Allow JSON transforms

CHANGELOG.md

@@ -5,6 +5,7 @@
 ### Fixes
 
 - `[jest-snapshot]` Inline snapshots: do not indent empty lines ([#8277](https://github.com/facebook/jest/pull/8277))
+- `[@jest/runtime, @jest/transform]` Allow custom transforms for JSON dependencies ([#2578](https://github.com/facebook/jest/pull/2578))
 
 ### Chore & Maintenance
 

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 +1,14 @@
+/**
+ * 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 = source => {
+  const json = JSON.parse(source);
+  Object.keys(json).forEach(k => (json[k] = k));
+  return JSON.stringify(json);
+};

packages/jest-runtime/src/index.ts

@@ -465,8 +465,16 @@ class Runtime {
     moduleRegistry: ModuleRegistry,
   ) {
     if (path.extname(modulePath) === '.json') {
+      const text = stripBOM(fs.readFileSync(modulePath, 'utf8'));
+
+      const transformedFile = this._scriptTransformer.transformJson(
+        modulePath,
+        options,
+        text,
+      );
+
       localModule.exports = this._environment.global.JSON.parse(
-        stripBOM(fs.readFileSync(modulePath, 'utf8')),
+        transformedFile,
       );
     } else if (path.extname(modulePath) === '.node') {
       localModule.exports = require(modulePath);

packages/jest-transform/src/ScriptTransformer.ts

@@ -405,6 +405,28 @@ export default class ScriptTransformer {
     return result;
   }
 
+  transformJson(
+    filename: Config.Path,
+    options: Pick<Options, 'isInternalModule' | 'isCoreModule'> | undefined,
+    fileSource: string,
+  ): string {
+    const isInternalModule = !!(options && options.isInternalModule);
+    const isCoreModule = !!(options && options.isCoreModule);
+    const willTransform =
+      !isInternalModule && !isCoreModule && this.shouldTransform(filename);
+
+    if (willTransform) {
+      const {code: transformedJsonSource} = this.transformSource(
+        filename,
+        fileSource,
+        false,
+      );
+      return transformedJsonSource;
+    }
+
+    return fileSource;
+  }
+
   /**
    * @deprecated use `this.shouldTransform` instead
    */