Writing local files using javascript or other technology

I have seen lots of posting about people wanting to write text files using Javascript. It looks like this used to be possible. I have tried lots and lots of the examples, but can't get any to work short of Downloadify.

I am looking for something that will let me write a text value out to a local file on my own machine. I am running the Javascript locally and not on a web server. I would like this to be written out immediately and not require pressing a Download button like Downloadify.

Are there any options?