TypeScript Re-export Utilities

Imagine you're building a large TypeScript library or application. You've organized your code into multiple modules, and you want to provide a clean, unified entry point for consumers of your library. This often involves re-exporting various components, types, and functions from internal modules to a single public interface. This challenge will guide you through creating flexible utilities to streamline this re-exporting process.

Problem Description

Your task is to create a set of TypeScript utilities that simplify the process of re-exporting multiple entities from one or more modules into a new module. These utilities should be generic and type-safe, allowing you to specify exactly which entities you want to re-export and how they should be exposed.

Key Requirements:

1. reExportAll Utility:

   • This function should take an object representing the module(s) you want to re-export from.
   • It should return a new object where all exported members of the source module(s) are re-exported.
   • The utility must preserve the original types of the re-exported members.

2. reExportSpecific Utility:

   • This function should take the source module object and a list of specific keys (names of members) to re-export.
   • It should return a new object containing only the specified members.
   • The utility must preserve the original types of the re-exported members.

3. Handling Multiple Sources:

   • Both utilities should be able to accept an array of source module objects, merging their exports into the new module.
   • In case of key collisions (the same export name exists in multiple source modules), the later module in the array should take precedence.

4. Type Safety:

   • The utilities should leverage TypeScript's type system to ensure that the returned object accurately reflects the types of the re-exported members.
   • When re-exporting, the types should not be lost or incorrectly inferred.

Expected Behavior:

When you use these utilities, the resulting exported object should behave identically to manually re-exporting each item. Consumers of your public API should be able to import entities from your main module with their correct types, regardless of which internal module they originate from.

Edge Cases:

• Empty Source Modules: What happens if a source module has no exports?
• No Exports Specified for reExportSpecific: What happens if an empty array of keys is provided to reExportSpecific?
• Key Collisions: How are conflicting export names handled when merging multiple sources?

Examples

Example 1: Re-exporting all from a single module

// --- Internal Module A ---
export const greet = (name: string): string => `Hello, ${name}!`;
export type GreetingFn = (name: string) => string;
export const PI = 3.14159;

// --- Main Entry Point (using reExportAll) ---
import * as ModuleA from './moduleA'; // Assume ModuleA contains the exports above

// Let's simulate the re-export
const reExportedModule = reExportAll([ModuleA]);

// Usage:
console.log(reExportedModule.greet('World')); // Output: Hello, World!
const myGreeting: typeof reExportedModule.GreetingFn = reExportedModule.greet; // Type should be GreetingFn
console.log(reExportedModule.PI); // Output: 3.14159

Output:

The reExportedModule object will contain greet, GreetingFn, and PI with their original types.

Explanation:

reExportAll takes ModuleA and creates reExportedModule that exposes all its members.

Example 2: Re-exporting specific members from a single module

// --- Internal Module B ---
export const add = (a: number, b: number): number => a + b;
export const subtract = (a: number, b: number): number => a - b;
export const multiply = (a: number, b: number): number => a * b;
export const divide = (a: number, b: number): number => a / b;

// --- Main Entry Point (using reExportSpecific) ---
import * as ModuleB from './moduleB';

// Let's simulate the re-export
const reExportedMath = reExportSpecific(ModuleB, ['add', 'multiply']);

// Usage:
console.log(reExportedMath.add(5, 3)); // Output: 8
console.log(reExportedMath.multiply(4, 6)); // Output: 24
// reExportedMath.subtract(10, 2); // This would be a TypeScript error!

Output:

The reExportedMath object will contain add and multiply with their original types.

Explanation:

reExportSpecific takes ModuleB and a list of keys, returning an object with only add and multiply.

Example 3: Merging exports from multiple modules with a collision

// --- Internal Module C ---
export const config = { theme: 'dark', version: '1.0.0' };
export const logMessage = (msg: string) => console.log(`[C] ${msg}`);

// --- Internal Module D ---
export const config = { timeout: 5000, retries: 3 }; // Different 'config'
export const logMessage = (msg: string) => console.warn(`[D] ${msg}`); // Different 'logMessage'

// --- Main Entry Point ---
import * as ModuleC from './moduleC';
import * as ModuleD from './moduleD';

// Let's simulate the re-export of all, merging C then D
const combinedExports = reExportAll([ModuleC, ModuleD]);

// Usage:
console.log(combinedExports.config); // Output: { timeout: 5000, retries: 3 }
combinedExports.logMessage('Something happened'); // Output: [D] Something happened
// combinedExports.version; // This would be a TypeScript error if ModuleC.version didn't exist

// Let's simulate re-exporting specific and merging
const specificCombined = reExportSpecific([ModuleC, ModuleD], ['config', 'logMessage']);
console.log(specificCombined.config); // Output: { timeout: 5000, retries: 3 }
specificCombined.logMessage('Another event'); // Output: [D] Another event

Output:

combinedExports and specificCombined will both have a config property from ModuleD and a logMessage function from ModuleD, as ModuleD was the last one provided in the array.

Explanation:

When merging, the exports from later modules in the array overwrite those from earlier modules if the keys are the same.

Constraints

• The utilities must be implemented in TypeScript.
• The generated utilities should be as performant as possible, avoiding unnecessary overhead.
• The solution should not rely on external libraries for the core re-exporting logic.
• The types of exported members must be accurately preserved.

Notes

• Consider how you will define the return type of your utility functions to ensure maximum type safety. Generics will be your friend here.
• Think about the structure of the input modules. They are likely to be imported using import * as ModuleName from './path/to/module'.
• For reExportAll, you'll need a way to infer all exported keys from a module object.
• For reExportSpecific, you'll need to filter the keys based on the provided list.
• When merging multiple sources, pay close attention to the order in which sources are processed to handle collisions correctly.