Stack with private fields

Stack.js

class Stack{
	#array = []
    push(val){
      return this.#array.push(val);
    }
    pop(){
      return this.#array.pop(); 
    }
    isEmpty(){
      return this.#array.length == 0
    }
    peek(){
      if(this.isEmpty()){
        return undefined;
      }
      return this.#array[this.#array.length - 1];
    }
}