Add ESM support for @azure/functions-core virtual module

[TsuyoshiUshio]

Add ESM support for @azure/functions-core virtual module

Resolves #104

Summary

The @azure/functions-core virtual module is currently only accessible via CJS require() through a Module.prototype.require Proxy. This PR adds ESM import support using Node.js loader hooks (module.register()), enabling ESM-based code to use import { ... } from '@azure/functions-core' at runtime.

This is Step 1 of the full-stack ESM support plan for the Azure Functions Node.js ecosystem.

Changes

src/setupCoreModule.ts (modified)

• Expose coreApi on globalThis via Symbol.for('azure-functions-core') so the ESM loader can access it
• Register ESM loader hooks via module.register() (feature-gated for Node.js >= 20.6)
• Existing CJS require() Proxy is unchanged -- zero risk to CJS consumers

src/esm-core-loader.mjs (new)

• ESM loader hooks file registered by module.register()
• resolve(): Maps @azure/functions-core to a virtual URL
• load(): Returns synthetic ESM source that re-exports properties from the globalThis coreApi object

src/esmCoreLoaderHooks.ts (new)

• TypeScript version of the loader hooks for unit test coverage
• Same logic as esm-core-loader.mjs but importable by Mocha/ts-node

webpack.config.js (modified)

• Added .mjs handling rule so the ESM loader file is not broken by webpack

test/setupCoreModule.test.ts (new)

• 8 unit tests covering:
  • CJS Proxy interception (2 tests)
  • globalThis Symbol exposure (1 test)
  • ESM resolve hook -- virtual URL and passthrough (2 tests)
  • ESM load hook -- synthetic source, passthrough, and error case (3 tests)

docs/design-esm-core-module.md (new)

• Design document covering architecture, alternatives considered, and open questions

How it works

setupCoreModule()
  |-- 1. CJS Proxy (existing, unchanged)
  |     require('@azure/functions-core') --> coreApi
  |
  |-- 2. ESM Loader (new)
        globalThis[Symbol.for('azure-functions-core')] = coreApi
        module.register('./esm-core-loader.mjs')
          |-- resolve(): '@azure/functions-core' --> virtual URL
          |-- load(): virtual URL --> synthetic ESM with named exports

Node.js version compatibility

Node.js	module.register()	Behavior
18.x	Not available	Graceful fallback -- CJS only
20.6+	Available	Full ESM + CJS support
22.x	Available	Full ESM + CJS support

Testing

• New tests: 8/8 passing
• Full suite: 140 passing, 8 pending, 0 failing
• No regressions in existing tests

Related issues

• Support ESM module export for @azure/functions-extensions-servicebus azure-functions-nodejs-extensions#61 -- ESM module support
• ESM Support azure-functions-nodejs-extensions#90 -- Extensions ESM PR
• Provide package as ESM alongside CommonJS module azure-functions-nodejs-library#287 -- Library ESM support (P1)
• [Placeholder] Node ESM Story and Samples azure-functions-nodejs-library#400 -- Node ESM Story placeholder

[TsuyoshiUshio]

Created PR first, however, we need to discuss the order of the merging. :)