CAAT, the custom audio algorithm tester is a page that let’s you try out your own simple audio filter algorithms.
Just (mis)use the textarea for coding and listen to what you get. There are some examples on how you would do basic things.
It helps me sometimes, when I just want to check something out very quickly.
Of course, this is a very unperformant way to implement audio filter algorithms for several reasons. This is just a demo. If you’re interested in how to implement algorithms the right way, I recommend using Web Audio API’s Audio Worklets or the talk “C++ in the Audio Industry” by Timur Doumler.