Why wouldnt this JS code be valid?

Hi, I'm currently learning arrays on the beginner JS course. One of the questions states:

"Use the array method that combines all of the items in an array into a single string. In the final string, the array items should be separated by a comma AND a space. Finally, log the final string value to the console."

They provide you this code:

var months = ['January','February','March','April','May','June','July','August','September','October','November','December'];

This was my solution which in my mind seems very reasonable but it doesnt seem to work for some reason:

months = months.join(', '); console.log(months);

Now, after researching, I found out that the answer was to create a new variable instead of reassigning the months.join() to the months variable but my question is why doesn't this work? it seems more efficient than creating an entirely new variable.

Thanks,