👀 Views: 3 💬 Answers: 1 📅 Created: 2025-06-19

typescript type-safety utility-types TypeScript

I've been researching this but I've looked through the documentation and I'm still confused about I'm trying to create a utility function in TypeScript that performs a deep merge of two objects while ensuring type safety. My current attempt works for shallow merges, but I've run into issues when dealing with nested objects. The goal is to merge properties from the second object into the first, recursively, while preserving the types of the resulting merged object. Here's what I have so far: ```typescript type DeepMerge<T, U> = { [K in keyof T | keyof U]: K extends keyof U ? K extends keyof T ? DeepMerge<T[K], U[K]> : U[K] : T[K]; }; function deepMerge<T, U>(target: T, source: U): DeepMerge<T, U> { const output = {...target}; for (const key in source) { if (source.hasOwnProperty(key)) { if (typeof source[key] === 'object' && source[key] !== null && !Array.isArray(source[key])) { output[key] = deepMerge(target[key] as object, source[key]); } else { output[key] = source[key]; } } } return output; } ``` When I call this function with the following code: ```typescript const obj1 = { a: { b: 1 }, c: 2 }; const obj2 = { a: { b: 2, d: 3 }, e: 4 }; const merged = deepMerge(obj1, obj2); ``` I end up with `merged` as expected, but I lose type information for the nested properties. TypeScript treats the merged result as having a structure of `any`, and I get the following behavior: ``` Argument of type 'any' is not assignable to parameter of type 'object'. ``` I've tried adding additional type constraints, but I keep running into issues with inferred types when using nested objects. Is there a way to modify the `DeepMerge` type to ensure better type inference on the merged output? Or is there a better approach altogether for deep merging with strict type safety? Any insights would be greatly appreciated! This is part of a larger service I'm building. I'd really appreciate any guidance on this. Cheers for any assistance!