React Context API is a way to essentially create global variables that can be passed around in a React app. This is the alternative to "prop drilling", or passing props from grandparent to parent to child, and so on. Context is often touted as a simpler, lighter solution to using Redux for state management. I haven't used Redux myself yet, but every time I use React's Context API, I have to look it up because it doesn't seem obvious to me.

I'm going to leave some brief, concise steps to getting started with Context here.

Prerequisite

Read Getting Started with React or Build a React App with Hooks if you don't know React or React Hooks yet.

Create Context

Imagine I have some information I want to be available anywhere or everywhere throughout a React app. A theme might be implemented using Context - for example, on this site I have Context serving three themes: dark mode, light mode, and MS-DOS mode (on the 404 page).In this simple example, I'll use a logged in user.

I'll create Context, and call it UserContext . This will also give me UserContext.Provider and UserContext.Consumer . What these two components do is straightforward:

Provider - The component that provides the value

- The component that provides the value Consumer - A component that is consuming the value

So I'll create it with React.createContext() in a new file called UserContext.js .

src/UserContext.js

import React from 'react' const UserContext = React . createContext ( ) export const UserProvider = UserContext . Provider export const UserConsumer = UserContext . Consumer export default UserContext

I'm passing in an empty object value here to represent that I might be filling in this data later with an API call. You can pre-populate this with whatever data you want, in case you're not retrieving the data through an API.

React . createContext ( true )

Providing Context

The provider always needs to exist as a wrapper around the parent element, no matter how you choose to consume the values. I'll wrap the entire App component in the Provider . I'm just creating some value ( user ) and passing it down as the Provider value prop.

src/App.js

import React from 'react' import HomePage from './HomePage' import { UserProvider } from './UserContext' function App ( ) { const user = { name : 'Tania' , loggedIn : true } return ( < UserProvider value = { user } > < HomePage /> </ UserProvider > ) }

Now any child, grandchild, great-grandchild, and so on will have access to user as a prop. Unfortunately, retrieving that value is slightly more involved than simply getting it like you might with this.props or this.state .

Consuming Context

The way you provide Context is the same for class and functional components, but consuming it is a little different for both.

Class component

The most common way to access Context from a class component is via the static contextType . If you need the value from Context outside of render , or in a lifecycle method, you'll use it this way.

src/HomePage.js (class example)

import React , { Component } from 'react' import UserContext from './UserContext' class HomePage extends Component { static contextType = UserContext componentDidMount ( ) { const user = this . context console . log ( user ) } render ( ) { return < div > { user . name } </ div > } }

The traditional way to retrieve Context values was by wrapping the child component in the Consumer . From there, you would be able to access the value prop as props . You may still see this, but it's more of a legacy way of accessing Context.

src/HomePage.js (class example)

import React , { Component } from 'react' import { UserConsumer } from './UserContext' class HomePage extends Component { render ( ) { return ( < UserConsumer > { ( props ) => { return < div > { props . name } </ div > } } </ UserConsumer > ) } }

Functional component and Hooks

For functional components, you'll use useContext , such as in the example below. This is the equivalent of static contextType .

src/HomePage.js

import React , { useContext } from 'react' import UserContext from './UserContext' export const HomePage = ( ) => { const user = useContext ( UserContext ) return < div > { user . name } </ div > }

Updating Context

Updating context is not much different than updating regular state. We can create a wrapper class that contains the state of Context and the means to update it.

src/UserContext.js

import React , { Component } from 'react' const UserContext = React . createContext ( ) class UserProvider extends Component { state = { user : { } , } setUser = ( user ) => { this . setState ( ( prevState ) => ( { user } ) ) } render ( ) { const { children } = this . props const { user } = this . state const { setUser } = this return ( < UserContext.Provider value = { { user , setUser , } } > { children } </ UserContext.Provider > ) } } export default UserContext export { UserProvider }

Now you can update and view the user from the Context method.

import React , { Component } from 'react' import UserContext from './UserContext' class HomePage extends Component { static contextType = UserContext render ( ) { const { user , setUser } = this . context return ( < div > < button onClick = { ( ) => { const newUser = { name : 'Joe' , loggedIn : true } setUser ( newUser ) } } > Update User </ button > < p > { ` Current User: ${ user . name } ` } </ p > </ div > ) } }

In my opinion, the biggest downfall of Context API with classes is that you cannot use multiple static contextTypes in one component. This leads to the necessity of having one really big Context for all global state in an application, so it's not sufficient for a large application. The method of creating a wrapper for Context is also difficult to test.

Conclusion

To summarize:

Use const ___Context = React.createContext() to create context.

to create context. Pull ___Context.Provider and ___Context.Consumer out of ___Context

and out of Wrap Provider around your parent component.

around your parent component. A class can consume with static contextType = ___Context

A functional component can consume with const x = useContext(___Context)

Hope this helps!