React-Redux connect(): when and how to use it

Hi, my name is Andrey Okhotnikov and I'm JavaScript developer. I develop single page applacations , and in my spare time I help professional developers become even more professional.

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React provides two major mechanisms for providing data to components namely: props and state. While props are read-only and allow a parent component to pass attributes to a child component, state is local and encapsulated within the component and can change at any time in the component’s lifecycle.

Since state is a very powerful mechanism for building powerful and dynamic React apps, it becomes necessary that state is properly managed in the application. Several libraries already exist, that provide a well-structured architecture for managing application state such as Flux, Redux, MobX.

Redux is a predictable state container for JavaScript apps ranging from vanilla apps to framework apps. It has a very tiny footprint and yet allows you to write consistent apps that can run in any environment:

This guide is based on state management in React applications with Redux using react-redux. It is not an introduction to either React or Redux.

It assumes you already have a fundamental understanding of React and the Redux architecture and API. If that’s not the case then you can check the React documentation here and the Redux documentation here.

React-Redux

The react-redux package provides React bindings for the Redux state container making it very easy for a React application to be connected to a Redux store. This allows you to separate your React application components based on their connection to the Redux store as follows:

1. Presentational Components — These components are only concerned with how things look and are not aware of the Redux state. They get their data from props and may trigger callbacks passed to them via props.
2. Container Components — These components are responsible for how things work and are fully aware of the Redux state. They are often created using React Redux and may dispatch Redux actions. They also subscribe to changes in the Redux state.

You can learn more about this separation of concerns from this article. In this guide, our major focus will be on container components that are connected to the Redux state using react-redux.

The react-redux package exposes a very simple interface, and all you should be interested in is just the following:

1. <Provider store> — Wraps the React application and makes the Redux state available to all container components in the application’s hierarchy
2. connect([mapStateToProps], [mapDispatchToProps], [mergeProps], [options]) — Creates a higher-order component for making container components out of base React components

You can install react-redux in your project as follows:

npm install react-redux --save

Given that you already have a Redux store setup for your React application, here is how you can connect the app to the Redux store:

With this setup, you can now create container components, that are connected to the Redux store, within the hierarchy of the AppRootComponentusing the connect() API.

When to use connect()

1. Creating container components.

As stated in the previous section, the react-redux connect() API is used for creating container elements that are connected to the Redux store. The Redux store to be connected to is derived from the topmost ancestor of the component using the React context mechanism. You have no need for connect() if you are only creating a presentational component.

Whether you want to just get data from the Redux store, or you want to dispatch actions on the Redux store, or you want to do both in your React component, you can make the component a container component by wrapping it in a higher-order component returned by react-redux connect():

2. Avoiding manual subscription to the Redux store.

You could create a container component yourself and manually subscribe the component to the Redux store using store.subscribe(). However, using react-redux connect() comes with some performance improvements and optimizations which you may not be able to implement in your application.

In the following code snippet, we attempt to manually create a container component and connect it to the Redux store by subscribing to the store, in order to achieve a similar functionality as with the previous code snippet:

react-redux connect() also provides additional flexibility, allowing you to configure container components to receive dynamic props based on the props initially passed to them. This is useful for selecting a slice of the Redux state based on props, or to bind action creators to a particular variable from props.

If your React application uses multiple Redux stores, react-redux connect()allows you to easily specify which store a container component should be connected to.

Anatomy of connect()

The connect() function provided by react-redux can take up to four arguments, all of which are optional. Calling the connect() function returns a higher-order component which can be used to wrap any React component.

Since a higher-order component is returned by connect(), it has to be invoked again with the base React component in order to convert it to a container component:

const ContainerComponent = connect()(BaseComponent);

Here is the signature of the connect() function:

connect([mapStateToProps], [mapDispatchToProps], [mergeProps], [options])

mapStateToProps(state, [ownProps]) => stateProps

This argument is a function that returns either a plain object or another function. Passing this argument subscribes the container component to the Redux store updates, which means that the mapStateToProps function will be invoked each time the store updates. If you are not interested in store updates, leave it as undefined or null.

mapStateToProps is declared with two parameters, the second one being optional. The first parameter is the current state of the Redux store. The second parameter, if passed, is an object of the props passed to the component:

If a plain object is returned from mapStateToProps, the returned statePropsobject is merged into the component’s props. You can access these props in the component as follows:

However, if a function is returned, that function is used as mapStateToPropsfor each instance of the component. This can be useful for improving the rendering performance and for memoization.

mapDispatchToProps(dispatch, [ownProps]) => dispatchProps

This argument can either be an object or a function that returns either a plain object or another function. To better illustrate how mapDispatchToProps works, you will need to have some action creators.

