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david marjanac4,851 Points
why you put in var=button array 
dont know why ther is  in var addButton=document.getElementByTagName("button");
Robert Karlsson8,021 Points
From the getElementsByTagName-method you get a nodelist back with all matching elements. The  at the end gives you the first element in the list.
An example of a method that doesn't need the  at the end is the getElementById, since it only returns a single element, instead of a list of elements.