Debugging JavaScript REPL-style?

Stand-alone REPL (no browser/DOM, just JavaScript): JavaScript Shell from the Rhino project.

To me, the most convenient debugger and REPL for JavaScript is Mozrepl. It is a Firefox/XULRunner extension that accesses the browser/application instance using telnet, and you can observe and manipulate everything in the browser; even the browser itself (remember, always talking about Firefox).

It is amazingly useful as a debugger (on standalone XUL applications it is the only bearable way to do real debugging) and as a tool to play around and understand the guts of your application, it speeds up your Javascript development time tenfold.

For an impressive demo of is possibilities, check out this video.

I've been using FireBug, i don't know if it is exactly what you need but i love debugging JavaScript through it.

Because you can print variables to its own console without having to always doing alert(var); you can just do console.log(var)

The Safari 4 beta has this ability in the error console (in the "Develop" menu). It's especially cool because when it returns an object or HTML node, it lets you delve into it with a little reveal arrow, showing its members, contents, etc.

I use firebug console window for this.

A guide to using Firebug's command-line API is here: Link.

Javascript REPL based on windows Script Host.

Also, there's a nice integration between that REPL and emacs.

For Chrome You can use jsshell - nice console:

https://chrome.google.com/extensions/detail/kmgmkbicahmbceidoidjbkbpkfogaldh

http://hugoware.net/projects/jsshell

I usually use Chrome's built in console. Even recent versions of IE have a decent dev tools window.

JRunscript is super cool (and I'm embarrassed I didn't know about it), but the issues I usually run into are due to variations in javascript implementation or DOM, not the language itself.