Add change listener on mitrhil prop

prop.js

const m = require('mithril')

function emitChangeToSubscribers(subscribers, newState, oldState){
  subscribers.forEach(function(subscriber){
    subscriber.call(null, newState, oldState)
  })
}

module.exports = function(initState){
  const store = m.prop(initState)
  const subscribers = []
  
  function prop(){
    if( arguments.length > 0 ) {
      // arguments[0] -> newState
      // store() -> oldState
      emitChangeToSubscribers(subscribers, arguments[0], store())
    }
    return store.apply(null, arguments)
  }
  
  prop.onChange = function(callbackFn){ subscribers.push(callbackFn) }
  
  return prop
}

Example of use:

const name = prop('John')

name.onChange(function(newValue, previousValue){ 
  console.log('The name change from ' + previousValue + ' to ' + newValue)
})