Tutorial: Tic-Tac-Toe

The tutorial is divided into several sections:

• Setup for the tutorial will give you a starting point to follow the tutorial.
• Overview will teach you the fundamentals of React: components, props, and state.
• Completing the game will teach you the most common techniques in React development.
• Adding time travel will give you a deeper insight into the unique strengths of React.

What are you building?

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.

Setup for the tutorial

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.

Overview

Now that you’re set up, let’s get an overview of React!

Inspecting the starter code

In CodeSandbox you’ll see three main sections:

1. The Files section with a list of files like App.js, index.js, styles.css and a folder called public
2. The code editor where you’ll see the source code of your selected file
3. The browser section where you’ll see how the code you’ve written will be displayed

The App.js file should be selected in the Files section. The contents of that file in the code editor should be:

export default function Square() {
  return <button className="square">X</button>;
}

The browser section should be displaying a square with a X in it like this:

Now let’s have a look at the files in the starter code.

App.js

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:

export default function Square() {
  return <button className="square">X</button>;
}

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.

export default function Square() {
  return <button className="square">X</button>;
}

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.

styles.css

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.

index.js

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.

import {StrictMode} from 'react';
import {createRoot} from 'react-dom/client';
import './styles.css';

import App from './App';

Lines 1-5 brings all the necessary pieces together:

• React
• React’s library to talk to web browsers (React DOM)
• the styles for your components
• the component you created in App.js.

The remainder of the file brings all the pieces together and injects the final product into index.html in the public folder.

Building the board

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:

export default function Square() {
  return <button className="square">X</button><button className="square">X</button>;
}

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:

export default function Square() {
  return (
    <>
      <button className="square">X</button>
      <button className="square">X</button>
    </>
  );
}

Now you should see:

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:

export default function Square() {
  return (
    <>
      <div className="board-row">
        <button className="square">1</button>
        <button className="square">2</button>
        <button className="square">3</button>
      </div>
      <div className="board-row">
        <button className="square">4</button>
        <button className="square">5</button>
        <button className="square">6</button>
      </div>
      <div className="board-row">
        <button className="square">7</button>
        <button className="square">8</button>
        <button className="square">9</button>
      </div>
    </>
  );
}

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:

export default function Board() {
  //...
}

At this point your code should look something like this:

Passing data through props

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 className="square">1</button>) from your Board component into a new Square component:

function Square() {
  return <button className="square">1</button>;
}

export default function Board() {
  // ...
}

Then you’ll update the Board component to render that Square component using JSX syntax:

// ...
export default function Board() {
  return (
    <>
      <div className="board-row">
        <Square />
        <Square />
        <Square />
      </div>
      <div className="board-row">
        <Square />
        <Square />
        <Square />
      </div>
      <div className="board-row">
        <Square />
        <Square />
        <Square />
      </div>
    </>
  );
}

Note how unlike the browser divs, your own components Board and Square must start with a capital letter.

Let’s take a look:

Oh no! You lost the numbered squares you had before. Now each square says “1”. To fix this, you will use props to pass the value each square should have from the parent component (Board) to its child (Square).

Update the Square component to read the value prop that you’ll pass from the Board:

function Square({ value }) {
  return <button className="square">1</button>;
}

function Square({ value }) indicates the Square component can be passed a prop called value.

Now you want to display that value instead of 1 inside every square. Try doing it like this:

function Square({ value }) {
  return <button className="square">value</button>;
}

Oops, this is not what you wanted:

You wanted to render the JavaScript variable called value from your component, not the word “value”. To “escape into JavaScript” from JSX, you need curly braces. Add curly braces around value in JSX like so:

function Square({ value }) {
  return <button className="square">{value}</button>;
}

For now, you should see an empty board:

