onload = function() { stripe () };

function getElementsByClassName(elm, tag, className){
	var testClass = new RegExp("(^|\\s)" + className + "(\\s|$)");
	var tag = tag || "*";
	var elm = elm || document;
	var elements = (tag == "*" && elm.all)? elm.all : elm.getElementsByTagName(tag);
	var returnElements = [];
	var current;
	var length = elements.length;
	for(var i=0; i<length; i++){
		current = elements[i];
		if(testClass.test(current.className)){
			returnElements.push(current);
		}	
	}
	return returnElements;
}

 function stripe() {
    
    var books = getElementsByClassName(document.getElementById("middle"), "div", "book");

    for (var i = 1; i < books.length; i += 2) {
      books[i].className += " zebra";
    }
  }