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Thank you @allObjects for clearing that up. I liked your concise explanation within the pp detail on battery differences.
Did you mean 100..2100ma to be 2.1A and not 1.2 as indicated?
I was surprised to learn that my PC had a USB3 port along with the USB2 ports. The 500ma value has a bit of latitude when on a USB3 port. 900 big ones!
https://en.wikipedia.org/wiki/USB_3.0
"As with earlier versions of USB, USB 3.0 provides power at 5 volts nominal. The available current for low-power (one unit load) SuperSpeed devices is 150 mA, an increase from the 100 mA defined in USB 2.0. For high-power SuperSpeed devices, the limit is six unit loads or 900 mA (4.5 watts), almost twice USB 2.0's 500 mA"
For what it's worth, I've not had an issue, yet, . . . running WiFi over USB2/USB3, but I heed your warning. That said, I chose to power my little critters with an external supply, lest I suffer the wrath of extra electrons wanting to take an undesired path while I'm on a productive breadboard design streak.
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Robin
http://www.espruino.com/WiFi Docimentation states that you have three (3) mutually exclusive powering options:
The pins are 2nd and 3rd in row next to micro USB. 1st pin - GND - is ground (-).
http://www.espruino.com/WiFi was made with breadboard friendliness and integrated Wifi communication in mind.
While you are developing and are connected, you can use the micro USB. Disconnected you can feed it over the VUSB (+5V) or +3.3V pins - or - some of these USB power packs to micro USB.
Most/Some of these power packs include a 3.7V Lithium cell that you also can connect with some extra caution to 3.3V. The extra caution is that a fully charged 3.7V cell can deliver up to 4.4V which will destroy the processor. To be safe, you can connect it directly to the VUSB pin, but towards the capacity end of the 3.7V cell and some load, the cell may not have enough voltage anymore to drive the voltage regulator so that the regulator still puts 3.3V out (as to @Robin's comment). USB power banks use internally a booster to always put out 5V, and since the booster's efficiency is always less than 100%, you spend some capacity on it, but on the other hand you are safe. You can also connect the USB power bank +5V to the VUSB pin.
The behavior of Espruino regarding serial / console out is different when powered connected / USB vs powered through pins. There are several forum entries about this (google espruino+console+usb+connected).
One last remark: to operate onboard ESP8266 Wifi, you need a good power source. It can run from a standard 500mA USB supply (with sufficient capacitor in parallel), but more than 500mA is better. Today's phone power adaptors deliver 1000..2100mA (1.2A) and are a great option. (To go elaborate, you modify a USB cable the way that it allows you to power not only from your connected computer but also from other source: cut the 5V line and crimp male and female connectors to the ends, crimp a connector also the the GND line. This gives you all the options... including the one when disconnecting form the computer on the computer side you still have power through the micro USB connector...)