How to use Battery Status API

function Battery() {
 observable(this);

 if (typeof navigator.getBattery !== 'function') {
 navigator.getBattery = function () {
 return new Promise(function (resolve, reject) {
 if (navigator.battery) {
 resolve(navigator.battery);
 } else {
 reject(new Error('Battery Status API not implemented.'));
 }
 });
 };
 }

 navigator.getBattery().then(function(batteryManager) {
 this._battery = batteryManager;
 this.trigger('ready', this);
 this._battery.addEventListener('chargingchange', function() {
 this.trigger('chargingChange', this.isCharging());
 }.bind(this));
 this._battery.addEventListener('chargingtimechange', function() {
 this.trigger('chargingTimeChange', this.getChargingTime());
 }.bind(this));
 this._battery.addEventListener('dischargingtimechange', function() {
 this.trigger('dischargingTimeChange', this.getDischargingTime());
 }.bind(this));
 this._battery.addEventListener('levelchange', function() {
 this.trigger('levelChange', this.getLevel());
 }.bind(this));
 }.bind(this), function(error) {
 this.trigger('error', error);
 }.bind(this));
}

Battery.prototype.getLevel = function() {
 return this._battery.level;
};

Battery.prototype.getPercentage = function() {
 return this.getLevel() * 100;
};

Battery.prototype.isCharging = function() {
 return this._battery.charging;
};

Battery.prototype.isDischarging = function() {
 return !this.isCharging();
};

Battery.prototype.getChargingTime = function() {
 var chargingTime = this._battery.chargingTime;
 return Number.isFinite(chargingTime) ? chargingTime : 0;
};

Battery.prototype.getDischargingTime = function() {
 var dischargingTime = this._battery.dischargingTime;
 return Number.isFinite(dischargingTime) ? dischargingTime : 0;
};

Battery.prototype.getStatus = function() {
 return this.isCharging() ? 'charging' : 'discharging';
};

function observable(el) {
 var callbacks = {};

 el.on = function(name, fn) {
 if (typeof fn !== 'function') {
 throw new TypeError('Second argument for "on" method must be a function.');
 }
 (callbacks[name] = callbacks[name] || []).push(fn);
 return el;
 };

 el.one = function(name, fn) {
 fn.one = true;
 return el.on.call(el, name, fn);
 };

 el.off = function(name, fn) {
 if (name === '*') return (callbacks = {}, callbacks);
 if (!callbacks[name]) return;
 if (fn) {
 if (typeof fn !== 'function') {
 throw new TypeError('Second argument for "off" method must be a function.');
 }
 callbacks[name] = callbacks[name].map(function(fm, i) {
 if (fm === fn) {
 callbacks[name].splice(i, 1);
 }
 });
 } else {
 delete callbacks[name];
 }
 };

 el.trigger = function(name) {
 if (!callbacks[name]) return;
 var args = [].slice.call(arguments, 1);

 callbacks[name].forEach(function(fn, i) {
 if (fn) {

 fn.apply(fn, args);
 if (fn.one) callbacks[name].splice(i, 1);
 }
 });
 return el;
 };

 return el;
}

var battery = new Battery();