This is because the Board component hasn’t passed the value prop to each Square component it renders yet. To fix it you’ll add the value prop to each Square component rendered by the Board component:

export default function Board() {
  return (
    <>
      <div className="board-row">
        <Square value="1" />
        <Square value="2" />
        <Square value="3" />
      </div>
      <div className="board-row">
        <Square value="4" />
        <Square value="5" />
        <Square value="6" />
      </div>
      <div className="board-row">
        <Square value="7" />
        <Square value="8" />
        <Square value="9" />
      </div>
    </>
  );
}

Now you should see a grid of numbers again:

Your updated code should look like this:

Making an interactive component

Let’s fill the Square component with an X when you click it. Declare a function called handleClick inside of the Square. Then, add onClick to the props of the button JSX element returned from the Square:

function Square({ value }) {
  function handleClick() {
    console.log('clicked!');
  }

  return (
    <button
      className="square"
      onClick={handleClick}
    >
      {value}
    </button>
  );
}

If you click on a square now, you should see a log saying "clicked!" in the Console tab at the bottom of the Browser section in CodeSandbox. Clicking the square more than once will log "clicked!" again. Repeated console logs with the same message will not create more lines in the console. Instead, you will see an incrementing counter next to your first "clicked!" log.

As a next step, you want the Square component to “remember” that it got clicked, and fill it with an “X” mark. To “remember” things, components use state.

React provides a special function called useState that you can call from your component to let it “remember” things. Let’s store the current value of the Square in state, and change it when the Square is clicked.

Import useState at the top of the file. Remove the value prop from the Square component. Instead, add a new line at the start of the Square that calls useState. Have it return a state variable called value:

import { useState } from 'react';

function Square() {
  const [value, setValue] = useState(null);

  function handleClick() {
    //...

value stores the value and setValue is a function that can be used to change the value. The null passed to useState is used as the initial value for this state variable, so value here starts off equal to null.

Since the Square component no longer accepts props anymore, you’ll remove the value prop from all nine of the Square components created by the Board component:

// ...
export default function Board() {
  return (
    <>
      <div className="board-row">
        <Square />
        <Square />
        <Square />
      </div>
      <div className="board-row">
        <Square />
        <Square />
        <Square />
      </div>
      <div className="board-row">
        <Square />
        <Square />
        <Square />
      </div>
    </>
  );
}

Now you’ll change Square to display an “X” when clicked. Replace the console.log("clicked!"); event handler with setValue('X');. Now your Square component looks like this:

function Square() {
  const [value, setValue] = useState(null);

  function handleClick() {
    setValue('X');
  }

  return (
    <button
      className="square"
      onClick={handleClick}
    >
      {value}
    </button>
  );
}

By calling this set function from an onClick handler, you’re telling React to re-render that Square whenever its <button> is clicked. After the update, the Square’s value will be 'X', so you’ll see the “X” on the game board. Click on any Square, and “X” should show up:

Each Square has its own state: the value stored in each Square is completely independent of the others. When you call a set function in a component, React automatically updates the child components inside too.

After you’ve made the above changes, your code will look like this:

React Developer Tools

React DevTools let you check the props and the state of your React components. You can find the React DevTools tab at the bottom of the browser section in CodeSandbox:

To inspect a particular component on the screen, use the button in the top left corner of React DevTools:

Completing the game

By this point, you have all the basic building blocks for your tic-tac-toe game. To have a complete game, you now need to alternate placing “X”s and “O”s on the board, and you need a way to determine a winner.

Lifting state up

Currently, each Square component maintains a part of the game’s state. To check for a winner in a tic-tac-toe game, the Board would need to somehow know the state of each of the 9 Square components.

How would you approach that? At first, you might guess that the Board needs to “ask” each Square for that Square’s state. Although this approach is technically possible in React, we discourage it because the code becomes difficult to understand, susceptible to bugs, and hard to refactor. Instead, the best approach is to store the game’s state in the parent Board component instead of in each Square. The Board component can tell each Square what to display by passing a prop, like you did when you passed a number to each Square.

