Operating on undefined variable

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0

function add(num) { 
  var sum;
  for (i = 1; i < num + 1; i++){
    sum = (sum || 0) + i;
  }; 
  return sum;     
}
add(9);

What is a keyword to describe the behavior for defining sum when it gets added to "i" in the for loop? Is this a ternary operation? Obviously the first time it loops through it is undefined, so javascript chooses 0. Why is that?

asked 44 secs ago

noob-in-need

40

Operating on undefined variable