TypeScript: How to Properly Type a Recursive Function for Deeply Nested Objects?
I'm working on a TypeScript project where I need to write a recursive function that deeply merges two nested objects. The challenge I'm facing is how to properly type this function so that TypeScript can infer the correct types when merging. Here's an example of what I've tried: ```typescript interface NestedObject { [key: string]: string | number | NestedObject; } function deepMerge<T extends NestedObject>(target: T, source: T): T { for (const key in source) { if (source.hasOwnProperty(key)) { if (typeof source[key] === 'object' && source[key] !== null) { if (!target[key]) { target[key] = {} as NestedObject; } deepMerge(target[key] as NestedObject, source[key] as NestedObject); } else { target[key] = source[key]; } } } return target; } ``` However, when I try to call this function, TypeScript throws a type error. For example: ```typescript const obj1 = { a: 1, b: { c: 3 } }; const obj2 = { a: 2, b: { d: 4 } }; const merged = deepMerge(obj1, obj2); ``` The error I'm getting is: `Argument of type '{ a: number; b: { c: number; }; }' is not assignable to parameter of type 'NestedObject'.` I understand that I need to refine the type definitions, but I'm unsure how to ensure that the merged object retains the proper nested structure without losing type safety. I'm currently using TypeScript version 4.5.2. Any guidance on how to type this recursive function correctly would be appreciated!