new stack implementation (ES6) fixes: #{246} (#247)

* new stack implementation (ES6) fixes: #{246}

* Update Data-Structures/Stack/StackES6.js

Co-authored-by: Tapajyoti Bose <44058757+ruppysuppy@users.noreply.github.com>

* Update StackES6.js

Co-authored-by: vinayak <itssvinayak@gmail.com>
Co-authored-by: Tapajyoti Bose <44058757+ruppysuppy@users.noreply.github.com>

Data-Structures/Stack/StackES6.js

@@ -0,0 +1,68 @@
+/* Stack data-structure. It's work is based on the LIFO method (last-IN-first-OUT).
+ * It means that elements added to the stack are placed on the top and only the
+ * last element (from the top) can be reached. After we get access to the last
+ * element, he pops from the stack.
+ * This is a class-based implementation of a Stack. It provides functions
+ * 'push' - to add an element, 'pop' - to remove an element from the top.
+ * Also it implements 'length', 'last' and 'isEmpty' properties and
+ * static isStack method to check is an object the instance of Stack class.
+ */
+
+// Class declaration
+class Stack {
+  constructor () {
+    this.stack = []
+    this.top = 0
+  }
+
+  // Adds a value to the end of the Stack
+  push (newValue) {
+    this.stack.push(newValue)
+    this.top += 1
+  }
+
+  // Returns and removes the last element of the Stack
+  pop () {
+    if (this.top !== 0) {
+      this.top -= 1
+      return this.stack.pop()
+    }
+    throw new Error('Stack Underflow')
+  }
+
+  // Returns the number of elements in the Stack
+  get length () {
+    return this.top
+  }
+
+  // Returns true if stack is empty, false otherwise
+  get isEmpty () {
+    return this.top === 0
+  }
+
+  // Returns the last element without removing it
+  get last () {
+    if (this.top !== 0) {
+      return this.stack[this.stack.length - 1]
+    }
+    return null
+  }
+
+  // Checks if an object is the instance os the Stack class
+  static isStack (el) {
+    return el instanceof Stack
+  }
+}
+const newStack = new Stack()
+console.log('Is it a Stack?,', Stack.isStack(newStack))
+console.log('Is stack empty? ', newStack.isEmpty)
+newStack.push('Hello world')
+newStack.push(42)
+newStack.push({ a: 6, b: 7 })
+console.log('The length of stack is ', newStack.length)
+console.log('Is stack empty? ', newStack.isEmpty)
+console.log('Give me the last one ', newStack.last)
+console.log('Pop the latest ', newStack.pop())
+console.log('Pop the latest ', newStack.pop())
+console.log('Pop the latest ', newStack.pop())
+console.log('Is stack empty? ', newStack.isEmpty)