Add your <script> to the bottom of your <body>, or add an event listener for DOMContentLoaded following this StackOverflow question.
If that script executes in the <head> section of the code, document.getElementsByClassName(...) will return an empty array because the DOM is not loaded yet.
You're getting the Type Error because you're referencing search_span[0], but search_span[0] is undefined.
This works when you execute it in Dev Tools because the DOM is already loaded.