I don't see running in the browser as a benefit. It means that a required step for a neophyte in producing meaningful output is getting a handle on the utterly brain-dead DOM API.
Its a programming language for the web. Not only is running in browser a benefit, its a necessity.
There's 'console.log' which outputs to the debuggers output screen.
But then the mere fact that a computer comes with a browser already installed is not a notable advantage. I can get a Python REPL running on a computer with an amount of effort and know-how comparable to that necessary to install Chrome.