To collect data from multiple children, or to have two child components communicate with each other, declare the shared state in their parent component instead. The parent component can pass that state back down to the children via props. This keeps the child components in sync with each other and with their parent.

Lifting state into a parent component is common when React components are refactored.

Let’s take this opportunity to try it out. Edit the Board component so that it declares a state variable named squares that defaults to an array of 9 nulls corresponding to the 9 squares:

// ...
export default function Board() {
  const [squares, setSquares] = useState(Array(9).fill(null));
  return (
    // ...
  );
}

Array(9).fill(null) creates an array with nine elements and sets each of them to null. The useState() call around it declares a squares state variable that’s initially set to that array. Each entry in the array corresponds to the value of a square. When you fill the board in later, the squares array will look like this:

['O', null, 'X', 'X', 'X', 'O', 'O', null, null]

Now your Board component needs to pass the value prop down to each Square that it renders:

export default function Board() {
  const [squares, setSquares] = useState(Array(9).fill(null));
  return (
    <>
      <div className="board-row">
        <Square value={squares[0]} />
        <Square value={squares[1]} />
        <Square value={squares[2]} />
      </div>
      <div className="board-row">
        <Square value={squares[3]} />
        <Square value={squares[4]} />
        <Square value={squares[5]} />
      </div>
      <div className="board-row">
        <Square value={squares[6]} />
        <Square value={squares[7]} />
        <Square value={squares[8]} />
      </div>
    </>
  );
}

Next, you’ll edit the Square component to receive the value prop from the Board component. This will require removing the Square component’s own stateful tracking of value and the button’s onClick prop:

function Square({value}) {
  return <button className="square">{value}</button>;
}

At this point you should see an empty tic-tac-toe board:

And your code should look like this:

Each Square will now receive a value prop that will either be 'X', 'O', or null for empty squares.

Next, you need to change what happens when a Square is clicked. The Board component now maintains which squares are filled. You’ll need to create a way for the Square to update the Board’s state. Since state is private to a component that defines it, you cannot update the Board’s state directly from Square.

Instead, you’ll pass down a function from the Board component to the Square component, and you’ll have Square call that function when a square is clicked. You’ll start with the function that the Square component will call when it is clicked. You’ll call that function onSquareClick:

function Square({ value }) {
  return (
    <button className="square" onClick={onSquareClick}>
      {value}
    </button>
  );
}

Next, you’ll add the onSquareClick function to the Square component’s props:

function Square({ value, onSquareClick }) {
  return (
    <button className="square" onClick={onSquareClick}>
      {value}
    </button>
  );
}

Now you’ll connect the onSquareClick prop to a function in the Board component that you’ll name handleClick. To connect onSquareClick to handleClick you’ll pass a function to the onSquareClick prop of the first Square component:

export default function Board() {
  const [squares, setSquares] = useState(Array(9).fill(null));

  return (
    <>
      <div className="board-row">
        <Square value={squares[0]} onSquareClick={handleClick} />
        //...
  );
}

Lastly, you will define the handleClick function inside the Board component to update the squares array holding your board’s state:

export default function Board() {
  const [squares, setSquares] = useState(Array(9).fill(null));

  function handleClick() {
    const nextSquares = squares.slice();
    nextSquares[0] = "X";
    setSquares(nextSquares);
  }

  return (
    // ...
  )
}

The handleClick function creates a copy of the squares array (nextSquares) with the JavaScript slice() Array method. Then, handleClick updates the nextSquares array to add X to the first ([0] index) square.

Calling the setSquares function lets React know the state of the component has changed. This will trigger a re-render of the components that use the squares state (Board) as well as its child components (the Square components that make up the board).

