Intermediate Algorithm - Arguments Optional. - Need an explanation [SPOILERS]

1

Hello,

I recently completed the Arguments Optional problem. However, I am not 100% sure why it works and I was hoping someone could explain it to me.

My solution:

 function addTogether(a,b) {
	  if (typeof a ==='number' && typeof b === 'number') {
		  return a + b;
	  } 
	  else if (typeof a ==='number' && typeof b ==='undefined') {
		  return function(b) {                                     
			  if (typeof b === 'object') {                         
				return undefined;
			  } else {
				return a + b;
			  }
		  };
		}
	}
	addTogether(2, 3);

If you look at this specific part below. I am checking typeof b for ‘undefined’ yet for some reason after I check it and it is undefined, if I check it again in the inner if statement, it is suddenly an object in some cases. Why would it randomly change from undefined to object?

else if (typeof a ==='number' && typeof b ==='undefined') {
		  return function(b) {                                     
			  if (typeof b === 'object') {                         
				return undefined;
			  } else {
				return a + b;
			  }
		  };
2

Notice what you’re returning.

return function(b) {                                     
    if (typeof b === 'object') {                         
      return undefined;
    } else {
      return a + b;
    }
}

If a is a number and b isn’t, the function returns another function and not another value. This function gets called later. Here’s how the program works:

var sum = addTogether(10,7); // 17
var nextStep = addTogether(15); // Returns a function with '15' as the first value
var finalStep = nextStep(8); // Returns 23 (15 + 8)

So, there needs to be another check for b to make sure that it’s a number when that returned function gets called.