created tests and push function for stack

spec/stack.js

@@ -13,10 +13,52 @@ describe('Stack', () => {
 
   context('push()', () => {
     it('pushes an element to the top of the stack.', () => {
-      const myStack = new Stack()
+      const stack = new Stack()
 
-      expect(() => myStack.push('foo'))
-        .to.alter(() => myStack.length(), { from: 0, to: 1 })
+      expect(() => stack.push('foo'))
+        .to.alter(() => stack.length(), { from: 0, to: 1 })
+    })
+
+    it('pushes an element to the top of the stack when there is preexisting data', () => {
+      const stack = new Stack()
+      stack.push('alice')
+      stack.push('bob')
+
+      expect(() => stack.push('foo'))
+        .to.alter(() => stack.peek(), {from: 'bob', to: 'foo'})
+    })
+
+  })
+  context('pop()', () => {
+    it('pushes an element to the top of the stack.', () => {
+      const stack = new Stack()
+
+      expect(() => stack.push('foo'))
+        .to.alter(() => stack.length(), { from: 0, to: 1 })
+    })
+  })
+  context('peek()', () => {
+    it('pushes an element to the top of the stack.', () => {
+      const stack = new Stack()
+
+      expect(() => stack.push('foo'))
+        .to.alter(() => stack.length(), { from: 0, to: 1 })
+    })
+  })
+  context('isEmpty()', () => {
+    it('pushes an element to the top of the stack.', () => {
+      const stack = new Stack()
+
+      expect(() => stack.push('foo'))
+        .to.alter(() => stack.length(), { from: 0, to: 1 })
+    })
+  })
+  context('length()', () => {
+    it('pushes an element to the top of the stack.', () => {
+      const stack = new Stack()
+
+      expect(() => stack.push('foo'))
+        .to.alter(() => stack.length(), { from: 0, to: 1 })
     })
   })
 })

src/stack.js

@@ -1,5 +1,29 @@
 'use strict'
 
 export default class Stack {
-  // your code here
+  constructor(){
+    this.size = 0
+    this.value = [ ]
+  }
+
+  push(element){
+    this.value.unshift(element)
+    this.size++
+  }
+
+  pop(){
+    return this.value.pop()
+  }
+
+  peek(){
+    return this.value[0]
+  }
+
+  isEmpty(){
+    return (this.size === 0)
+  }
+
+  length(){
+    return this.size
+  }
 }