Now you can add X’s to the board… but only to the upper left square. Your handleClick function is hardcoded to update the index for the upper left square (0). Let’s update handleClick to be able to update any square. Add an argument i to the handleClick function that takes the index of the square to update:

export default function Board() {
  const [squares, setSquares] = useState(Array(9).fill(null));

  function handleClick(i) {
    const nextSquares = squares.slice();
    nextSquares[i] = "X";
    setSquares(nextSquares);
  }

  return (
    // ...
  )
}

Next, you will need to pass that i to handleClick. You could try to set the onSquareClick prop of square to be handleClick(0) directly in the JSX like this, but it won’t work:

<Square value={squares[0]} onSquareClick={handleClick(0)} />

Here is why this doesn’t work. The handleClick(0) call will be a part of rendering the board component. Because handleClick(0) alters the state of the board component by calling setSquares, your entire board component will be re-rendered again. But this runs handleClick(0) again, leading to an infinite loop:

Why didn’t this problem happen earlier?

When you were passing onSquareClick={handleClick}, you were passing the handleClick function down as a prop. You were not calling it! But now you are calling that function right away—notice the parentheses in handleClick(0)—and that’s why it runs too early. You don’t want to call handleClick until the user clicks!

You could fix by creating a function like handleFirstSquareClick that calls handleClick(0), a function like handleSecondSquareClick that calls handleClick(1), and so on. You would pass (rather than call) these functions down as props like onSquareClick={handleFirstSquareClick}. This would solve the infinite loop.

However, defining nine different functions and giving each of them a name is too verbose. Instead, let’s do this:

export default function Board() {
  // ...
  return (
    <>
      <div className="board-row">
        <Square value={squares[0]} onSquareClick={() => handleClick(0)} />
        // ...
  );
}

Notice the new () => syntax. Here, () => handleClick(0) is an arrow function, which is a shorter way to define functions. When the square is clicked, the code after the => “arrow” will run, calling handleClick(0).

Now you need to update the other eight squares to call handleClick from the arrow functions you pass. Make sure that the argument for each call of the handleClick corresponds to the index of the correct square:

export default function Board() {
  // ...
  return (
    <>
      <div className="board-row">
        <Square value={squares[0]} onSquareClick={() => handleClick(0)} />
        <Square value={squares[1]} onSquareClick={() => handleClick(1)} />
        <Square value={squares[2]} onSquareClick={() => handleClick(2)} />
      </div>
      <div className="board-row">
        <Square value={squares[3]} onSquareClick={() => handleClick(3)} />
        <Square value={squares[4]} onSquareClick={() => handleClick(4)} />
        <Square value={squares[5]} onSquareClick={() => handleClick(5)} />
      </div>
      <div className="board-row">
        <Square value={squares[6]} onSquareClick={() => handleClick(6)} />
        <Square value={squares[7]} onSquareClick={() => handleClick(7)} />
        <Square value={squares[8]} onSquareClick={() => handleClick(8)} />
      </div>
    </>
  );
};

Now you can again add X’s to any square on the board by clicking on them:

But this time all the state management is handled by the Board component!

This is what your code should look like:

Now that your state handling is in the Board component, the parent Board component passes props to the child Square components so that they can be displayed correctly. When clicking on a Square, the child Square component now asks the parent Board component to update the state of the board. When the Board’s state changes, both the Board component and every child Square re-renders automatically. Keeping the state of all squares in the Board component will allow it to determine the winner in the future.

Let’s recap what happens when a user clicks the top left square on your board to add an X to it:

1. Clicking on the upper left square runs the function that the button received as its onClick prop from the Square. The Square component received that function as its onSquareClick prop from the Board. The Board component defined that function directly in the JSX. It calls handleClick with an argument of 0.
2. handleClick uses the argument (0) to update the first element of the squares array from null to X.
3. The squares state of the Board component was updated, so the Board and all of its children re-render. This causes the value prop of the Square component with index 0 to change from null to X.

In the end the user sees that the upper left square has changed from empty to having a X after clicking it.

Why immutability is important

Note how in handleClick, you call .slice() to create a copy of the squares array instead of modifying the existing array. To explain why, we need to discuss immutability and why immutability is important to learn.

