Woop

Scratchpad apps generally tend to have a broad range of usages (simple notes, to-dos, tasks, and so forth). They're also very adept at helping developers transform text.
Woop is one such application - a scriptable scratchpad that allows you to run near-instant transformations to your texts directly from your computer's desktop and without requiring you to rely on potentially unsafe online tools.

Woop, a Boop Windows port

If you're a macOS developer, there's a good chance you've heard of Boop, one of the best Apple Swift-based and JavaScript-powered scratchpad apps out there.
Boop is safe, lightweight, and fast. It allows macOS developers to bring their scripts and extend the app's functionality, it features syntax highlighting, and it's also free and open-source.
Thankfully, everything that makes Boop great is also what defines Woop.

Instantly transform texts using .js scripts

The best part of Woop is definitely the fact that it uses .js scripts. Not only does the app have a plethora of useful built-in scripts that can help you transform text, but it's also extendable via your own self-written scripts.
Custom scripts can be added to any folder on your computer. Once the scripts have been added, go to Woop's menu, select Settings, and select the desired location for the custom scripts from the pop-up menu.
Another advantage of Woop is definitely its simplicity. To transform text, simply paste the text in question into Woop's main window, press CTRL+B and select the action (the script you want to run) from the provided list, and that's pretty much it.
All changes should be done instantaneously. To re-execute the last script, use CTRL+SHIFT+B or select the action from the app's menu.

Conclusion

Taking everything into account, Woop manages to provide Windows users with everything that Boop offers for macOS developers. It's modern-looking, very customizable, lightweight, and easy-to-use, as well as free. In a few simple words, it's a remarkably useful scriptable scratchpad.