I know that if we want to find a group of elements, getElementsByTagName is the method for us and it returns a NodeList. but if we are looking for tag name with body , then why we need to add [0] after the (body) element? There is only one body tag in an HTML document.

 var body = document.getElementsByTagName(body)[0];

 body.className = unreadable;

why we cant write this code without index[0] like this

 var body = document.getElementsByTagName(body);

 body.className = unreadable;

If i write this code the class unreadable will not be added with body tag ...why?