I recently found out, using javascript i can find out the battery level of the current systems, i was really fascianted when i find out.
It wasnt ...
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strictly speaking, this can be done not by js, but by the browser. This is the API of the platform. Modern browsers can also monitor the network status, access Bluetooth/contacts/camera/speech recognition/geo/generate binary files (this is one of the most frequent cases of my practice).
The full list of progressive features of the platform can be found here developer.mozilla.org/ru/docs/Web/API