diff --git a/index.html b/index.html
index 0a32121..8729d1b 100644
--- a/index.html
+++ b/index.html
@@ -81,7 +81,7 @@
           </a>
         </li>
         <li class="Tile">
-          <a target="_blank" href="/step2-03/" class="Tile-link">
+          <a target="_blank" href="/step2-03/demo/" class="Tile-link">
             Step 3<br />
             UI Fabric: Theming and Styling
           </a>
diff --git a/step2-04/README.md b/step2-04/README.md
new file mode 100644
index 0000000..b7197b4
--- /dev/null
+++ b/step2-04/README.md
@@ -0,0 +1,11 @@
+# Step 2.4
+
+Testing Typescript code with jest.
+
+# Exercise
+
+1. copy the generic `Stack<T>` code you have developed in Step 2.1
+
+2. import the `Stack<T>` class into `stack.spec.ts`
+
+3. Follow the instructions inside the file to complete the two tests
diff --git a/step2-04/index.html b/step2-04/demo/index.html
similarity index 100%
rename from step2-04/index.html
rename to step2-04/demo/index.html
diff --git a/step2-04/demo/src/index.spec.ts b/step2-04/demo/src/index.spec.ts
new file mode 100644
index 0000000..87620da
--- /dev/null
+++ b/step2-04/demo/src/index.spec.ts
@@ -0,0 +1,32 @@
+import { square } from '.';
+
+describe('jest example', () => {
+  beforeEach(() => {
+    jest.resetModules();
+  });
+
+  it('should be able to give the square of two numbers', () => {
+    console.log('test');
+    expect(square(5)).toBe(25);
+  });
+
+  it('should increment counter', () => {
+    const { increment } = require('.');
+    expect(increment()).toBe(1);
+  });
+
+  it('should decrement counter', () => {
+    const { decrement } = require('.');
+    expect(decrement()).toBe(-1);
+  });
+
+  it('should retrieve count', () => {
+    const { decrement, getCount, increment } = require('.');
+    increment();
+    increment();
+    decrement();
+    increment();
+
+    expect(getCount()).toBe(2);
+  });
+});
diff --git a/step2-04/demo/src/index.ts b/step2-04/demo/src/index.ts
new file mode 100644
index 0000000..0ac7fff
--- /dev/null
+++ b/step2-04/demo/src/index.ts
@@ -0,0 +1,17 @@
+let counter = 0;
+
+export function getCount() {
+  return counter;
+}
+
+export function increment() {
+  return ++counter;
+}
+
+export function decrement() {
+  return --counter;
+}
+
+export function square(x: number) {
+  return x * x;
+}
diff --git a/step2-04/demo/src/store/index.ts b/step2-04/demo/src/store/index.ts
new file mode 100644
index 0000000..221b5f4
--- /dev/null
+++ b/step2-04/demo/src/store/index.ts
@@ -0,0 +1,14 @@
+export type FilterTypes = 'all' | 'active' | 'completed';
+
+export interface TodoItem {
+  label: string;
+  completed: boolean;
+}
+
+export interface Store {
+  todos: {
+    [id: string]: TodoItem;
+  };
+
+  filter: FilterTypes;
+}
diff --git a/step2-04/exercise/src/stack.spec.ts b/step2-04/exercise/src/stack.spec.ts
new file mode 100644
index 0000000..8b89b72
--- /dev/null
+++ b/step2-04/exercise/src/stack.spec.ts
@@ -0,0 +1,16 @@
+describe('stack', () => {
+  it('should push item to the top of the stack', () => {
+    // implement test here:
+    // 1. require the stack
+    // 2. create stack push calls to place some items in the stack
+    // 3. write assertions via the expect() API
+  });
+
+  it('should pop the item from the top of stack', () => {
+    // implement test here:
+    // 1. require the stack
+    // 2. create stack push calls to place some items in the stack
+    // 3. pop a few items off the stack
+    // 4. write assertions via the expect() API
+  });
+});
diff --git a/step2-04/exercise/src/stack.ts b/step2-04/exercise/src/stack.ts
new file mode 100644
index 0000000..52c8311
--- /dev/null
+++ b/step2-04/exercise/src/stack.ts
@@ -0,0 +1 @@
+// Place your implementation from Step 2-01 exercise here
diff --git a/step2-04/index.spec.ts b/step2-04/index.spec.ts
deleted file mode 100644
index 2b48abb..0000000
--- a/step2-04/index.spec.ts
+++ /dev/null
@@ -1,16 +0,0 @@
-import { square } from '.';
-
-describe('square', () => {
-  beforeEach(() => {
-    console.log('this happens before each test');
-  });
-
-  afterEach(() => {
-    console.log('this happens after each test');
-  });
-
-  it('should be able to give the square of two numbers', () => {
-    console.log('test');
-    expect(square(5)).toBe(25);
-  });
-});
diff --git a/step2-04/index.ts b/step2-04/index.ts
deleted file mode 100644
index 4d99754..0000000
--- a/step2-04/index.ts
+++ /dev/null
@@ -1,3 +0,0 @@
-export function square(x: number) {
-  return x * x;
-}