There are lot of ways to develop using Processing. Using Processing language and create pde file, using p5.js locally or editing online.
1) Today I used Processing language using P3 sketch, which is different from p5.js. For example, setting the canvas size is “size(600, 400);” whereas p5.js is “createCanvas(600, 400);”. P3 creates pde file and play button generate visuals. However, Processing is written in Java and it creates applets, which often crashes systems according to Daniel Shiffman.
2) I also used p5.js by downloading p5.js library and folder set up.
3) I can also use p5 editor at https://alpha.editor.p5js.org/
You can view the visual by open the index.html file, which opens sketch.js to show the visual.
But it is better to set up an local host in your development computer, the instruction are at https://github.com/processing/p5.js/wiki/Local-server.
In case of Mac, run terminal to run Python local server by entering:
“python -m SimpleHTTPServer”.
It will set up 0.0.0.0 local IP address on port:8000.
In case of browser p5.js editor, you can save code if you have an account.