adding bonus content for jest

bonus-jest/exercise/src/stack.ts

@@ -0,0 +1,27 @@
+export class Stack<T> {
+  private _items: T[] = [];
+
+  /** Add an item to the top of the stack. */
+  push(item: T) {
+    this._items.push(item);
+  }
+
+  /** Remove the top item from the stack and return it. */
+  pop(): T {
+    if (this._items.length > 0) {
+      return this._items.pop();
+    }
+  }
+
+  /** Return the top item from the stack without removing it. */
+  peek(): T {
+    if (this._items.length > 0) {
+      return this._items[this._items.length - 1];
+    }
+  }
+
+  /** Get the number of items in the stack/ */
+  get count(): number {
+    return this._items.length;
+  }
+}