Tutorial: Using the connect API

We now recommend using the React-Redux hooks API as the default. However, the connect API still works fine.

This tutorial also shows some older practices we no longer recommend, like separating Redux logic into folders by type. We’ve kept this tutorial as-is for completeness, but recommend reading through the “Redux Essentials” tutorial and the Redux Style Guide in the Redux docs for our current best practices.

We’re working on a new tutorial that will introduce the hooks APIs. Until then, we suggest reading Redux Fundamentals, Part 5: UI and React for a hooks tutorial.

To see how to use React Redux in practice, we’ll show a step-by-step example by creating a todo list app.

A Todo List Example

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The React UI Components

We have implemented our React UI components as follows:

TodoApp is the entry component for our app. It renders the header, the AddTodo, TodoList, and VisibilityFilters components.
AddTodo is the component that allows a user to input a todo item and add to the list upon clicking its “Add Todo” button:
- It uses a controlled input that sets state upon onChange.
- When the user clicks on the “Add Todo” button, it dispatches the action (that we will provide using React Redux) to add the todo to the store.
TodoList is the component that renders the list of todos:
- It renders the filtered list of todos when one of the VisibilityFilters is selected.
Todo is the component that renders a single todo item:
- It renders the todo content, and shows that a todo is completed by crossing it out.
- It dispatches the action to toggle the todo’s complete status upon onClick.
VisibilityFilters renders a simple set of filters: all, completed, and incomplete. Clicking on each one of them filters the todos:
- It accepts an activeFilter prop from the parent that indicates which filter is currently selected by the user. An active filter is rendered with an underscore.
- It dispatches the setFilter action to update the selected filter.
constants holds the constants data for our app.
And finally index renders our app to the DOM.

The Redux Store

The Redux portion of the application has been set up using the patterns recommended in the Redux docs:

Store
- todos: A normalized reducer of todos. It contains a byIds map of all todos and a allIds that contains the list of all ids.
- visibilityFilters: A simple string all, completed, or incomplete.
Action Creators
- addTodo creates the action to add todos. It takes a single string variable content and returns an ADD_TODO action with payload containing a self-incremented id and content
- toggleTodo creates the action to toggle todos. It takes a single number variable id and returns a TOGGLE_TODO action with payload containing id only
- setFilter creates the action to set the app’s active filter. It takes a single string variable filter and returns a SET_FILTER action with payload containing the filter itself
Reducers
- The todos reducer
  - Appends the id to its allIds field and sets the todo within its byIds field upon receiving the ADD_TODO action
  - Toggles the completed field for the todo upon receiving the TOGGLE_TODO action
- The visibilityFilters reducer sets its slice of store to the new filter it receives from the SET_FILTER action payload
Action Types
- We use a file actionTypes.js to hold the constants of action types to be reused
Selectors
- getTodoList returns the allIds list from the todos store
- getTodoById finds the todo in the store given by id
- getTodos is slightly more complex. It takes all the ids from allIds, finds each todo in byIds, and returns the final array of todos
- getTodosByVisibilityFilter filters the todos according to the visibility filter

You may check out this CodeSandbox for the source code of the UI components and the unconnected Redux store described above.

We will now show how to connect this store to our app using React Redux.

Providing the Store

First we need to make the store available to our app. To do this, we wrap our app with the <Provider /> API provided by React Redux.

// index.js
import React from 'react'
import ReactDOM from 'react-dom'
import TodoApp from './TodoApp'

import { Provider } from 'react-redux'
import store from './redux/store'

const rootElement = document.getElementById('root')
ReactDOM.render(
  <Provider store={store}>
    <TodoApp />
  </Provider>,
  rootElement
)

Notice how our <TodoApp /> is now wrapped with the <Provider /> with store passed in as a prop.

Connecting the Components

React Redux provides a connect function for you to read values from the Redux store (and re-read the values when the store updates).

The connect function takes two arguments, both optional:

mapStateToProps: called every time the store state changes. It receives the entire store state, and should return an object of data this component needs.
mapDispatchToProps: this parameter can either be a function, or an object.
- If it’s a function, it will be called once on component creation. It will receive dispatch as an argument, and should return an object full of functions that use dispatch to dispatch actions.
- If it’s an object full of action creators, each action creator will be turned into a prop function that automatically dispatches its action when called. Note: We recommend using this “object shorthand” form.

