How to add event listener for html class?

Question

If I have HTML like this:

Answer 1

No tags with classname preview so

var preview = document.getElementsByClassName("preview");

will cause that error

this line must be as follows

var preview = document.getElementsByClassName("previewBulk");

and as @raym0nd said the number of div tags must be equal to the number of a tags

EDIT

the problem with your code is you used the index of for loop inside the anonymous function but this function is called with mouseout and mouseover event and after finishing the for loop try the following

var movie = document.getElementsByClassName("movieImg");

    for(var i = 0, j=movie.length; i

or test in jsFiddle