diff --git a/db.ts b/db.ts
index a9613d4e6..9dfae1496 100644
--- a/db.ts
+++ b/db.ts
@@ -1,51 +1,83 @@
-export const messages = [
-  {
-    id: '1',
-    content: 'You on your way?',
-    createdAt: new Date(new Date('1-1-2019').getTime() - 60 * 1000 * 1000),
-  },
-  {
-    id: '2',
-    content: "Hey, it's me",
-    createdAt: new Date(new Date('1-1-2019').getTime() - 2 * 60 * 1000 * 1000),
-  },
-  {
-    id: '3',
-    content: 'I should buy a boat',
-    createdAt: new Date(new Date('1-1-2019').getTime() - 24 * 60 * 1000 * 1000),
-  },
-  {
-    id: '4',
-    content: 'This is wicked good ice cream.',
-    createdAt: new Date(
-      new Date('1-1-2019').getTime() - 14 * 24 * 60 * 1000 * 1000
-    ),
-  },
-];
+export type Message = {
+  id: string;
+  content: string;
+  createdAt: Date;
+};
 
-export const chats = [
-  {
-    id: '1',
-    name: 'Ethan Gonzalez',
-    picture: 'https://randomuser.me/api/portraits/thumb/men/1.jpg',
-    messages: ['1'],
-  },
-  {
-    id: '2',
-    name: 'Bryan Wallace',
-    picture: 'https://randomuser.me/api/portraits/thumb/men/2.jpg',
-    messages: ['2'],
-  },
-  {
-    id: '3',
-    name: 'Avery Stewart',
-    picture: 'https://randomuser.me/api/portraits/thumb/women/1.jpg',
-    messages: ['3'],
-  },
-  {
-    id: '4',
-    name: 'Katie Peterson',
-    picture: 'https://randomuser.me/api/portraits/thumb/women/2.jpg',
-    messages: ['4'],
-  },
-];
+export type Chat = {
+  id: string;
+  name: string;
+  picture: string;
+  messages: string[];
+};
+
+export const messages: Message[] = [];
+export const chats: Chat[] = [];
+
+export const resetDb = () => {
+  messages.splice(
+    0,
+    Infinity,
+    ...[
+      {
+        id: '1',
+        content: 'You on your way?',
+        createdAt: new Date(new Date('1-1-2019').getTime() - 60 * 1000 * 1000),
+      },
+      {
+        id: '2',
+        content: "Hey, it's me",
+        createdAt: new Date(
+          new Date('1-1-2019').getTime() - 2 * 60 * 1000 * 1000
+        ),
+      },
+      {
+        id: '3',
+        content: 'I should buy a boat',
+        createdAt: new Date(
+          new Date('1-1-2019').getTime() - 24 * 60 * 1000 * 1000
+        ),
+      },
+      {
+        id: '4',
+        content: 'This is wicked good ice cream.',
+        createdAt: new Date(
+          new Date('1-1-2019').getTime() - 14 * 24 * 60 * 1000 * 1000
+        ),
+      },
+    ]
+  );
+
+  chats.splice(
+    0,
+    Infinity,
+    ...[
+      {
+        id: '1',
+        name: 'Ethan Gonzalez',
+        picture: 'https://randomuser.me/api/portraits/thumb/men/1.jpg',
+        messages: ['1'],
+      },
+      {
+        id: '2',
+        name: 'Bryan Wallace',
+        picture: 'https://randomuser.me/api/portraits/thumb/men/2.jpg',
+        messages: ['2'],
+      },
+      {
+        id: '3',
+        name: 'Avery Stewart',
+        picture: 'https://randomuser.me/api/portraits/thumb/women/1.jpg',
+        messages: ['3'],
+      },
+      {
+        id: '4',
+        name: 'Katie Peterson',
+        picture: 'https://randomuser.me/api/portraits/thumb/women/2.jpg',
+        messages: ['4'],
+      },
+    ]
+  );
+};
+
+resetDb();
diff --git a/schema/resolvers.ts b/schema/resolvers.ts
index a02e87f63..836424a3f 100644
--- a/schema/resolvers.ts
+++ b/schema/resolvers.ts
@@ -34,7 +34,7 @@ const resolvers = {
       if (chatIndex === -1) return null;
 
       const chat = chats[chatIndex];
-      
+
       const messagesIds = messages.map(currentMessage => Number(currentMessage.id));
       const messageId = String(Math.max(...messagesIds) + 1);
       const message = {
diff --git a/tests/mutations/__snapshots__/addMessage.test.ts.snap b/tests/mutations/__snapshots__/addMessage.test.ts.snap
new file mode 100644
index 000000000..0e883493f
--- /dev/null
+++ b/tests/mutations/__snapshots__/addMessage.test.ts.snap
@@ -0,0 +1,22 @@
+// Jest Snapshot v1, https://goo.gl/fbAQLP
+
+exports[`Mutation.addMessage should add message to specified chat 1`] = `
+Object {
+  "addMessage": Object {
+    "content": "Hello World",
+    "id": "5",
+  },
+}
+`;
+
+exports[`Mutation.addMessage should add message to specified chat 2`] = `
+Object {
+  "chat": Object {
+    "id": "1",
+    "lastMessage": Object {
+      "content": "Hello World",
+      "id": "5",
+    },
+  },
+}
+`;
diff --git a/tests/mutations/addMessage.test.ts b/tests/mutations/addMessage.test.ts
new file mode 100644
index 000000000..c2465d402
--- /dev/null
+++ b/tests/mutations/addMessage.test.ts
@@ -0,0 +1,49 @@
+import { createTestClient } from 'apollo-server-testing';
+import { ApolloServer, gql } from 'apollo-server-express';
+import schema from '../../schema';
+import { resetDb } from '../../db';
+
+describe('Mutation.addMessage', () => {
+  beforeEach(resetDb);
+
+  it('should add message to specified chat', async () => {
+    const server = new ApolloServer({ schema });
+
+    const { query, mutate } = createTestClient(server);
+
+    const addMessageRes = await mutate({
+      variables: { chatId: '1', content: 'Hello World' },
+      mutation: gql`
+        mutation AddMessage($chatId: ID!, $content: String!) {
+          addMessage(chatId: $chatId, content: $content) {
+            id
+            content
+          }
+        }
+      `,
+    });
+
+    expect(addMessageRes.data).toBeDefined();
+    expect(addMessageRes.errors).toBeUndefined();
+    expect(addMessageRes.data).toMatchSnapshot();
+
+    const getChatRes = await query({
+      variables: { chatId: '1' },
+      query: gql`
+        query GetChat($chatId: ID!) {
+          chat(chatId: $chatId) {
+            id
+            lastMessage {
+              id
+              content
+            }
+          }
+        }
+      `,
+    });
+
+    expect(getChatRes.data).toBeDefined();
+    expect(getChatRes.errors).toBeUndefined();
+    expect(getChatRes.data).toMatchSnapshot();
+  });
+});