javascriptundefinedaddition

Why does adding numbers to undefined to result in NaN?

I understand that when a variable is declared it is given the value "undefined", but I don't understand why the following code doesn't work (returns NaN):

function sum() {
// return the sum of all arguments given.
  var answer;
  for (var i = 0; i<arguments.length; i++){
    answer += (arguments[i]);
  }
  console.log(answer);
}
sum(4,5,3,2);

In order to get the correct answer (14), I had to initialize the answer variable:

var answer = 0;
Solution

  • In essence, you're asking why undefined + 5 + 4 + 3 + 2 is NaN (not a number). The answer is that, when you treat undefined as a number, it is implicitly converted to NaN. Any operation on NaN will, in turn, result in NaN.