TypeScript Inference Issue with Mapped Types and Conditional Properties in an Express Middleware
I'm currently working on an Express.js middleware in TypeScript that processes incoming requests. I want to create a mapped type that transforms the request object based on the presence of certain query parameters. The challenge I'm facing is that TypeScript isn't inferring the types correctly when I apply conditional properties based on these query parameters. Here’s a simplified version of what I've tried: ```typescript import { Request, Response, NextFunction } from 'express'; type QueryParams = { filter?: string; limit?: string; }; type TransformedRequest<T> = T extends { filter: infer F } ? { filter: F; } : {}; type CustomRequest = Request<{}, {}, {}, QueryParams> & TransformedRequest<QueryParams>; const queryMiddleware = (req: CustomRequest, res: Response, next: NextFunction) => { const { filter, limit } = req.query; if (filter) { // Here, TypeScript should know filter is a string, but it throws an error. console.log(`Filtering by: ${filter}`); } if (limit) { // This also throws an error because TypeScript sees limit as possibly undefined. console.log(`Limit set to: ${limit}`); } next(); }; ``` When I attempt to access `filter` and `limit`, I get the error: `Property 'filter' does not exist on type 'CustomRequest'.` It seems like TypeScript isn't recognizing the conditional types correctly. I've tried using `as` assertions in various places to force TypeScript to understand the shape of my request object, but that hasn't resolved the issue either. I also experimented with using `Partial` and other utility types, but nothing seems to work as expected. Is there a better way to achieve the desired type inference for my transformed request without running into these type errors? Any insights or alternative approaches would be greatly appreciated!