How to deal with react router url params and type script

Question

In react router, I am going through the tutorial but trying to use typescript and have this code

let params = useParams();
let id:string = params.invoiceId
let invoice = getInvoice(parseInt(id, 10));

The second line has an error

TS2322: Type 'string | undefined' is not assignable to type 'string'.   Type 'undefined' is not assignable to type 'string'

How do I do this properly to lookup and display the invoice? Also, how to reroute to the catch all * if the string is not an integer.

thanks, Dean