Output a separate "legacy ESM" artifact for Webpack 4

Author: markerikson

.github/workflows/test.yml

@@ -145,7 +145,7 @@ jobs:
         run: yarn test
         if: matrix.example != 'are-the-types-wrong'
 
-      - name: Run test step
+      - name: Run test step (attw)
         working-directory: ./redux-toolkit/examples/publish-ci/${{ matrix.example }}
         # Ignore "FalseCJS" errors in the `attw` job
         run: yarn test -n FalseCJS

package.json

@@ -15,7 +15,7 @@
   ],
   "author": "Dan Abramov <dan.abramov@me.com>",
   "main": "dist/cjs/redux-thunk.cjs",
-  "module": "dist/redux-thunk.mjs",
+  "module": "dist/redux-thunk.legacy-esm.js",
   "types": "dist/redux-thunk.d.ts",
   "exports": {
     "./package.json": "./package.json",

tsup.config.ts

@@ -1,4 +1,5 @@
 import { defineConfig, Options } from 'tsup'
+import fs from 'fs'
 
 export default defineConfig(options => {
   const commonOptions: Partial<Options> = {
@@ -14,7 +15,14 @@ export default defineConfig(options => {
       format: ['esm'],
       outExtension: () => ({ js: '.mjs' }),
       dts: true,
-      clean: true
+      clean: true,
+      onSuccess() {
+        // Support Webpack 4 by pointing `"module"` to a file with a `.js` extension
+        fs.copyFileSync(
+          'dist/redux-thunk.mjs',
+          'dist/redux-thunk.legacy-esm.js'
+        )
+      }
     },
     {
       ...commonOptions,