RxJS has these 4 parts:

1. callback
2. creating new callbacks from original
3. callback caller
4. startup

I like to think of it like this:

//observer
const callback = value => {
  console.log(value)
}
//observable
const callbackCaller = callback => {
  document.addEventListener('click', callback)
}
//map
const transform = x => x + 1
//internals
const createNewCallbackFromTransform = (transform, callback) => {
  const newCallback = value => callback(transform(value))
  return newCallback
}
//subscribe
callbackCaller(createNewCallbackFromTransform(transform, callback))