diff --git a/tests/lib/cache-helper.js b/tests/lib/cache-helper.js
new file mode 100644
index 0000000..5a323a8
--- /dev/null
+++ b/tests/lib/cache-helper.js
@@ -0,0 +1,57 @@
+/**
+ * Copyright 2022 Google LLC
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *      http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+import test from 'ava';
+import { readFromCache, writeToCache } from '../../tools/lib/cache-helper.js';
+
+const filename = 'file.txt';
+const message = 'abc';
+
+test('readFromCache', (t) => {
+  let rfc;
+
+  // Returns message if file is found
+  writeToCache(filename, message);
+  rfc = readFromCache(filename);
+  t.deepEqual(rfc, Buffer.from(message));
+
+  // Returns null if file has expired
+  writeToCache(filename, message);
+  rfc = readFromCache(filename, -100);
+  t.is(rfc, null);
+
+  // Returns null if file is not found
+  rfc = readFromCache(String(Math.random()));
+  t.is(rfc, null);
+
+  // Returns null if no file name is provided
+  rfc = readFromCache('');
+  t.is(rfc, null);
+});
+
+test('writeToCache', (t) => {
+  // `writeToCache` doesn't throw an error with valid arguments
+  t.notThrows(() => {
+    writeToCache(filename, message);
+    writeToCache(filename, Buffer.from(message));
+  });
+
+  // `writeToCache` throws an error with invalid arguments
+  t.throws(() => {
+    //@ts-ignore
+    writeToCache(filename, new Promise());
+  });
+});
diff --git a/tools/lib/cache-helper.js b/tools/lib/cache-helper.js
index 73cc68d..dcff20c 100644
--- a/tools/lib/cache-helper.js
+++ b/tools/lib/cache-helper.js
@@ -30,7 +30,7 @@ export const { pathname: cacheDir } = new URL('../../.cache', import.meta.url);
 
 /**
  * @param {string} name to fetch
- * @param {number=} expiry default 48hr
+ * @param {number} [expiry] default 48hr
  */
 export function readFromCache(name, expiry = 48 * hourMs) {
   const p = path.join(cacheDir, name);