For example, say you have the following action creators:

1. Default Implementation

If you don’t supply your own mapDispatchToProps object or function, the default implementation will be used, which simply injects the store’s dispatch method as a prop to the component.

You can use the dispatch prop in your component as follows:

2. Passing an object

If an object is passed for this argument, each function in the object will be taken to be a Redux action creator and will be wrapped into a call to the store’s dispatch method so that it can be called directly. The resulting dispatchProps object of action creators will be merged into the component’s props.

The following code snippet shows how you can define mapDispatchToProps by supplying an object of action creators, and how the action creators can be used as props to your React component:

3. Passing a function

If a function is passed, it is left to you to return an object of dispatchPropsthat binds action creators using the store’s dispatch method. The function takes the store’s dispatch as its first parameter. As with mapStateToProps, it can also take an optional ownProps second parameter that maps to the original props passed to the component.

If this function returns another function, then the returned function is used as mapDispatchToProps instead, which can be useful for improving rendering performance and memoization.

The bindActionCreators() helper provided by Redux can be used within this function to bind action creators to the store’s dispatch method.

The following code snippet shows how you can define mapDispatchToProps by supplying a function, and how the bindActionCreators() helper can be used to bind the comment action creators to a React component’s props.actions:

mergeProps(stateProps, dispatchProps, ownProps) => props

This argument, if passed, is a function that takes three parameters — namely:

- stateProps — the props object returned from a call to mapStateToProps()

- dispatchProps — the action creators props object from mapDispatchToProps()

- ownProps — the original props received by the component.

This function returns a plain object of props that will be passed to the wrapped component. This is useful for conditionally mapping part of the Redux store’s state or action creators based on props.

When this function is not supplied, the default implementation is as follows:

options

The options object, if specified, contains options for modifying the behavior of connect(). connect() is a special implementation of connectAdvanced(), it accepts most of the options available to connectAdvanced() with some additional options.

You can refer to this documentation to see all the options available to connect() and how they can modify its behavior.

How to use connect()

Setting the store

Before converting a regular React component to a container component using connect(), you have to specify the Redux store the component will be connected to.

Assume that you have a container component named NewComment for adding a new comment to a post and also showing a button to submit the comment. The component might look like the following code snippet:

For you to actually use this component in your application, you will have to specify the Redux store the component must be connected to, otherwise, you will get an error.

This can be done in two ways:

1. Set the store prop on the container component

The first way is to specify the Redux store on the component by passing a reference to the Redux store as the value of the store prop of the component:

2. Set the store prop on a <Provider> component

If you wish to set the Redux store once for your application, then this is the way to go. This is usually the case for apps that use only one Redux store.

react-redux provides a <Provider> component which can be used to wrap the root application component. It accepts a store prop which expects a reference to the Redux store you want to use for your application. The storeis passed down to container components down the app’s hierarchy using React’s context mechanism:

Accessing ownProps

As stated earlier, the mapStateToProps and mapDispatchToProps functions passed to connect() can be declared with the ownProps of the component as the second parameter.

However, there is a caveat. If the number of mandatory parameters of the declared function is less than 2, then ownProps will never be passed. But if the function is declared with no mandatory parameters or at least 2parameters, then ownProps is passed.

Here are a few scenarios:

1. Declared with no parameters

Here, ownProps is passed because the function is declared with no mandatory parameters. Hence, the following will also work in a similar fashion, using the new ES6 rest parameters syntax:

2. Declared with one parameter

Here, there is only one parameter, state. Hence, arguments[1] is undefinedbecause ownProps is not passed.

3. Declared with default parameter

Here, there is only one mandatory parameter, state, because the second ownProps parameter is optional since a default value has been specified for it. Hence, since there is only one mandatory parameter, ownProps is not passed and as a result it maps to the default value that was assigned to it— {}.

4. Declared with two parameters

This is pretty straightforward. ownProps is passed here since the function is declared with two mandatory parameters.

Conclusion

In this guide, you have seen when and how to use the connect() API provided by the react-redux package to create container components that are connected to the Redux state.

While this guide, covers much of the anatomy of the connect() API and its usage, it did not extensively show use case examples. You can find more of these in this documentation.

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