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, ...
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JavaScript Canvas - Blobs & Files

A JavaScript File is just a Blob with a few additional read only properties to indicate the file’s characteristics: where data is an array of strings, ArrayBuffer, ArrayBufferView and blobs, name is ...