There are generally two approaches to changing data. The first approach is to mutate the data by directly changing the data’s values. The second approach is to replace the data with a new copy which has the desired changes. Here is what it would look like if you mutated the squares array:

const squares = [null, null, null, null, null, null, null, null, null];
squares[0] = 'X';
// Now `squares` is ["X", null, null, null, null, null, null, null, null];

And here is what it would look like if you changed data without mutating the squares array:

const squares = [null, null, null, null, null, null, null, null, null];
const nextSquares = ['X', null, null, null, null, null, null, null, null];
// Now `squares` is unchanged, but `nextSquares` first element is 'X' rather than `null`

The result is the same but by not mutating (changing the underlying data) directly, you gain several benefits.

Immutability makes complex features much easier to implement. Later in this tutorial, you will implement a “time travel” feature that lets you review the game’s history and “jump back” to past moves. This functionality isn’t specific to games—an ability to undo and redo certain actions is a common requirement for apps. Avoiding direct data mutation lets you keep previous versions of the data intact, and reuse them later.

There is also another benefit of immutability. By default, all child components re-render automatically when the state of a parent component changes. This includes even the child components that weren’t affected by the change. Although re-rendering is not by itself noticeable to the user (you shouldn’t actively try to avoid it!), you might want to skip re-rendering a part of the tree that clearly wasn’t affected by it for performance reasons. Immutability makes it very cheap for components to compare whether their data has changed or not. You can learn more about how React chooses when to re-render a component in the memo API reference.

Taking turns

It’s now time to fix a major defect in this tic-tac-toe game: the “O”s cannot be marked on the board.

You’ll set the first move to be “X” by default. Let’s keep track of this by adding another piece of state to the Board component:

function Board() {
  const [xIsNext, setXIsNext] = useState(true);
  const [squares, setSquares] = useState(Array(9).fill(null));

  // ...
}

Each time a player moves, xIsNext (a boolean) will be flipped to determine which player goes next and the game’s state will be saved. You’ll update the Board’s handleClick function to flip the value of xIsNext:

export default function Board() {
  const [xIsNext, setXIsNext] = useState(true);
  const [squares, setSquares] = useState(Array(9).fill(null));

  function handleClick(i) {
    const nextSquares = squares.slice();
    if (xIsNext) {
      nextSquares[i] = "X";
    } else {
      nextSquares[i] = "O";
    }
    setSquares(nextSquares);
    setXIsNext(!xIsNext);
  }

  return (
    //...
  );
}

Now, as you click on different squares, they will alternate between X and O, as they should!

But wait, there’s a problem. Try clicking on the same square multiple times:

The X is overwritten by an O! While this would add a very interesting twist to the game, we’re going to stick to the original rules for now.

When you mark a square with a X or an O you aren’t first checking to see if the square already has a X or O value. You can fix this by returning early. You’ll check to see if the square already has a X or an O. If the square is already filled, you will return in the handleClick function early—before it tries to update the board state.

function handleClick(i) {
  if (squares[i]) {
    return;
  }
  const nextSquares = squares.slice();
  //...
}

Now you can only add X’s or O’s to empty squares! Here is what your code should look like at this point:

Declaring a winner

Now that you show which player’s turn is next, you should also show when the game is won and there are no more turns to make. To do this you’ll add a helper function called calculateWinner that takes an array of 9 squares, checks for a winner and returns 'X', 'O', or null as appropriate. Don’t worry too much about the calculateWinner function; it’s not specific to React:

export default function Board() {
  //...
}

function calculateWinner(squares) {
  const lines = [
    [0, 1, 2],
    [3, 4, 5],
    [6, 7, 8],
    [0, 3, 6],
    [1, 4, 7],
    [2, 5, 8],
    [0, 4, 8],
    [2, 4, 6]
  ];
  for (let i = 0; i < lines.length; i++) {
    const [a, b, c] = lines[i];
    if (squares[a] && squares[a] === squares[b] && squares[a] === squares[c]) {
      return squares[a];
    }
  }
  return null;
}

