redux.js

var currentReducer = reducer
var currentState = preloadedState
var currentListeners = []
var nextListeners = currentListeners
var isDispatching = false

function dispatch(action) {
  if (!isPlainObject(action)) {
    throw new Error(
      'Actions must be plain objects. ' +
      'Use custom middleware for async actions.'
    )
  }

  if (typeof action.type === 'undefined') {
    throw new Error(
      'Actions may not have an undefined "type" property. ' +
      'Have you misspelled a constant?'
    )
  }

  if (isDispatching) {
    throw new Error('Reducers may not dispatch actions.')
  }

  try {
    isDispatching = true
    currentState = currentReducer(currentState, action)
  } finally {
    isDispatching = false
  }

  var listeners = currentListeners = nextListeners
  for (var i = 0; i < listeners.length; i++) {
    var listener = listeners[i]
    listener()
  }

  return action
}