feat: add files to reproduce ESM issue

related: kulshekhar/ts-jest#1709

.gitignore

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+node_modules

jest.config.js

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+export default {
+    // NOTE: This workaround seems to work, thanks to https://github.com/gadicc
+    // moduleNameMapper: {
+    //     '^(\\./.*)\\.js$': '$1',
+    // },
+    globals: {
+        'ts-jest': {
+            useESM: true,
+        },
+    },
+    testEnvironment: 'node',
+    transform: {'^.+\\.tsx?$': 'ts-jest'},
+    testMatch: ['<rootDir>/src/**/*.spec.ts'],
+    extensionsToTreatAsEsm: ['.ts'],
+    moduleFileExtensions: ['ts', 'js'],
+};

package.json

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+{
+    "name": "ts-jest-playground",
+    "version": "1.0.0",
+    "description": "",
+    "exports": "dist/a.js",
+    "type": "module",
+    "engines": {
+        "node": ">=12.20"
+    },
+    "scripts": {
+        "test": "jest --config=jest.config.js"
+    },
+    "author": "",
+    "license": "ISC",
+    "devDependencies": {
+        "@types/jest": "^26.0.23",
+        "jest": "^27.0.0-next.9",
+        "ts-jest": "^27.0.0-next.12",
+        "typescript": "^4.2.4"
+    }
+}

src/a.spec.ts

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+import * as a from './a.js';
+
+describe('a', () => {
+    test('foo', () => {
+        expect(a.foo()).toEqual(42);
+    })
+})

src/a.ts

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+import {bar} from './b.js';
+
+export const foo = bar;

src/b.ts

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+export const bar = () => 42;

tsconfig.json

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+{
+    "compilerOptions": {
+        "target": "esnext",
+        "module": "ES2020",
+        "moduleResolution": "node",
+        "esModuleInterop": true,
+        "noEmit": true
+    },
+    "include": [ "src" ]
+}