Pixl.js + grove sensors

Is the PWR led on on that shield? For it to work you'd need the switch set to 3.3v OR for Vin and 5v to be shorted together (either with the Pixl's solder jumper, or just with a wire on the Arduino headers).

And how are you powering the Pixl.js? via a 3v battery, or USB?

http://www.espruino.com/DHT11 says:

If no data received at all: {"temp": -1, "rh": -1, err : true, "checksumError": false}

So that could be what you're getting? The DHT11 is a digital device, so Espruino can detect if it's working/connected correctly or not. If something goes wrong it'll report back -1 for the values.