diff --git a/__tests__/inMemoryStorage.js b/__tests__/inMemoryStorage.js
index 1bedd39..c228573 100644
--- a/__tests__/inMemoryStorage.js
+++ b/__tests__/inMemoryStorage.js
@@ -38,6 +38,19 @@ describe('inMemoryStorage', () => {
     expect(inMemoryStorage.length).toBe(1)
   })
 
+  test('key', () => {
+    inMemoryStorage.setItem('key-1', 'value-1')
+
+    expect(() => {
+      inMemoryStorage.key()
+    }).toThrow(
+      "Failed to execute 'key' on 'Storage': 1 argument required, but only 0 present."
+    )
+
+    expect(inMemoryStorage.key(0)).toBe('key-1')
+    expect(inMemoryStorage.key(1)).toBe(null)
+  })
+
   test('clear', () => {
     inMemoryStorage.setItem('key-1', 'value-1')
     inMemoryStorage.setItem('key-2', 'value-2')
diff --git a/src/inMemoryStorage.js b/src/inMemoryStorage.js
index 771bce7..03e66cb 100644
--- a/src/inMemoryStorage.js
+++ b/src/inMemoryStorage.js
@@ -27,6 +27,22 @@ class InMemoryStorage {
     return null
   }
 
+  key(index) {
+    if (index === undefined) {
+      throw new TypeError(
+        "Failed to execute 'key' on 'Storage': 1 argument required, but only 0 present."
+      )
+    }
+
+    const key = Object.keys(this._data)[index]
+
+    if (key !== undefined) {
+      return key
+    }
+
+    return null
+  }
+
   clear() {
     this._data = {}
     this.length = 0