example-no-typeguards.ts

// not great
interface IGenericRequest {
    requestId: string;
}

interface IGetUserRequest extends IGenericRequest {
    username: string;
}

function handler(request: IGenericRequest) {
    const getUserRequest = request as IGetUserRequest;
    if (getUserRequest.username && getUserRequest.username.length > 0) {
        const username = getUserRequest.username;
        // ...the rest of the handler...
    }
}