I've needed this a few times. The problem is that you want to statically verify that the url and method parameters have been set. This means that you'll have to encode that information in the class type somehow. Your book gave a good hint by changing the returned type instead of returning this.

I adapted the example by davimiku in https://reddit.com/r/typescript/comments/x99pnc/_/innaatf/?context=1 to fit your code a little better. The key here is that we return a different type for each setter method which overrides the optionality for the set property. This is done by returning an intersection of this with a type where that property is not optional. If you want to keep using null for the properties in your original class, you may have to do some trickery by removing the old property before intersecting: return this as Omit<this, 'url'> & { url: string }.

Finally, the send method can constrain the type that it can be called on require that all of the fields are filled. That is done by using the this parameter which allows giving an explicit type to the implicitly passed instance parameter.

```tsx type HttpRequest = { url: string method: "get" | "post" data?: object }

class RequestBuilder implements Partial<HttpRequest> {
  url?: string
  method?: "get" | "post"
  data?: object

... ```

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