feat: Add merge utility

README.md

@@ -40,14 +40,14 @@ In your `jest.config.json`:
 
 ### Advanced usage
 
-If you are already have another preset in your configuration, you can [merge them together](https://stackoverflow.com/questions/51002460/is-it-possible-to-use-jest-with-multiple-presets-at-the-same-time):
+If you already have another preset in your configuration, you can use `@cloudscape-design/jest-preset/merge` helper to merge them together:
 
 ```js
-const merge = require('merge');
+const mergePresets = require('@cloudscape-design/jest-preset/merge');
 const tsPreset = require('ts-jest/jest-preset');
 const cloudscapePreset = require('@cloudscape-design/jest-preset');
 
-module.exports = merge.recursive(tsPreset, cloudscapePreset, {
+module.exports = mergePresets(tsPreset, cloudscapePreset, {
   // ... your config
 });
 ```

merge.js

@@ -0,0 +1,13 @@
+// Copyright Amazon.com, Inc. or its affiliates. All Rights Reserved.
+// SPDX-License-Identifier: Apache-2.0
+const { mergeWith } = require('lodash');
+
+function mergeArrays(dest, src) {
+  if (Array.isArray(dest)) {
+    return dest.concat(src);
+  }
+}
+
+module.exports = function mergePresets(...presets) {
+  return mergeWith({}, ...presets, mergeArrays);
+};

package.json

@@ -23,7 +23,8 @@
     "jest": ">=24"
   },
   "dependencies": {
-    "@babel/plugin-transform-modules-commonjs": "^7.9.0"
+    "@babel/plugin-transform-modules-commonjs": "^7.9.0",
+    "lodash": "^4.17.21"
   },
   "devDependencies": {
     "eslint": "^8.56.0",

test/merge.test.js

@@ -0,0 +1,79 @@
+// Copyright Amazon.com, Inc. or its affiliates. All Rights Reserved.
+// SPDX-License-Identifier: Apache-2.0
+const tap = require('tap');
+const { strict: assert } = require('assert');
+const merge = require('../merge');
+
+tap.test('merging simple configurations', t => {
+  assert.deepEqual(
+    merge(
+      {
+        transformIgnorePatterns: ['project-ignore'],
+        transform: {
+          'project-js': 'project-babel-jest',
+        },
+      },
+      {
+        transformIgnorePatterns: ['our-ignore'],
+        transform: {
+          'cloudscape-js': 'our-babel-jest',
+        },
+      },
+    ),
+    {
+      transform: {
+        'project-js': 'project-babel-jest',
+        'cloudscape-js': 'our-babel-jest',
+      },
+      transformIgnorePatterns: ['project-ignore', 'our-ignore'],
+    },
+  );
+  t.end();
+});
+
+tap.test('merging multiple configurations', t => {
+  assert.deepEqual(
+    merge(
+      {
+        transform: { 'project-js': 'project-babel-jest' },
+      },
+      {
+        transform: { 'project-ts': 'ts-transformer' },
+      },
+
+      {
+        transform: { 'cloudscape-js': 'our-babel-jest' },
+      },
+    ),
+    {
+      transform: {
+        'project-js': 'project-babel-jest',
+        'project-ts': 'ts-transformer',
+        'cloudscape-js': 'our-babel-jest',
+      },
+    },
+  );
+  t.end();
+});
+
+tap.test('does not mutate objects', t => {
+  const src = {
+    transform: { 'project-js': 'project-babel-jest' },
+  };
+  const dest = {
+    transform: { 'cloudscape-js': 'our-babel-jest' },
+  };
+  assert.deepEqual(merge(src, dest), {
+    transform: {
+      'project-js': 'project-babel-jest',
+      'cloudscape-js': 'our-babel-jest',
+    },
+  });
+  assert.deepEqual(src, {
+    transform: { 'project-js': 'project-babel-jest' },
+  });
+  assert.deepEqual(dest, {
+    transform: { 'cloudscape-js': 'our-babel-jest' },
+  });
+  t.end();
+});