Real-world promise demo

[raq929]

I did this in 016, and @jrhorn424 thinks it (or something similar) maybe should be added to the lesson.

I wrote an IOU ("Yen owes me keys") on a piece of paper. I gave my keys to Yen, and explained that the piece of paper is now the "Promise". I can't drive home until I get my keys back, and getting my keys back can set things in motion. A failure to get my keys will cause a different course of action.

I made a psuedo-code promise:

new Promise((resolve, reject) {
  if(time > 5 pm) {
     reject(error)
  }

 resolve(keys)
})

And when it became relevant, a promise chain:

keysPromise()
  .then(putKeysInCoat)
  .then(putOnCoat)
  .then(goToCar)
  .then(driveHome)
  .catch(callYen)

And I explained that each of these has a return value that is passed to the next one. (With arrows and the values).