Weight Sensor (Load Cell)

The module above looks good - then it's just a matter of finding a load cell.

As @DrAzzy said, it's worth checking Google for ways that people have done it with an Arduino, as they will almost certainly also work on an Espruino.

There may well be some kitchen scales you could 'hack' - for instance I just did a quick check and found this: http://www.avdweb.nl/arduino/hardware-interfacing/scale-interface.html

I'm sure there are others, but there's a pin on those scales that outputs a simple square wave, and then you just have to measure the pulse width. On Espruino it's actually even more simple than in Arduino - you just do:

setWatch(function(e) {
  var time = e.time - e.lastTime;
  // ...
}, A0, {repeat:true,edge:"falling"});