Stack Implementation using ES6

stack-es6.js

class Stack {
  constructor() {
      this.items = [];
  }
  
  push(element) {
    this.items.push(element);
  }
  
  pop() {
    if(this.items.length === 0) {
      return 'Error: Underflow'
    }
    return this.items.pop();
  }
  
  peek() {
    if(this.isEmpty()) {
      return 'Error: Empty'
    }
    return this.items[this.items.length - 1];
  }
  
  printStack() {
    if(this.isEmpty()) {
      return 'Error: Empty'
    }
    let i = this.items.length;
    while(i > 0) {
      console.log(this.items[--i]);
    }
  }
  
  isEmpty(){
    return this.items.length === 0;
  }
}

const stack = new Stack();

console.log('POPED :: ' + stack.pop());
stack.push(1);
stack.push(2);
stack.push(3);
console.log('PEEK :: '+ stack.peek());
stack.push(4);
stack.push(5);
stack.printStack();
console.log('POPED :: '+ stack.pop());
stack.push(6);
stack.printStack();
console.log('PEEK :: '+ stack.peek());
console.log('POPED :: '+ stack.pop());
console.log('POPED :: '+ stack.pop());
console.log('POPED :: '+ stack.pop());
console.log('POPED :: '+ stack.pop());
console.log('POPED :: '+ stack.pop());
console.log('POPED :: '+ stack.pop());

stack-output.txt

"POPED :: Error: Underflow"
"PEEK :: 3"
5
4
3
2
1
"POPED :: 5"
6
4
3
2
1
"PEEK :: 6"
"POPED :: 6"
"POPED :: 4"
"POPED :: 3"
"POPED :: 2"
"POPED :: 1"
"POPED :: Error: Underflow"