fm - minimal observer with model

fm.js

const fm = (() => {
  let model = {}
  let observers = { fm_observe_all: [] }
  return {
    save: (key, val) => {
      model[key] = val
      emit(key)
    },
    silentSave:  (key, val) => {
      model[key] = val
    },
    emit: (key) => {
      if (typeof observers[key] !== 'undefined') {
        observers[key].forEach((observerCallback) => {
          observerCallback(model[key])
        })
      }
      observers['fm_observe_all'].forEach((observerCallback) => {
        observerCallback({[key]: model[key]})
      })
    },
    sub: (key, observerCallback) => {
      if (typeof observers[key] === 'undefined') {
        observers[key] = []
      }
      if (!observers[key].includes(observerCallback)) {
        observers[key].push(observerCallback)
      }
    },
    unsub:  (key, observerCallback) => {
      var index = observers[key].indexOf(observerCallback)
      if (index > -1) {
        observers[key].splice(index, 1)
      }
    }
  }
})()
export default fm