Uncaught Error: Function "substring" not found!

Pretty sure this is a noob question, but in my watch face I want to replace something inside a string. So I just did

mystring = mystring .replace('.', ''); 

and got

Uncaught Error: Function "substring" not found!

Any ideas why that wouldn't work?
Same with substring.

I just tried it on the banglejs 2 emulator and it worked fine, I'm pretty new to Espruino myself but 2 things I'd check, is the firmware up to date and can you run it directly in the terminal?

It is actually working in the emulator console.

OK, never mind, I had an array instead of a string in my code.
Thanks for your help though.

OK, and if you send your code to the emulator, does it work there?
If not, can you then share your code via pastebin.com or similar? :)

Heh, yeah that will certainly confuse the poor thing xD