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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What Is JavaScript?

JavaScript is a critical tool in web development. While CSS and HTML provide web pages with static content, including the style and structure that appear on the page, JavaScript enables dynamic ...