// much better
interface IGenericRequest {
    requestId: string;
}

interface IGetUserRequest extends IGenericRequest {
    username: string;
}

function isUserRequest(request: IGenericRequest): request is IGetUserRequest {
    const maybeUserRequest = request as IGetUserRequest;
    return maybeUserRequest.username && maybeUserRequest.username.length > 0;
}

function handler(request: IGenericRequest) {
    if (isUserRequest(request)) {
        const username = request.username;
        // ...the rest of the handler...
    }
}