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React Fundamentals / Thinking in React

Learning objectives

  • Differentiate between declarative and imperative programming
  • Identify what's state and which components should hold it
  • Determine when to lift state up

Teaching method

  1. We try to foster critical thinking.
    • ⬆️ Discovery ⬇️ Instruction
  2. We don’t explain everything you need to know before the exercise:
    • Learn by doing (and teaching ;)
    • The exercise is meant to help you build a mental model

More about our teaching method here.


You need to be comfortable writing JavaScript and HTML to do this exercise. The exercise uses the following ES6 & ES5 features:

You need to have node and npm installed on your computer.

If you find this exercise too difficult

If you find the exercise too difficult we recommend you to do the following basic React course from freeCodeCamp before.

🥑 Before we start the exercise

Before you start, we are going to use the useState hook in this exercise.

import React, { useState } from 'react';

function Example() {
  const [count, setCount] = useState(0);

  return (
      <p>You clicked {count} times</p>
      <button onClick={() => setCount(count + 1)}>
        Click me

The goal of this exercise is to understand what's state and how to reason about it. The goal of this exercise is not to learn how the useState hook works. How we store the state is an implementation detail. The principles you'll learn in this exercise can be applied to classes & this.setState, hooks, and some other state management libraries.

Getting started

git clone
cd thinking-in-react
yarn start

🤸‍♀️ Exercise

  • 1. Refactor the “about” and “footer” sections by creating a function component for each. Make sure everything works. Hint, you can look at the src/components/Header.js as an example.

  • 2. Refactor the navbar by creating a Function Component. Pass the dependencies (toggleMenu in this case) via props. Make sure everything works by clicking on the "Training" button at the top right of the screen. Hint, you can look at the src/components/Header.js as an example (heads up, use curly brackets to pass a prop as a function)

  • 3. Refactor the books section in App.js by creating a new function component called Books. The <Books> component will have the JSX related to books. This task is a stepping stone, keep all the state in <App> and pass any needed state from <App> to <Books> via props. You'll refactor and improve this code again in the next task. Make sure everything works.

  • 4. Is there any state in App.js that should be in the <Books> component? Refactor <Books> if appropriate.

  • 5. Break <Books> down into two smaller components: <BookList> and <BookFilter>. <BookList> will be responsible for displaying the books. <BookFilter> will be responsible for filtering the books. Is there any state in <Books> that should be moved into <BookList> or <BookFilter>?

🏋️‍♀️ Bonus exercise

  • Can we move the isMenuOpen state inside the menu? Does it conflict with the idea of "lifting state up".
  • If you look at the React Profiler when you open and close the menu, is the whole app being rendered? If so, how can we avoid that and still lift the state up?

Articles and links


This material is available for private, non-commercial use under the GPL version 3.


The goal of this exercise is to learn how to think in React, by



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