[anykey]

Re: the multi-line JS stuff in Chrome, there's snippets!

https://developer.chrome.com/devtools/docs/authoring-develop...

Probably not 100% what you wanted, but they're really handy for sketching out code fragments.

[mnemonik]

And for Firefox Developer Tools, we have the Scratchpad. I like ours better due to the keyboard shortcuts for eval/display/inspect/re-eval function/etc, but I may be biased ;)

https://developer.mozilla.org/en-US/docs/Tools/Scratchpad

Much of the new JS that I write these days starts as a scratchpad that I evolve over many iterations, right in the browser.