        }
    }
}

async function fetchPerson(payload: any): Promise<Result<Person, string>> {
    const x = await get("https://potato.com", payload);
    const parsed = parsePerson(x);
    if (parsed.success) {
        return Ok(parsed.data);
    } else {
        return Err(`Failed to parse into Person: ${parsed.error}`);
    }
}

async function main() {
    const payloads = [
        12,
        {fname: "William", lname: "Carroll"},
        {fname: "William", lname: "Carroll", age: 32},
    ];

    for (const payload of payloads) {
        console.log("Fetching...");
        const result = await fetchPerson(payload)
        if (result.success) {
            console.log(`Success: ${JSON.stringify(result.data)}`);
        } else {
            console.log(`Failed to fetch Person: ${result.error}`);
        }
    }
}
main();