Allow JSON transforms

CHANGELOG.md

@@ -4,6 +4,8 @@
 
 ### Fixes
 
+- `[@jest/runtime]` Allow custom transforms for JSON dependencies
+
 ### Chore & Maintenance
 
 ### Performance

packages/jest-runtime/src/__tests__/runtime_internal_module.test.js

@@ -44,5 +44,29 @@ describe('Runtime', () => {
         const exports = runtime.requireInternalModule(modulePath);
         expect(exports()).toBe('internal-module-data');
       }));
+
+    it('loads JSON modules and applies transforms', () =>
+      createRuntime(__filename, {
+        transform: {'^.+\\.json$': './test_json_preprocessor'},
+      }).then(runtime => {
+        const modulePath = path.resolve(
+          path.dirname(runtime.__mockRootPath),
+          'internal-root.json',
+        );
+        const exports = runtime.requireModule(modulePath);
+        expect(exports).toEqual({foo: 'foo'});
+      }));
+
+    it('loads internal JSON modules without applying transforms', () =>
+      createRuntime(__filename, {
+        transform: {'^.+\\.json$': './test_json_preprocessor'},
+      }).then(runtime => {
+        const modulePath = path.resolve(
+          path.dirname(runtime.__mockRootPath),
+          'internal-root.json',
+        );
+        const exports = runtime.requireInternalModule(modulePath);
+        expect(exports).toEqual({foo: 'bar'});
+      }));
   });
 });

packages/jest-runtime/src/__tests__/test_root/internal-root.json

@@ -0,0 +1,3 @@
+{
+  "foo": "bar"
+}

packages/jest-runtime/src/__tests__/test_root/test_json_preprocessor.js

@@ -0,0 +1,10 @@
+/**
+ * Copyright (c) Facebook, Inc. and its affiliates. All Rights Reserved.
+ *
+ * This source code is licensed under the MIT license found in the
+ * LICENSE file in the root directory of this source tree.
+ */
+
+'use strict';
+
+module.exports.process = () => `{"foo": "foo"}`;

packages/jest-runtime/src/index.ts

@@ -27,7 +27,6 @@ import {
   shouldInstrument,
 } from '@jest/transform';
 import fs from 'graceful-fs';
-import stripBOM from 'strip-bom';
 import {run as cliRun} from './cli';
 import {options as cliOptions} from './cli/args';
 import {findSiblingsWithFileExtension} from './helpers';
@@ -464,11 +463,7 @@ class Runtime {
     options: InternalModuleOptions | undefined,
     moduleRegistry: ModuleRegistry,
   ) {
-    if (path.extname(modulePath) === '.json') {
-      localModule.exports = this._environment.global.JSON.parse(
-        stripBOM(fs.readFileSync(modulePath, 'utf8')),
-      );
-    } else if (path.extname(modulePath) === '.node') {
+    if (path.extname(modulePath) === '.node') {
       localModule.exports = require(modulePath);
     } else {
       // Only include the fromPath if a moduleName is given. Else treat as root.

packages/jest-transform/src/ScriptTransformer.ts

@@ -331,6 +331,8 @@ export default class ScriptTransformer {
       !isInternalModule &&
       !isCoreModule &&
       (this.shouldTransform(filename) || instrument);
+    const isJson = path.extname(filename) === '.json';
+    const prefix = isJson ? 'module.exports = ' : '';
 
     try {
       const extraGlobals = (options && options.extraGlobals) || [];
@@ -342,11 +344,14 @@ export default class ScriptTransformer {
           instrument,
         );
 
-        wrappedCode = wrap(transformedSource.code, ...extraGlobals);
+        wrappedCode = wrap(
+          `${prefix}${transformedSource.code}`,
+          ...extraGlobals,
+        );
         sourceMapPath = transformedSource.sourceMapPath;
         mapCoverage = transformedSource.mapCoverage;
       } else {
-        wrappedCode = wrap(content, ...extraGlobals);
+        wrappedCode = wrap(`${prefix}${content}`, ...extraGlobals);
       }
 
       return {