Yesterday P5js reached version 1.0.0. This is of major, major, major importance for the community. P5js is library that promises to do in the browser what Processing did in Java, and allow the Processing community to do the transition to modern web technologies without relearning a new syntax.
But better than my words, go and read the announcement published in Medium by Laureen Mccarthy.
And finally, this is all good and fine but what P5 is there for is for the sketches. Starting with P5js is simple. Just use the online editor where everything is there waiting for your ideas.