The third parameter of dispatch is undefined.

[yywc]

Version

3.1.1

Reproduction link

https://codesandbox.io/s/vue-template-4fv3d

Steps to reproduce

1. go to link;
2. open Console of page in the bottom;
3. see the undefined;

What is expected?

In theory,it should show the code of cb.

What is actually happening?

In fact,it's undefined.

---

According to Vuex code, wrappedActionHandler function provide two params.One is payload,the other is cb.

  // notice the cb
  entry.push(function wrappedActionHandler (payload, cb) {
    let res = handler.call(store, {
      dispatch: local.dispatch,
      commit: local.commit,
      getters: local.getters,
      state: local.state,
      rootGetters: store.getters,
      rootState: store.state
    }, payload, cb) // pass it and do nothing
    if (!isPromise(res)) {
      res = Promise.resolve(res)
    }
    if (store._devtoolHook) {
      return res.catch(err => {
        store._devtoolHook.emit('vuex:error', err)
        throw err
      })
    } else {
      return res
    }
  })

  // dispatch function only use payload
  dispatch (_type, _payload) {
    // check object-style dispatch
    const {
      type,
      payload
    } = unifyObjectStyle(_type, _payload)

    const action = { type, payload }
    const entry = this._actions[type]
    // ...
    const result = entry.length > 1
      ? Promise.all(entry.map(handler => handler(payload))) // only payload parameter
      : entry[0](payload)
  }

So I think cb parameter is useless and it should be delete from code in order to
eliminate misunderstanding.

This may be a duplicate of #1571.