This feature is non-standard and is not on a standards track. Do not use it on production sites facing the Web: it will not work for every user. There may also be large incompatibilities between implementations and the behavior may change in the future.
txPower read-only property of the
BluetoothAdvertisingData interface returns the transmission power at which the device is broadcasting, measured in dBm. This is used to compute the path loss as
this.txPower - this.rssi.
var power = BluetoothAdvertisingData.txPower;
The definition of 'txPower' in that specification.