Hi, I'm the author playcode.io. I make it one and try to make it really convenient, fast and flexible in order to quickly test javascript or learn it. I've been doing it for about 5 years. It is used by 150,000 users a month and earns $30,000 a year. Not much, but nice, too.

It's really awesome! And it still blows my mind that it was created by one dev while it is smooth and fast.

Really impressive piece of software!