className only changing every other class

Question

I\'m performing a small text with JavaScript with the getElementsByClassName() and I am getting some unwanted results. I would like the script to change each CS

Answer 1

document.getElementsByClassName returns a HTMLCollection object, which is live

An HTMLCollection in the HTML DOM is live; it is automatically updated when the underlying document is changed.

So when you call

blockSet[0].className = "block-selected";

You changed the underlying document and that item is not in the collection anymore (the blockSet[0] is now the second item in your original selection).