Fix Jest ESM compat with Babel

Author: rekmarks

babel.config.cjs

@@ -0,0 +1,10 @@
+// We use Babel to transpile down ESM dependencies to CommonJS for our tests
+// using babel-jest.
+module.exports = {
+  env: {
+    test: {
+      presets: ['@babel/preset-env', '@babel/preset-typescript'],
+      plugins: ['@babel/plugin-transform-modules-commonjs']
+    }
+  }
+};

jest.config.cjs

@@ -3,6 +3,32 @@
  * https://jestjs.io/docs/configuration
  */
 
+/**
+ * Dependencies that are ESM-only, and need to be transpiled by Babel.
+ * This list is used in the `transformIgnorePatterns` option below.
+ * 
+ * You probably need to add a dependency to this list if the tests fail with something like:
+ * - `SyntaxError: Cannot use import statement outside a module`
+ * - `SyntaxError: Unexpected token 'export'`
+ * If so, identify the dependency that's causing the error via the stack trace, and add it
+ * to this list.
+ * 
+ * No, we do not live in the best of all possible worlds. Why do you ask?
+ *
+ * For details on Jest's currently experimental ESM support see: https://github.com/jestjs/jest/issues/9430
+ */
+const ESM_DEPENDENCIES = [
+  'execa',
+  'strip-final-newline',
+  'npm-run-path',
+  'path-key',
+  'onetime',
+  'mimic-fn',
+  'human-signals',
+  'is-stream',
+  'get-stream',
+];
+
 // This file needs to be .cjs for compatibility with jest-it-up.
 module.exports = {
   // All imported modules in your tests should be mocked automatically
@@ -103,8 +129,9 @@ module.exports = {
   // An enum that specifies notification mode. Requires { notify: true }
   // notifyMode: "failure-change",
 
-  // A preset that is used as a base for Jest's configuration
-  preset: 'ts-jest',
+  // Disabled in favor of babel-jest, configured via "transform" below.
+  // // A preset that is used as a base for Jest's configuration
+  // preset: 'ts-jest',
 
   // Run tests from one or more projects
   // projects: undefined,
@@ -189,13 +216,12 @@ module.exports = {
   // timers: "real",
 
   // A map from regular expressions to paths to transformers
-  // transform: undefined,
+  transform: {
+    "\\.[jt]sx?$": "babel-jest"
+  },
 
   // An array of regexp pattern strings that are matched against all source file paths, matched files will skip transformation
-  // transformIgnorePatterns: [
-  //   "/node_modules/",
-  //   "\\.pnp\\.[^\\/]+$"
-  // ],
+  transformIgnorePatterns: [`node_modules/(?!(${ESM_DEPENDENCIES.join('|')}))`],
 
   // An array of regexp pattern strings that are matched against all modules before the module loader will automatically return a mock for them
   // unmockedModulePathPatterns: undefined,

package.json

@@ -36,6 +36,10 @@
     "yargs": "^17.7.1"
   },
   "devDependencies": {
+    "@babel/core": "^7.23.5",
+    "@babel/plugin-transform-modules-commonjs": "^7.23.3",
+    "@babel/preset-env": "^7.23.5",
+    "@babel/preset-typescript": "^7.23.3",
     "@lavamoat/allow-scripts": "^2.3.1",
     "@metamask/auto-changelog": "^3.0.0",
     "@metamask/eslint-config": "^10.0.0",
@@ -51,6 +55,7 @@
     "@types/yargs": "^17.0.10",
     "@typescript-eslint/eslint-plugin": "^5.62.0",
     "@typescript-eslint/parser": "^5.62.0",
+    "babel-jest": "^29.7.0",
     "deepmerge": "^4.2.2",
     "eslint": "^8.27.0",
     "eslint-config-prettier": "^8.5.0",

src/misc-utils.test.ts

@@ -137,7 +137,7 @@ describe('misc-utils', () => {
   describe('runCommand', () => {
     it('runs the command, discarding its output', async () => {
       const execaSpy = jest
-        .spyOn(execaModule, 'default')
+        .spyOn(execaModule, 'execa')
         // Typecast: It's difficult to provide a full return value for execa
         .mockResolvedValue({ stdout: '   some output  ' } as any);
 
@@ -155,7 +155,7 @@ describe('misc-utils', () => {
   describe('getStdoutFromCommand', () => {
     it('executes the given command and returns a version of the standard out from the command with whitespace trimmed', async () => {
       const execaSpy = jest
-        .spyOn(execaModule, 'default')
+        .spyOn(execaModule, 'execa')
         // Typecast: It's difficult to provide a full return value for execa
         .mockResolvedValue({ stdout: '   some output  ' } as any);
 
@@ -175,7 +175,7 @@ describe('misc-utils', () => {
   describe('getLinesFromCommand', () => {
     it('executes the given command and returns the standard out from the command split into lines', async () => {
       const execaSpy = jest
-        .spyOn(execaModule, 'default')
+        .spyOn(execaModule, 'execa')
         // Typecast: It's difficult to provide a full return value for execa
         .mockResolvedValue({ stdout: 'line 1\nline 2\nline 3' } as any);
 
@@ -193,7 +193,7 @@ describe('misc-utils', () => {
 
     it('does not strip leading and trailing whitespace from the output, but does remove empty lines', async () => {
       const execaSpy = jest
-        .spyOn(execaModule, 'default')
+        .spyOn(execaModule, 'execa')
         // Typecast: It's difficult to provide a full return value for execa
         .mockResolvedValue({
           stdout: '  line 1\nline 2\n\n   line 3   \n',

src/repo.ts

@@ -309,6 +309,7 @@ export async function hasChangesInDirectorySinceGitTag(
       tagName,
     );
 
+    /* istanbul ignore else */
     if (!(tagName in CHANGED_FILE_PATHS_BY_TAG_NAME)) {
       CHANGED_FILE_PATHS_BY_TAG_NAME[tagName] = changedFilePaths;
     }