Get battery level JavaScript

From the English version of that page:

This feature has been removed from the Web standards. Though some browsers may still support it, it is in the process of being dropped. Do not use it in old or new projects. Pages or Web apps using it may break at any time.

and

The battery property is deprecated and has been replaced by a Navigator.getBattery() method returning a battery Promise. Its support is still partial though.

WARNING

Do not use the following answer in production, as it has been deprecated since then. The spec will be reworked due to privacy and security issues, so expect to see changes and malfunctions.

ANSWER

To answer my own question, thanks to the help of the document that @Quentin provided to me, I reached my goal using BatteryManager.level function the the promise Navigator.getBattery(). The link that gives the working snippet can be found here : https://developer.mozilla.org/en-US/docs/Web/API/BatteryManager/level.

The following JSFiddle display in the console (F12) the value of the remaining battery (I could not verify the ouput in a desktop yet, so please correct me if it is not working on desktop) :

JSFiddle

JavaScript

navigator.getBattery().then(function(battery) {

    var level = battery.level;

    console.log(level);
});

event.target.level is the level value in the event handlers.

navigator.getBattery().then((battery) => {

    battery.ondischargingtimechange = (event) => { 
       console.warn(`Discharging : `, event.target.level) 
    };

    battery.onchargingtimechange = (event) => { 
       console.info(`Charging : `, event.target.level) ;
    };
});

REal Example: JSFIDDLE