example-with-type-assertions.ts

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

interface IGetUserRequest extends IGenericRequest {
    username: string;
}

function mustBeUserRequest(request: IGenericRequest): asserts request is IGetUserRequest {
    if (!maybeUserRequest.username || !maybeUserRequest.username.length > 0) {
        throw new Error('Bad input!');
    }
}

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