How to use an I2C chip? (Adafruit MMA8451)

In the documentation, I wonder if I'm getting "static acceleration" data rather than actual acceleration data

It could be... It might help to graph the data somehow when it's moving to see if it makes sense.

Normally from an accelerometer you'd expect to have the value including gravity, so when you lie it flat on a surface one axis should be 1g and the others 0g (ish).

Looks like you're not doing any checks for the sign of the value either, so it'd likely negative values will just show up as large positive numbers. It might be worth looking at some of Espruino's other accelerometer modules to see how that gets handled usually.