Tutorial: Tic-Tac-Toe – React

You will build a small tic-tac-toe game during this tutorial. This tutorial does not assume any existing React knowledge. The techniques you’ll learn in the tutorial are fundamental to building any React app, and fully understanding it will give you a deep understanding of React.
This tutorial is designed for people who prefer to learn by doing and want to quickly try making something tangible. If you prefer learning each concept step by step, start with Describing the UI.
The tutorial is divided into several sections:
In this tutorial, you’ll build an interactive tic-tac-toe game with React.
You can see what it will look like when you’re finished here:
If the code doesn’t make sense to you yet, or if you are unfamiliar with the code’s syntax, don’t worry! The goal of this tutorial is to help you understand React and its syntax.
We recommend that you check out the tic-tac-toe game above before continuing with the tutorial. One of the features that you’ll notice is that there is a numbered list to the right of the game’s board. This list gives you a history of all of the moves that have occurred in the game, and it is updated as the game progresses.
Once you’ve played around with the finished tic-tac-toe game, keep scrolling. You’ll start with a simpler template in this tutorial. Our next step is to set you up so that you can start building the game.
In the live code editor below, click Fork in the top-right corner to open the editor in a new tab using the website CodeSandbox. CodeSandbox lets you write code in your browser and preview how your users will see the app you’ve created. The new tab should display an empty square and the starter code for this tutorial.
You can also follow this tutorial using your local development environment. To do this, you need to:
If you get stuck, don’t let this stop you! Follow along online instead and try a local setup again later.
Now that you’re set up, let’s get an overview of React!
In CodeSandbox you’ll see three main sections:
The App.js file should be selected in the Files section. The contents of that file in the code editor should be:
The browser section should be displaying a square with an X in it like this:
Now let’s have a look at the files in the starter code.
The code in App.js creates a component. In React, a component is a piece of reusable code that represents a part of a user interface. Components are used to render, manage, and update the UI elements in your application. Let’s look at the component line by line to see what’s going on:
The first line defines a function called Square. The export JavaScript keyword makes this function accessible outside of this file. The default keyword tells other files using your code that it’s the main function in your file.
The second line returns a button. The return JavaScript keyword means whatever comes after is returned as a value to the caller of the function. <button> is a JSX element. A JSX element is a combination of JavaScript code and HTML tags that describes what you’d like to display. className="square" is a button property or prop that tells CSS how to style the button. X is the text displayed inside of the button and </button> closes the JSX element to indicate that any following content shouldn’t be placed inside the button.
Click on the file labeled styles.css in the Files section of CodeSandbox. This file defines the styles for your React app. The first two CSS selectors (* and body) define the style of large parts of your app while the .square selector defines the style of any component where the className property is set to square. In your code, that would match the button from your Square component in the App.js file.
Click on the file labeled index.js in the Files section of CodeSandbox. You won’t be editing this file during the tutorial but it is the bridge between the component you created in the App.js file and the web browser.
Lines 1-5 bring all the necessary pieces together:
The remainder of the file brings all the pieces together and injects the final product into index.html in the public folder.
Let’s get back to App.js. This is where you’ll spend the rest of the tutorial.
Currently the board is only a single square, but you need nine! If you just try and copy paste your square to make two squares like this:
You’ll get this error:
React components need to return a single JSX element and not multiple adjacent JSX elements like two buttons. To fix this you can use Fragments (<> and </>) to wrap multiple adjacent JSX elements like this:
Great! Now you just need to copy-paste a few times to add nine squares and…
Oh no! The squares are all in a single line, not in a grid like you need for our board. To fix this you’ll need to group your squares into rows with divs and add some CSS classes. While you’re at it, you’ll give each square a number to make sure you know where each square is displayed.
In the App.js file, update the Square component to look like this:
The CSS defined in styles.css styles the divs with the className of board-row. Now that you’ve grouped your components into rows with the styled divs you have your tic-tac-toe board:
But you now have a problem. Your component named Square, really isn’t a square anymore. Let’s fix that by changing the name to Board:
At this point your code should look something like this:
Psssst… That’s a lot to type! It’s okay to copy and paste code from this page. However, if you’re up for a little challenge, we recommend only copying code that you’ve manually typed at least once yourself.
Next, you’ll want to change the value of a square from empty to “X” when the user clicks on the square. With how you’ve built the board so far you would need to copy-paste the code that updates the square nine times (once for each square you have)! Instead of copy-pasting, React’s component architecture allows you to create a reusable component to avoid messy, duplicated code.
First, you are going to copy the line defining your first square (<button