You will call calculateWinner(squares) in the Board component’s handleClick function to check if a player has won. You can perform this check at the same time you check if a user has clicked a square that already has a X or and O. We’d like to return early in both cases:

function handleClick(i) {
  if (squares[i] || calculateWinner(squares)) {
    return;
  }
  const nextSquares = squares.slice();
  //...
}

To let the players know when the game is over, you can display text such as “Winner: X” or “Winner: O”. To do that you’ll add a status section to the Board component. The status will display the winner if the game is over and if the game is ongoing you’ll display which player’s turn is next:

export default function Board() {
  // ...
  const winner = calculateWinner(squares);
  let status;
  if (winner) {
    status = "Winner: " + winner;
  } else {
    status = "Next player: " + (xIsNext ? "X" : "O");
  }

  return (
    <div>
      <div className="status">{status}</div>
      <div className="board-row">
        // ...
  )
}

Congratulations! You now have a working tic-tac-toe game. And you’ve just learned the basics of React too. So you are the real winner here. Here is what the code should look like:

Adding time travel

As a final exercise, let’s make it possible to “go back in time” to the previous moves in the game.

Storing a history of moves

If you mutated the squares array, implementing time travel would be very difficult.

However, you used slice() to create a new copy of the squares array after every move, and treated it as immutable. This will allow you to store every past version of the squares array, and navigate between the turns that have already happened.

You’ll store the past squares arrays in another array called history, which you’ll store as a new state variable. The history array represents all board states, from the first to the last move, and has a shape like this:

[
  // Before first move
  [null, null, null, null, null, null, null, null, null],
  // After first move
  [null, null, null, null, 'X', null, null, null, null],
  // After second move
  [null, null, null, null, 'X', null, null, null, 'O'],
  // ...
]

Lifting state up, again

You will now write a new top-level component called Game to display a list of past moves. That’s where you will place the history state that contains the entire game history.

Placing the history state into the Game component will let you remove the squares state from its child Board component. Just like you “lifted state up” from the Square component into the Board component, you will now lift it up from the Board into the top-level Game component. This gives the Game component full control over the Board’s data and lets it instruct the Board to render previous turns from the history.

First, add a Game component with export default. Have it render the Board component and some markup:

function Board() {
  // ...
}

export default function Game() {
  return (
    <div className="game">
      <div className="game-board">
        <Board />
      </div>
      <div className="game-info">
        <ol>{/*TODO*/}</ol>
      </div>
    </div>
  );
}

Note that you are removing the export default keywords before the function Board() { declaration and adding them before the function Game() { declaration. This tells your index.js file to use the Game component as the top-level component instead of your Board component. The additional divs returned by the Game component are making room for the game information you’ll add to the board later.

Add some state to the Game component to track which player is next and the history of moves:

export default function Game() {
  const [xIsNext, setXIsNext] = useState(true);
  const [history, setHistory] = useState([Array(9).fill(null)]);
  // ...

Notice how [Array(9).fill(null)] is an array with a single item, which itself is an array of 9 nulls.

To render the squares for the current move, you’ll want to read the last squares array from the history. You don’t need useState for this—you already have enough information to calculate it during rendering:

export default function Game() {
  const [xIsNext, setXIsNext] = useState(true);
  const [history, setHistory] = useState([Array(9).fill(null)]);
  const currentSquares = history[history.length - 1];
  // ...

