By using Tailspin which provides access to the inner workings of the interpreter it is possible to expand what can be done in these online interactive coding websites.
Among other things it provides:
- Live code errors and warnings
- A range of programming puzzles
- Automatic saving of your progress
- A visualisation of your program running that shows the values and lists in your program
- A visualisation that animates as you scrub through the program execution
Check it out here: JS problems — How will you fare?