Normally, you’ll call connect in this way:

const mapStateToProps = (state, ownProps) => ({
  // ... computed data from state and optionally ownProps
})

const mapDispatchToProps = {
  // ... normally is an object full of action creators
}

// `connect` returns a new function that accepts the component to wrap:
const connectToStore = connect(mapStateToProps, mapDispatchToProps)
// and that function returns the connected, wrapper component:
const ConnectedComponent = connectToStore(Component)

// We normally do both in one step, like this:
connect(mapStateToProps, mapDispatchToProps)(Component)

Let’s work on <AddTodo /> first. It needs to trigger changes to the store to add new todos. Therefore, it needs to be able to dispatch actions to the store. Here’s how we do it.

Our addTodo action creator looks like this:

// redux/actions.js
import { ADD_TODO } from './actionTypes'

let nextTodoId = 0
export const addTodo = (content) => ({
  type: ADD_TODO,
  payload: {
    id: ++nextTodoId,
    content,
  },
})

// ... other actions

By passing it to connect, our component receives it as a prop, and it will automatically dispatch the action when it’s called.

// components/AddTodo.js

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

class AddTodo extends React.Component {
  // ... component implementation
}

export default connect(null, { addTodo })(AddTodo)

Notice now that <AddTodo /> is wrapped with a parent component called <Connect(AddTodo) />. Meanwhile, <AddTodo /> now gains one prop: the addTodo action.

We also need to implement the handleAddTodo function to let it dispatch the addTodo action and reset the input

// components/AddTodo.js

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

class AddTodo extends React.Component {
  // ...

  handleAddTodo = () => {
    // dispatches actions to add todo
    this.props.addTodo(this.state.input)

    // sets state back to empty string
    this.setState({ input: '' })
  }

  render() {
    return (
      <div>
        <input
          onChange={(e) => this.updateInput(e.target.value)}
          value={this.state.input}
        />
        <button className="add-todo" onClick={this.handleAddTodo}>
          Add Todo
        </button>
      </div>
    )
  }
}

export default connect(null, { addTodo })(AddTodo)

Now our <AddTodo /> is connected to the store. When we add a todo it would dispatch an action to change the store. We are not seeing it in the app because the other components are not connected yet. If you have the Redux DevTools Extension hooked up, you should see the action being dispatched:

You should also see that the store has changed accordingly:

The <TodoList /> component is responsible for rendering the list of todos. Therefore, it needs to read data from the store. We enable it by calling connect with the mapStateToProps parameter, a function describing which part of the data we need from the store.

Our <Todo /> component takes the todo item as props. We have this information from the byIds field of the todos. However, we also need the information from the allIds field of the store indicating which todos and in what order they should be rendered. Our mapStateToProps function may look like this:

// components/TodoList.js

// ...other imports
import { connect } from "react-redux";

const TodoList = // ... UI component implementation

const mapStateToProps = state => {
  const { byIds, allIds } = state.todos || {};
  const todos =
    allIds && allIds.length
      ? allIds.map(id => (byIds ? { ...byIds[id], id } : null))
      : null;
  return { todos };
};

export default connect(mapStateToProps)(TodoList);

Luckily we have a selector that does exactly this. We may simply import the selector and use it here.

// redux/selectors.js

export const getTodosState = (store) => store.todos

export const getTodoList = (store) =>
  getTodosState(store) ? getTodosState(store).allIds : []

export const getTodoById = (store, id) =>
  getTodosState(store) ? { ...getTodosState(store).byIds[id], id } : {}

export const getTodos = (store) =>
  getTodoList(store).map((id) => getTodoById(store, id))

// components/TodoList.js

// ...other imports
import { connect } from "react-redux";
import { getTodos } from "../redux/selectors";

const TodoList = // ... UI component implementation

export default connect(state => ({ todos: getTodos(state) }))(TodoList);

We recommend encapsulating any complex lookups or computations of data in selector functions. In addition, you can further optimize the performance by using Reselect to write “memoized” selectors that can skip unnecessary work. (See the Redux docs page on Computing Derived Data and the blog post Idiomatic Redux: Using Reselect Selectors for Encapsulation and Performance for more information on why and how to use selector functions.)

