how do i detect windows 10 and office 365 versions in word js add-ins?

I am not sure about windows 10 build number, but can provide a way to get word host full version on desktop platform.

Generally, following solution will work,

const context = window.external.GetContext()
context.GetHostFullVersion()

But in some of the cases, where add-in opens in iframe, it needs accessed by following way,

const context = parent.window.external.GetContext()
context.GetHostFullVersion()

I have tried this on windows platform, this will provide you version string like “16.0.13328.20292”.