src/third_party/web_platform_tests/battery-status/battery-promise.html

<!DOCTYPE html>
<meta charset="utf-8">
<title>Battery Test: navigator.getBattery() returns BatteryManager as a promise</title>
<link rel="author" title="YuichiNukiyama" href="https://github.com/YuichiNukiyama">
<script src="/resources/testharness.js"></script>
<script src="/resources/testharnessreport.js"></script>

<script>
    function returnBattery() {
        return navigator.getBattery();
    }

    promise_test(function () {
        return returnBattery()
          .then(function (result) {
              assert_class_string(result, "BatteryManager", "getBattery should return BatteryManager Object.");
          });
    }, "navigator.getBattery() return BatteryManager");

    test(function () {
        assert_equals(navigator.getBattery(), navigator.getBattery());
    }, "navigator.getBattery() shall always return the same promise");
</script>

<div id="log"></div>