I'm a beginner JavaScript developer and I've been working on creating responsive web apps lately. I've learned a ton about the importance of making sure your app looks great on all devices, and I wanted to share some tips I've picked up along the way.

1. Use CSS media queries to adjust the layout of your app based on the size of the screen. This is a great way to make sure your app looks great on phones, tablets, and desktop computers.

2. Consider using a responsive framework like Bootstrap or Foundation to save time and make your life easier. These frameworks provide pre-designed, tested, and responsive components that you can easily incorporate into your app.

3. Test your app on a variety of devices to make sure it looks and functions as expected. It's easy to get caught up in development and forget to test, but trust me, it's worth it to make sure your app looks great on all devices.

I hope these tips are helpful for other beginner JavaScript developers out there! Happy coding!