Сложение числе в объекте с отображением в инпуте

var num1, num2, result; 
var calc = { 
  numbers: [0,1,2,3,4,5,6,7,8,9], 
  first: function(){ 
    var that = this; 
    document.addEventListener('keypress', function(){ 
      if(event.keyCode == 49) { 
        document.getElementById('field').value = that.numbers[1]; 
        num1 = that.numbers[1]; 
      } 
      else if(event.keyCode == 50) { 
        document.getElementById('field').value = that.numbers[2]; 
        num2 = that.numbers[2]; 
      } 
    function plus(){ 
      result = num1 + num2; 
      document.getElementById('field').value = result; 
    } 
    plus(); 
    }) 
 
  } 
} 
calc.first(); 
console.log(num1);  

<input type="text" id="field" disabled>