Stack implementation in javascript (es6)

Stack.js

let Stack = (()=>{
  
 let map = new WeakMap();
 let _items = [];

  class Stack {
  constructor(...items){
    // let's store our items array inside the weakmap
    map.set(this, []);
    // let's get the items array from map, and store it in _items for easy access elsewhere
    _items = map.get(this);
    
    //if constructor receives any items, it should be stacked up
    if(items.length>0)
      items.forEach(item => _items.push(item) )
      
  }
  
  push(...items){
    //push item to the stack
     items.forEach(item => _items.push(item) )
     return this;
    
  }
  
  pop(count=0){
    //pull out the topmost item (last item) from stack
    if(count===0)
       _items.pop()
    else
       _items.splice( -count, count )
    return this
  }
  
  peek(){
    // see what's the last item in stack
    return _items[_items.length-1]
  }
  
  size(){
    //no. of items in stack
    return _items.length
  }
  
  isEmpty(){
    // return whether the stack is empty or not
    return _items.length==0
  }
    
  toArray(){
    // return items of the queue
    return _items
  }
}

  
return Stack;

})();