I should add that http://www.espruino.com/Reference shows features that are in Espruino 2v04 but not the features that have been added since then (eg for the beta Bangle.js firmware). For that, there's: https://banglejs.com/reference
The specific one that would be most useful to you is https://banglejs.com/reference#l_Storage_open
Some examples of it here: https://github.com/espruino/Espruino/blob/master/tests/test_storage_file.js
It's still early days, but is intended for exactly the kind of thing you're after (eg. logging)
@Gordon started
Espruino is a JavaScript interpreter for low-power Microcontrollers. This site is both a support community for Espruino and a place to share what you are working on.
I should add that http://www.espruino.com/Reference shows features that are in Espruino 2v04 but not the features that have been added since then (eg for the beta Bangle.js firmware). For that, there's: https://banglejs.com/reference
The specific one that would be most useful to you is https://banglejs.com/reference#l_Storage_open
Some examples of it here: https://github.com/espruino/Espruino/blob/master/tests/test_storage_file.js
It's still early days, but is intended for exactly the kind of thing you're after (eg. logging)