This is an older thread, but I just searched and found it. I am new to using Web Developer Tools: primarily Firefox Developer Tools (Firefox v.51), but also Chrome DevTools (Chrome v.56)].
I wasn't able to run functions from the Developer Tools console, but I then found this
https://developer.mozilla.org/en-US/docs/Tools/Scratchpad
and I was able to add code to the Scratchpad, highlight and run a function, outputted to console per the attched screenshot.
I also added the Chrome "Scratch JS" extension: it looks like it provides the same functionality as the Scratchpad in Firefox Developer Tools (screenshot below).
https://chrome.google.com/webstore/detail/scratch-js/alploljligeomonipppgaahpkenfnfkn
Image 1 (Firefox): http://imgur.com/a/ofkOp
Image 2 (Chrome): http://imgur.com/a/dLnRX