How to Optimize JavaScript for Better Website Loading Speed?

I'm currently working on improving my website's speed, but the JavaScript files (both inline and external) seem to be one of the biggest issues based on PageSpeed Insights and GTmetrix reports. I've already tried minifying and combining them, but I'm still seeing delays in first contentful paint and time to interactive.
Can anyone share best practices or coding techniques to better handle JS without compromising functionality?
Do async/defer attributes always help, or are there better patterns depending on the structure of the site? Also, any common mistakes to avoid when optimizing scripts?