[Typescript] Create a Type-Safe Request Handler with Zod and Express

import express, { RequestHandler } from 'express';
import { it } from 'vitest';
import { z, ZodError } from 'zod';
import { Equal, Expect } from '../helpers/type-utils';
import { ParsedQs } from 'qs';

const makeTypeSafeHandler = <
  TQuery extends ParsedQs = any,
  TBody extends Record<string, any> = any
>(
  config: {
    query?: z.Schema<TQuery>;
    body?: z.Schema<TBody>;
  },
  handler: RequestHandler<any, any, TBody, TQuery>
): RequestHandler<any, any, TBody, TQuery> => {
  return (req, res, next) => {
    const { query, body } = req;
    if (config.query) {
      try {
        config.query.parse(query);
      } catch (e) {
        return res.status(400).send((e as ZodError).message);
      }
    }
    if (config.body) {
      try {
        config.body.parse(body);
      } catch (e) {
        return res.status(400).send((e as ZodError).message);
      }
    }
    return handler(req, res, next);
  };
};

const app = express();

it('Should make the query AND body type safe', () => {
  app.get(
    '/users',
    makeTypeSafeHandler(
      {
        query: z.object({
          id: z.string(),
        }),
        body: z.object({
          name: z.string(),
        }),
      },
      (req, res) => {
        type tests = [
          Expect<Equal<typeof req.query, { id: string }>>,
          Expect<Equal<typeof req.body, { name: string }>>
        ];
      }
    )
  );
});

it('Should default them to any if not passed in config', () => {
  app.get(
    '/users',
    makeTypeSafeHandler({}, (req, res) => {
      type tests = [
        Expect<Equal<typeof req.query, any>>,
        Expect<Equal<typeof req.body, any>>
      ];
    })
  );
});