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.
https://developer.chrome.com/devtools/docs/authoring-develop...
Probably not 100% what you wanted, but they're really handy for sketching out code fragments.