React useContext

React Context

React Context is a way to manage state globally.

It can be used together with the useState Hook to share state between deeply nested components more easily than with useState alone.

The Problem

State should be held by the highest parent component in the stack that requires access to the state.

To illustrate, we have many nested components. The component at the top and bottom of the stack need access to the state.

To do this without Context, we will need to pass the state as "props" through each nested component. This is called "prop drilling".

Example:

import { useState } from 'react';
import { createRoot } from 'react-dom/client';

function Component1() {
  const [user, setUser] = useState("Linus");

  return (
    <>
      <h1>{`Hello ${user}!`}</h1>
      <Component2 user={user} />
    </>
  );
}

function Component2({ user }) {
  return (
    <>
      <h1>Component 2</h1>
      <Component3 user={user} />
    </>
  );
}

function Component3({ user }) {
  return (
    <>
      <h1>Component 3</h1>
      <h2>{`Hello ${user} again!`}</h2>
    </>
  );
}

createRoot(document.getElementById('root')).render(
  <Component1 />
);

Even though component 2 did not need the state, it had to pass the state along so that it could reach component 3.

The Solution

The solution is to create context.

Create Context

To create context, you must Import createContext and initialize it:

import { useState, createContext, useContext } from 'react';
import { createRoot } from 'react-dom/client';

const UserContext = createContext();

Next we'll use the Context Provider to wrap the tree of components that need the state Context.

Context Provider

Wrap child components in the Context Provider and supply the state value.

function Component1() {
  const [user, setUser] = useState("Linus");

  return (
    <UserContext.Provider value={user}>
      <h1>{`Hello ${user}!`}</h1>
      <Component2 />
    </UserContext.Provider>
  );
}

Now, all components in this tree will have access to the user Context.

Use the useContext Hook

In order to use the Context in a child component, we need to access it using the useContext Hook.

First, include the useContext in the import statement:

import { useState, createContext, useContext } from "react";

Then you can access the user Context in all components:

function Component3() {
  const user = useContext(UserContext);

  return (
    <>
      <h1>Component 3</h1>
      <h2>{`Hello ${user} again!`}</h2>
    </>
  );
}

Full Example

Example:

import { useState, createContext, useContext } from 'react';
import { createRoot } from 'react-dom/client';

const UserContext = createContext();

function Component1() {
  const [user, setUser] = useState("Linus");

  return (
    <UserContext.Provider value={user}>
      <h1>{`Hello ${user}!`}</h1>
      <Component2 />
    </UserContext.Provider>
  );
}

function Component2() {
  return (
    <>
      <h1>Component 2</h1>
      <Component3 />
    </>
  );
}

function Component3() {
  const user = useContext(UserContext);

  return (
    <>
      <h1>Component 3</h1>
      <h2>{`Hello ${user} again!`}</h2>
    </>
  );
}

createRoot(document.getElementById('root')).render(
  <Component1 />
);