How to override a property to be non-nullable in Typescript

As of TypeScript 2.1, you can use a lookup type to access an interface property.

IncomingMessage['url'] // string | undefined

You can combine that with NonNullable to fit your use case.

NonNullable<IncomingMessage['url']> // string

https://www.typescriptlang.org/docs/handbook/release-notes/typescript-2-1.html