fix(transformer): transpile mjs files from node_modules for CJS mode

Author: Noyabronok

src/legacy/ts-jest-transformer.spec.ts

@@ -414,7 +414,7 @@ describe('TsJestTransformer', () => {
       },
       {
         filePath: 'my-project/node_modules/foo.mjs',
-        expectedResult: `export default foo;`,
+        expectedResult: `exports.default = foo;`,
       },
     ])('should transpile js file from node_modules for CJS', ({ filePath, expectedResult }) => {
       const result = tr.process(
@@ -432,6 +432,20 @@ describe('TsJestTransformer', () => {
       expect(omitLeadingWhitespace(result.code)).toContain(expectedResult)
     })
 
+    it('should transpile .mjs file with import statements from node_modules to CJS without SyntaxError', () => {
+      const result = tr.process(
+        `
+          import { LensList } from "./lens-list.mjs";
+          export default LensList;
+        `,
+        'my-project/node_modules/rettime/build/index.mjs',
+        baseTransformOptions,
+      )
+
+      expect(omitLeadingWhitespace(result.code)).toContain(`require("./lens-list.mjs")`)
+      expect(omitLeadingWhitespace(result.code)).not.toContain('import {')
+    })
+
     it('should transpile js file from node_modules for ESM', () => {
       const result = tr.process(
         `

src/legacy/ts-jest-transformer.ts

@@ -235,11 +235,15 @@ export class TsJestTransformer implements SyncTransformer<TsJestTransformerOptio
           compilerOptions: {
             ...configs.parsedTsConfig.options,
             module:
-              transformOptions.supportsStaticESM && transformOptions.transformerConfig.useESM
+              transformOptions.supportsStaticESM && transformOptions.transformerConfig?.useESM
                 ? ts.ModuleKind.ESNext
                 : ts.ModuleKind.CommonJS,
           },
-          fileName: sourcePath,
+          // .mjs fileName causes ts.transpileModule to preserve ESM syntax even with module: CommonJS
+          fileName:
+            transformOptions.supportsStaticESM && transformOptions.transformerConfig?.useESM
+              ? sourcePath
+              : sourcePath.replace(/\.mjs$/, '.js'),
         })
         result = {
           code: updateOutput(transpiledResult.outputText, sourcePath, transpiledResult.sourceMapText),

website/docs/guides/troubleshooting.md

@@ -67,20 +67,41 @@ SyntaxError: Cannot use import statement outside a module
 
 ### SOLUTION
 
-One of the node modules hasn't the correct syntax for Jests execution step. It needs to
-be transformed first.
+One of the node modules doesn't have the correct syntax for Jest's execution step. It needs to be transformed first.
 
-There is a good chance that the error message shows which module is affected:
+The error message usually shows which module is affected:
 
 ```shell
     SyntaxError: Cannot use import statement outside a module
     > 22 | import Component from "../../node_modules/some-module/lib";
          | ^
 ```
 
-In this case **some-module** is the problem and needs to be transformed.
-By adding the following line to the configuration file it will tell Jest which modules
-shouldnt be ignored during the transformation step:
+#### If the offending files are `.mjs` files
+
+Use when individual files have a `.mjs` extension. Add a dedicated transform rule — ts-jest will transpile all `.mjs` files in `node_modules` to CommonJS without needing to list packages individually:
+
+```ts title="jest.config.ts"
+import type { Config } from 'jest'
+import { createDefaultPreset } from 'ts-jest'
+
+const presetConfig = createDefaultPreset()
+
+const config: Config = {
+  ...presetConfig,
+  transformIgnorePatterns: ['node_modules/(?!.*\\.mjs$)'],
+  transform: {
+    ...presetConfig.transform,
+    '^.+/node_modules/.+\\.mjs$': ['ts-jest', {}],
+  },
+}
+
+export default config
+```
+
+#### If the offending package uses `"type": "module"` in its `package.json`
+
+Use when a package declares `"type": "module"`, causing its `.js` files to be treated as ESM. Name each affected package explicitly in `transformIgnorePatterns`:
 
 ```ts title="jest.config.ts"
 import type { Config } from 'jest'