js-test-4.js

// In this function we’re incrementing the count 
// property on the object that was passed in
// but there's a bug..

function incrementCount(counter) {
  if (counter.count) {
    counter.count++;
  } else {
    var counter = 1;
    counter.count = counter;
  }
}

// What happens when we call it in these 3 ways? comment each one.
incrementCount(0);

incrementCount({
    count: 10
});

incrementCount();

The 3rd give this error: Uncaught TypeError: Cannot read property 'count' of undefined
The first 2 do nothing, since there's no "return" in the function.