add unit test

package.json

@@ -12,7 +12,8 @@
     "start": "react-scripts start",
     "build": "react-scripts build",
     "test": "react-scripts test",
-    "eject": "react-scripts eject"
+    "eject": "react-scripts eject",
+    "test:coverage": "npm run test -- --coverage"
   },
   "eslintConfig": {
     "extends": "react-app"

src/lib.js

@@ -0,0 +1,26 @@
+export const joinString = (delimiter, ...params) => {
+  const results = [];
+  for (let param of params) {
+    if (!param) {
+      continue;
+    }
+
+    const paramType = typeof param;
+
+    if (paramType == 'string' || paramType == 'number') {
+      results.push(param);
+      continue;
+    }
+
+    if (Array.isArray(param) && param.length > 0) {
+      const innerResult = joinString(delimiter, ...param);
+      if (innerResult) {
+        results.push(innerResult);
+      }
+    }
+  }
+
+  return results.join(delimiter);
+};
+
+export const classNames = joinString.bind(null, ' ');

src/lib.test.js

@@ -0,0 +1,9 @@
+import { classNames } from './lib';
+
+test('classNames', () => {
+  expect(classNames('btn', 'btn--default')).toBe('btn btn--default');
+  expect(
+    classNames('btn', true && 'btn--default', false && 'btn--raised', null)
+  ).toBe('btn btn--default');
+  expect(classNames(['btn', null, 'btn--default'])).toBe('btn btn--default');
+});