diff --git a/src/__tests__/keyOf.spec.ts b/src/__tests__/keyOf.spec.ts
new file mode 100644
index 00000000..e69645fd
--- /dev/null
+++ b/src/__tests__/keyOf.spec.ts
@@ -0,0 +1,35 @@
+import keyOf from '../keyOf';
+
+describe('keyOf', () => {
+  test('should return the key of the first value matched', () => {
+    const obj = {
+      a: 1,
+      b: 2,
+      c: 3,
+    };
+
+    expect(keyOf(obj, 1)).toBe('a');
+    expect(keyOf(obj, 2)).toBe('b');
+    expect(keyOf(obj, 3)).toBe('c');
+  });
+
+  test('should return undefined if the value is not matched', () => {
+    const obj = {
+      a: 1,
+    };
+
+    expect(keyOf(obj, 2)).toBeUndefined();
+  });
+
+  test('should throw TypeError for invalid arguments', () => {
+    const invalidObj: unknown = 'testing';
+    const invalidFollow: unknown = 'testing';
+
+    expect(() => keyOf(invalidObj as OObject, 'test'))
+      .toThrow(new TypeError('Expected Object, got string testing'));
+    expect(() => keyOf({}, 'test', {
+      follow: invalidFollow as boolean,
+    }))
+      .toThrow(new TypeError('Expected Boolean, got string testing'));
+  });
+});