You have the principle resource in front of you... The browser. All you need now is a text editor so you create HTML pages to run the code in.
When functions are entered they go into memory and remain there until the end of the session. If you edit a function, just enter it again. Statements are executed immediately upon entering.
Dig around for IDE's that can execute JS. Myself, I just use HTML and the browser for my projects, and the console for debugging and testing.