Recompose "withReducer": justification of async reducer function call

Question

I'm using withReducer HOC and noticed this behaviour: Calling this on click handler for example:

import React from 'react'
import { withReducer } from 'recompose'
import { compose } from 'ramda'

export default compose(
  withReducer('state', 'dispatch', (state, { value }) => {
    console.log(value)
    return { ...state, value }
  }, { value: 'zero' })
)((props) => {
  const { dispatch, state } = props,
    onClick = () => {
      console.log('Hello')
      dispatch({ value: 'one' })
      dispatch({ value: 'two' })
      dispatch({ value: 'three' })
      console.log('World')
    }
  return (
    <div>
      <div>{state.value}</div>
      <button onClick={onClick}>Click me</button>
    </div>
  )
})

It will produce

Hello

World

one

two

three

This means that reduce function is called asynchronously. What is justification for calling it async rather than applying changes to store right away?

Answer 1

In this case, dispatch is actually a wrapper for the vanilla API method setState.

React implements setState asynchronously because state transitions are sometimes batched together for performance reasons.

According to the React docs:

setState() does not immediately mutate this.state but creates a pending state transition....There is no guarantee of synchronous operation of calls to setState and calls may be batched for performance gains.

Answer 2

The reducer is called asynchronously because we can only use setState to update the tree and setState is asynchronous.

If we call the reducer synchronously, we will need to save the new state somewhere, then call setState and get the new state asynchronously from where we save it. In the end, your tree still get updated asynchronously.

This is why recompose's withReducer() is slightly different from redux. You can think of that withReducer is a simplified version of redux + react-redux's connect().