feat(build): add a helper to merge mocha config objects

Signed-off-by: Miroslav Bajtoš <mbajtoss@gmail.com>

packages/build/index.js

@@ -20,3 +20,5 @@ exports.typeScriptPath = path.resolve(
   require.resolve('typescript/package.json'),
   '..',
 );
+
+exports.mergeMochaConfigs = require('./src/merge-mocha-configs');

packages/build/package.json

@@ -26,6 +26,7 @@
     "eslint": "^7.3.1",
     "fs-extra": "^9.0.1",
     "glob": "^7.1.6",
+    "lodash": "^4.17.15",
     "mocha": "^8.0.1",
     "nyc": "^15.1.0",
     "prettier": "^2.0.5",

packages/build/src/merge-mocha-configs.js

@@ -0,0 +1,39 @@
+// Copyright IBM Corp. 2017,2020. All Rights Reserved.
+// Node module: @loopback/build
+// This file is licensed under the MIT License.
+// License text available at https://opensource.org/licenses/MIT
+
+'use strict';
+
+const debug = require('debug')('loopback:build:merge-mocha-configs');
+const {assignWith} = require('lodash');
+
+module.exports = mergeMochaConfigs;
+
+/**
+ * Merge multiple Mocha configuration files into a single one.
+ *
+ * @param {MochaConfig[]} configs A list of Mocha configuration objects
+ * as provided by `.mocharc.js` files.
+ */
+function mergeMochaConfigs(...configs) {
+  debug('Merging mocha configurations', ...configs);
+  const result = assignWith({}, ...configs, assignMochaConfigEntry);
+  debug('Merged config:', result);
+  return result;
+}
+
+function assignMochaConfigEntry(targetValue, sourceValue, key) {
+  switch (key) {
+    case 'timeout':
+      return Math.max(targetValue || 0, sourceValue);
+    case 'require':
+      if (Array.isArray(sourceValue)) {
+        debug('Adding an array of files to require:', sourceValue);
+        return [...(targetValue || []), ...sourceValue];
+      } else {
+        debug('Adding a single file to require:', sourceValue);
+        return [...(targetValue || []), sourceValue];
+      }
+  }
+}