The try.gambitscheme.org web app has most of these features and more:

- REPL history with up/down arrows (explained on the landing page's README)

- You can use the "help" procedure to browse the full documentation

- TAB key for autocompletion

- In addition to the REPL there is a code editor to edit programs stored in the browser

- The system supports multiple threads, one REPL per thread

- There is a builtin tutorial to learn the basics of Scheme with examples executable in the REPL with a single click on a "run" button

- The REPL supports single stepping the execution of code

- Easy interface to JS with the SIX syntax extension, e.g. (let ((msg "hello")) \console.log(`msg, 1+2/3))

- The error messages are clear and precise giving the file and line number and highlighting in yellow the piece of code (in the REPL or file) that raised the exception. Just try this in both systems to see the difference:

    > (define (f x) (/ 1 (* x x)))
    > (f 5)
    > (f "hello") ;; the * procedure will raise an exception and highlight "(* x x)"