Now that our <TodoList /> is connected to the store. It should receive the list of todos, map over them, and pass each todo to the <Todo /> component. <Todo /> will in turn render them to the screen. Now try adding a todo. It should come up on our todo list!

We will connect more components. Before we do this, let’s pause and learn a bit more about connect first.

Common ways of calling `connect`

Depending on what kind of components you are working with, there are different ways of calling connect , with the most common ones summarized as below:

Do Not Subscribe to the Store

Subscribe to the Store

If you call connect without providing any arguments, your component will:

not re-render when the store changes
receive props.dispatch that you may use to manually dispatch action
// ... Component export default connect()(Component) // Component will receive dispatch (just like our !)

If you call connect with only mapStateToProps, your component will:

subscribe to the values that mapStateToProps extracts from the store, and re-render only when those values have changed
receive props.dispatch that you may use to manually dispatch action
// ... Component const mapStateToProps = (state) => state.partOfState export default connect(mapStateToProps)(Component)

If you call connect with only mapDispatchToProps, your component will:

not re-render when the store changes
receive each of the action creators you inject with mapDispatchToProps as props and automatically dispatch the actions upon being called
import { addTodo } from './actionCreators' // ... Component export default connect(null, { addTodo })(Component)

If you call connect with both mapStateToProps and mapDispatchToProps, your component will:

subscribe to the values that mapStateToProps extracts from the store, and re-render only when those values have changed
receive all of the action creators you inject with mapDispatchToProps as props and automatically dispatch the actions upon being called.
import * as actionCreators from './actionCreators' // ... Component const mapStateToProps = (state) => state.partOfState export default connect(mapStateToProps, actionCreators)(Component)

These four cases cover the most basic usages of connect. To read more about connect, continue reading our API section that explains it in more detail.

Now let’s connect the rest of our <TodoApp />.

How should we implement the interaction of toggling todos? A keen reader might already have an answer. If you have your environment set up and have followed through up until this point, now is a good time to leave it aside and implement the feature by yourself. There would be no surprise that we connect our <Todo /> to dispatch toggleTodo in a similar way:

// components/Todo.js

// ... other imports
import { connect } from "react-redux";
import { toggleTodo } from "../redux/actions";

const Todo = // ... component implementation

export default connect(
  null,
  { toggleTodo }
)(Todo);

Now our todo’s can be toggled complete. We’re almost there!

Finally, let’s implement our VisibilityFilters feature.

The <VisibilityFilters /> component needs to be able to read from the store which filter is currently active, and dispatch actions to the store. Therefore, we need to pass both a mapStateToProps and mapDispatchToProps. The mapStateToProps here can be a simple accessor of the visibilityFilter state. And the mapDispatchToProps will contain the setFilter action creator.

// components/VisibilityFilters.js

// ... other imports
import { connect } from "react-redux";
import { setFilter } from "../redux/actions";

const VisibilityFilters = // ... component implementation

const mapStateToProps = state => {
  return { activeFilter: state.visibilityFilter };
};
export default connect(
  mapStateToProps,
  { setFilter }
)(VisibilityFilters);

Meanwhile, we also need to update our <TodoList /> component to filter todos according to the active filter. Previously the mapStateToProps we passed to the <TodoList /> connect function call was simply the selector that selects the whole list of todos. Let’s write another selector to help filtering todos by their status.

// redux/selectors.js

// ... other selectors
export const getTodosByVisibilityFilter = (store, visibilityFilter) => {
  const allTodos = getTodos(store)
  switch (visibilityFilter) {
    case VISIBILITY_FILTERS.COMPLETED:
      return allTodos.filter((todo) => todo.completed)
    case VISIBILITY_FILTERS.INCOMPLETE:
      return allTodos.filter((todo) => !todo.completed)
    case VISIBILITY_FILTERS.ALL:
    default:
      return allTodos
  }
}

And connecting to the store with the help of the selector:

// components/TodoList.js

// ...

const mapStateToProps = (state) => {
  const { visibilityFilter } = state
  const todos = getTodosByVisibilityFilter(state, visibilityFilter)
  return { todos }
}

export default connect(mapStateToProps)(TodoList)