Why does JavaScript sometimes work and sometimes not? Understanding the basics of loading order

I created a process to open a menu when a button is pressed, but nothing happens.Even with the same JavaScript, it suddenly started working when I changed where the file was loaded.In web development, ...
Ars Technica

Client-side JavaScript file processing may come via File API

A new JavaScript API aims to facilitate limited local filesystem access in Web applications. A draft that documents the new API, authored by Mozilla’s Arun Ranganathan, has been submitted to the World ...