What is useContext hook in react

Posted 4 years ago by Ryan Dhungel

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Estimated read time: 8 seconds

You can create context using React.createContext such as const ThemeContext = React.createContext()
Once created, you will provide it to the components using <ThemeContext.Provider>
useContext hook > useContext(MyContext) accepts object/value returned from React.createContext
A component calling useContext will always re-render when the context value changes

Background color toggle app using context value

Try this example. This will allow you to change the background color of your page by accessing value from context:

import React, { useContext, useState, useEffect } from "react";

// theme object
const themes = {
  red: "#ff0000",
  green: "#00ff00"
};

// create context
const ThemeContext = React.createContext(themes);

// provide context to components
const App = () => {
  return (
    <ThemeContext.Provider value={themes}>
      <Home />
    </ThemeContext.Provider>
  );
};

const Home = () => {
  const themes = useContext(ThemeContext);
  const [toggle, setToggle] = useState(false);

  useEffect(() => {
    console.log(themes);
    // value of toggle in the state is true or not?
    document.body.style.backgroundColor = toggle ? themes.green : themes.red;
  }, [toggle]); // run useEffect when value of toggle changes

  // on button click set toggle state
  return <button onClick={() => setToggle(!toggle)}>Switch color</button>;
};

export default App;

Example of updating react context from child components

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

// create a context with default value of string and function
const UserContext = React.createContext({
  name: "",
  setName: f => f // () => {}
});

// provide context to components
const App = () => {
  // state
  const [name, setName] = useState("");
  // destructure both the state value (language) and state set method (setLanguage)
  // to send down value using context provider to context consumer/components
  const value = { name, setName };

  // provide context to child components
  return (
    <>
      {name && <p>Your name is: {name}</p>}
      <UserContext.Provider value={value}>
        <UserForm />
      </UserContext.Provider>
    </>
  );
};

// component with its own state for handling form data
// it also has access to parent state (name) and methods (setName)
const UserForm = () => {
  const { name, setName } = useContext(UserContext);
  const [typedName, setTypedName] = useState("");

  const resetName = () => {
    setName("");
    setTypedName("");
  };

  return (
    <div>
      <input
        type="text"
        onChange={event => setTypedName(event.target.value)}
        value={typedName}
      />
      <button onClick={() => setName(typedName)}>
        Click here to show name in parent component
      </button>
      <button onClick={resetName}>Reset</button>
    </div>
  );
};

export default App;

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