How does combineReducers in Redux determine which specific portion of the application state to send to the reducer?

While going through the Redux basics tutorial, I found myself a bit confused about how the code snippet below determines which part of the application state should be passed to each reducer mentioned in the combineReducers function. Does it simply rely on matching the state property name with the reducer function name?

import { combineReducers } from 'redux'
import { ADD_TODO, COMPLETE_TODO, SET_VISIBILITY_FILTER, VisibilityFilters } from './actions'
const { SHOW_ALL } = VisibilityFilters

function visibilityFilter(state = SHOW_ALL, action) {
  switch (action.type) {
    case SET_VISIBILITY_FILTER:
      return action.filter
    default:
      return state
  }
}

function todos(state = [], action) {
  switch (action.type) {
    case ADD_TODO:
      return [
        ...state,
        {
          text: action.text,
          completed: false
        }
      ]
    case COMPLETE_TODO:
      return state.map((todo, index) => {
        if (index === action.index) {
          return Object.assign({}, todo, {
            completed: true
          })
        }
        return todo
      })
    default:
      return state
  }
}

const todoApp = combineReducers({
  visibilityFilter,
  todos
})

export default todoApp

Answer №1

If you're curious about how the combineReducers function works, take a look at the source code. You can find more information in combineReducers.js in the redux repository. As each action goes through all the combined reducers, each reducer receives the specific portion of state that matches its designated key in the object passed to combineReducers.

In the given example, both the visibilityFilter and todos reducers have keys with matching names (due to the ES6 object property shorthand being used). These keys are utilized to provide the relevant sections of state to each respective reducer.

Answer №2

The snippet you shared contains two reducers, but their actual usage in an application is not apparent.

Consider the following code example:

import React from 'react'
import { connect } from 'react-redux'
import { addTodo } from '../actions'

let AddTodo = ({ dispatch }) => {
  let input

  return (
    <div>
      <input ref={node => {
        input = node
      }} />
      <button onClick={() => {
        dispatch(addTodo(input.value))
        input.value = ''
      }}>
        Add Todo
      </button>
    </div>
  )
}
AddTodo = connect()(AddTodo)

export default AddTodo

When the button within the component is clicked, it triggers the action addTodo:

const addTodo = (text) => {
  return {
    type: 'ADD_TODO',
    id: nextTodoId++,
    text
  }
}

This action is then handled by one of the aforementioned reducers:

const todo = (state, action) => {
  switch (action.type) {
    case 'ADD_TODO':
      return {
        id: action.id,
        text: action.text,
        completed: false
      }
    case 'TOGGLE_TODO':
      if (state.id !== action.id) {
        return state
      }

      return Object.assign({}, state, {
        completed: !state.completed
      })

    default:
      return state
  }
}

The reducer determines the new state of the application based on the action and returns it as the updated state. The dispatcher passes the current state to the reducer for processing.

Note: The code snippets provided are excerpts from the same tutorial referenced by the original poster.

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