charAt not evaluating

I am trying to write a function that will evaluate equality of characters in a string and return true if 3 in a row match. The charAt() doesn't seem to be working as the if statement always goes to the else block.

function myFunction(num1) 
{
var checkNum1;
  for (var i = 0; i < num1.length; i++)
  {
    if (num1.charAt(i) == num1.charAt(i+1) && num1.charAt(i) == num1.charAt(i+2))
    {
    checkNum1 = true;
    break;
    }
  }
  if (checkNum1 == true)
  {
  return true;
  }

  else
  {
  return false;
  }
}

What should I be doing to get the last "if" block to return true?