I am trying to build a Calculator using JavaScript and i am using an object oriented approach.
In the constructor function, I have a method that is supposed to display the values of the numbers clicked on screen. In the displayNum method, i used ‘+=’ to append the new number clicked to the already existing numbers but i keep getting ‘undefined’ before the first number i clicked. For example, if i click 8 and 9, i will get “undefined89”. Here is my code

function Calculator(element){
      this.numberBtns= [...element.querySelectorAll('[data-numbers]')]
      this.firstOperand= element.querySelector('.first-operand')
      this.secondOperand=element.querySelector('.second-operand')
    
      this.numberBtns.forEach((number)=>{
        number.addEventListener('click',()=>{
          this.displayNum(number.textContent);
          this.updateDisplay()
          
          
        })
      })
    
    }
    
    Calculator.prototype.updateDisplay= function (){
      this.secondOperand.textContent=this.currentOperand
    
    }
    Calculator.prototype.displayNum= function(number){
      this.currentOperand +=number
  
    }