added small exercise for jest testing

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
+  });
+});

step2-04/exercise/src/stack.ts

@@ -0,0 +1 @@
+// Place your implementation from Step 2-01 exercise here