TypeScript - How to Properly Type a Generic Function that Transforms Nested Objects Without Losing Type Information?
I'm reviewing some code and I'm working on a project using TypeScript 4.9, and I need to create a generic function that transforms nested objects. The scenario I'm working with is that when I try to map over these objects, I lose the type information of the nested properties. Here's what I have so far: ```typescript type NestedObject<T> = { [key: string]: T | NestedObject<T>; }; function transformNestedObject<T, U>(obj: NestedObject<T>, transformFn: (value: T) => U): NestedObject<U> { const transformed: NestedObject<U> = {}; for (const key in obj) { if (typeof obj[key] === 'object' && obj[key] !== null) { transformed[key] = transformNestedObject(obj[key] as NestedObject<T>, transformFn); } else { transformed[key] = transformFn(obj[key] as T); } } return transformed; } ``` I'm trying to use this function with the following example: ```typescript const example = { user: { name: 'John', age: 30, address: { city: 'New York', zip: '10001' } } }; const transformed = transformNestedObject(example, value => value); ``` The scenario I'm working with is that TypeScript infers the resulting type of `transformed` to be `NestedObject<unknown>`, which doesn't retain the specific property types. I would like the resulting type to reflect the same structure as the input object but with types transformed. This is important for ensuring type safety across my application. I've tried adding generic constraints and using `as` assertions, but I still run into errors such as: ``` Argument of type 'T' is not assignable to parameter of type 'unknown'. ``` How can I enhance my `transformNestedObject` function to ensure it retains the correct types for the transformed nested properties? Any suggestions or best practices would be greatly appreciated! The project is a CLI tool built with Typescript.