/* 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

}

}

export { Stack }