Next, create a handlePlay function inside the Game component that will be called by the Board component to update the game. Pass xIsNext, currentSquares and handlePlay as props to the Board component:

export default function Game() {
  const [xIsNext, setXIsNext] = useState(true);
  const [history, setHistory] = useState([Array(9).fill(null)]);
  const currentSquares = history[history.length - 1];

  function handlePlay(nextSquares) {
    // TODO
  }

  return (
    <div className="game">
      <div className="game-board">
        <Board xIsNext={xIsNext} squares={currentSquares} onPlay={handlePlay} />
        //...
  )
}

Let’s make the Board component fully controlled by the props it receives. Change the Board component to take three props: xIsNext, squares, and a new onPlay function that Board can call with the updated squares array when a player makes a move. Next, remove the first two lines of the Board function that call useState:

function Board({ xIsNext, squares, onPlay }) {
  function handleClick(i) {
    //...
  }
  // ...
}

Now replace the setSquares and setXIsNext calls in handleClick in the Board component with a single call to your new onPlay function so the Game component can update the Board when the user clicks a square:

function Board({ xIsNext, squares, onPlay }) {
  function handleClick(i) {
    if (calculateWinner(squares) || squares[i]) {
      return;
    }
    const nextSquares = squares.slice();
    if (xIsNext) {
      nextSquares[i] = "X";
    } else {
      nextSquares[i] = "O";
    }
    onPlay(nextSquares);
  }
  //...
}

The Board component is fully controlled by the props passed to it by the Game component. You need to implement the handlePlay function in the Game component to get the game working again.

What should handlePlay do when called? Remember that Board used to call setSquares with an updated array; now it passes the updated squares array to onPlay.

The handlePlay function needs to update Game’s state to trigger a re-render, but you don’t have a setSquares function that you can call any more—you’re now using the history state variable to store this information. You’ll want to update history by appending the updated squares array as a new history entry. You also want to toggle xIsNext, just as Board used to do:

export default function Game() {
  //...
  function handlePlay(nextSquares) {
    setHistory([...history, nextSquares]);
    setXIsNext(!xIsNext);
  }
  //...
}

Here, [...history, nextSquares] creates a new array that contains all the items in history, followed by nextSquares. (You can read the ...history spread syntax as “enumerate all the items in history”.)

For example, if history is [[null,null,null], ["X",null,null]] and nextSquares is ["X",null,"O"], then the new [...history, nextSquares] array will be [[null,null,null], ["X",null,null], ["X",null,"O"]].

At this point, you’ve moved the state to live in the Game component, and the UI should be fully working, just as it was before the refactor. Here is what the code should look like at this point:

Showing the past moves

Since you are recording the tic-tac-toe game’s history, you can now display a list of past moves to the player.

React elements like <button> are regular JavaScript objects; you can pass them around in your application. To render multiple items in React, you can use an array of React elements.

You already have an array of history moves in state, so now you need to transform it to an array of React elements. In JavaScript, to transform one array into another, you can use the array map method:

[1, 2, 3].map((x) => x * 2) // [2, 4, 6]

You’ll use map to transform your history of moves into React elements representing buttons on the screen, and display a list of buttons to “jump” to past moves. Let’s map over the history in the Game component:

export default function Game() {
  const [xIsNext, setXIsNext] = useState(true);
  const [history, setHistory] = useState([Array(9).fill(null)]);
  const currentSquares = history[history.length - 1];

  function handlePlay(nextSquares) {
    setHistory([...history, nextSquares]);
    setXIsNext(!xIsNext);
  }

  function jumpTo(nextMove) {
    // TODO
  }

  const moves = history.map((squares, move) => {
    let description;
    if (move > 0) {
      description = 'Go to move #' + move;
    } else {
      description = 'Go to game start';
    }
    return (
      <li>
        <button onClick={() => jumpTo(move)}>{description}</button>
      </li>
    );
  });

  return (
    <div className="game">
      <div className="game-board">
        <Board xIsNext={xIsNext} squares={currentSquares} onPlay={handlePlay} />
      </div>
      <div className="game-info">
        <ol>{moves}</ol>
      </div>
    </div>
  );
}

You can see what your code should look like below. Note that you should see an error in the developer tools console that says: Warning: Each child in an array or iterator should have a unique "key" prop. Check the render method of `Game`. You’ll fix this error in the next section.