Tutorial: Intro to React

This tutorial doesn’t assume any existing React knowledge.

Before We Start the Tutorial

We will build a small game during this tutorial. You might be tempted to skip it because you’re not building games — but give it a chance. The techniques you’ll learn in the tutorial are fundamental to building any React app, and mastering it will give you a deep understanding of React.

Tip

This tutorial is designed for people who prefer to learn by doing. If you prefer learning concepts from the ground up, check out our step-by-step guide. You might find this tutorial and the guide complementary to each other.

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.

You don’t have to complete all of the sections at once to get the value out of this tutorial. Try to get as far as you can — even if it’s one or two sections.

What Are We Building?

In this tutorial, we’ll show how to build an interactive tic-tac-toe game with React.

You can see what we’ll be building here: Final Result. If the code doesn’t make sense to you, 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 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.

You can close the tic-tac-toe game once you’re familiar with it. We’ll be starting from a simpler template in this tutorial. Our next step is to set you up so that you can start building the game.

Prerequisites

We’ll assume that you have some familiarity with HTML and JavaScript, but you should be able to follow along even if you’re coming from a different programming language. We’ll also assume that you’re familiar with programming concepts like functions, objects, and arrays.

If you need to review JavaScript, we recommend reading this guide. Note that we’re also using some features from ES6 — a recent version of JavaScript. In this tutorial, we’re using arrow functions, let, and const statements. You can use the Babel REPL to check what ES6 code compiles to.

Setup for the Tutorial

There are two ways to complete this tutorial: you can either write the code in your browser, or you can set up a local development environment on your computer.

Setup Option 1: Write Code in the Browser

This is the quickest way to get started!

First, open this Starter Code in a new tab. The new tab should display an empty tic-tac-toe game board and React code. We will be editing the React code in this tutorial.

You can now skip the second setup option, and go to the Overview section to get an overview of React.

Setup Option 2: Local Development Environment

This is completely optional and not required for this tutorial!

npx create-react-app my-app

cd my-app
cd src

# If you're using a Mac or Linux:
rm -f *

# Or, if you're on Windows:
del *

# Then, switch back to the project folder
cd ..

import React from 'react';
import ReactDOM from 'react-dom';
import './index.css';

Help, I’m Stuck!

If you get stuck, check out the community support resources. In particular, Reactiflux Chat is a great way to get help quickly. If you don’t receive an answer, or if you remain stuck, please file an issue, and we’ll help you out.

Overview

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

What Is React?

React is a declarative, efficient, and flexible JavaScript library for building user interfaces. It lets you compose complex UIs from small and isolated pieces of code called “components”.

React has a few different kinds of components, but we’re going to work with the newer, function-style components:

function ShoppingList(props) {
  return (
    <div className="shopping-list">
      <h1>Shopping List for {props.name}</h1>
      <ul>
        <li>Instagram</li>
        <li>WhatsApp</li>
        <li>Oculus</li>
      </ul>
    </div>
  );
}

// Example usage: <ShoppingList name="Mark" />

We’ll get to the funny XML-like tags soon. We use components to tell React what we want to see on the screen. When our data changes, React will efficiently update and re-render our components.

Here, ShoppingList is a React component. A component takes in parameters, called props (short for “properties”), and returns a hierarchy of views to display via its return value.

The function returns a description of what you want to see on the screen. React takes the description and displays the result. In particular, it returns a React element, which is a lightweight description of what to render. Most React developers use a special syntax called “JSX” which makes these structures easier to write. The <div /> syntax is transformed at build time to React.createElement('div'). The example above is equivalent to:

return React.createElement('div', {className: 'shopping-list'},
  React.createElement('h1', /* ... h1 children ... */),
  React.createElement('ul', /* ... ul children ... */)
);

See full expanded version.

If you’re curious, createElement() is described in more detail in the API reference, but we won’t be using it in this tutorial. Instead, we will keep using JSX.

JSX comes with the full power of JavaScript. You can put any JavaScript expressions within braces inside JSX. Each React element is a JavaScript object that you can store in a variable or pass around in your program.

The ShoppingList component above only renders built-in DOM components like <div /> and <li />. But you can compose and render custom React components too. For example, we can now refer to the whole shopping list by writing <ShoppingList />. Each React component is encapsulated and can operate independently; this allows you to build complex UIs from simple components.

Inspecting the Starter Code

If you’re going to work on the tutorial in your browser, open this code in a new tab: Starter Code. If you’re going to work on the tutorial locally, instead open src/index.js in your project folder (you have already touched this file during the setup).

This Starter Code is the base of what we’re building. We’ve provided the CSS styling so that you only need to focus on learning React and programming the tic-tac-toe game.

By inspecting the code, you’ll notice that we have three React components:

• Square
• Board
• Game

The Square component renders a single <button> and the Board renders 9 squares. The Game component renders a board with placeholder values which we’ll modify later. There are currently no interactive components.

Passing Data Through Props

To get our feet wet, let’s try passing some data from our Board component to our Square component.

We strongly recommend typing code by hand as you’re working through the tutorial and not using copy/paste. This will help you develop muscle memory and a stronger understanding.

In Board’s renderSquare function, change the code to pass a prop called value to the Square:

function Board() {
  function renderSquare(i) {
    return <Square value={i} />;  }
}

Change Square’s return value to show that value by replacing {/* TODO */} with {props.value}:

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

Before:

After: You should see a number in each square in the rendered output.

View the full code at this point

Congratulations! You’ve just “passed a prop” from a parent Board component to a child Square component. Passing props is how information flows in React apps, from parents to children.

Making an Interactive Component

Let’s fill the Square component with an “X” when we click it. First, change the button tag that is returned by the Square component to this:

function Square(props) {
  return (
    <button className="square" onClick={function() { alert('click'); }}>      {props.value}
    </button>
  );
}

If you click on a Square now, you should see an alert in your browser.

Note

To save typing, we will use the arrow function syntax for event handlers here and further below:

function Square(props) {
 return (
   <button className="square" onClick={() => alert('click')}>     {props.value}
   </button>
 );
}

Notice how with onClick={() => alert('click')}, we’re passing a function as the onClick prop. React will only call this function after a click. Forgetting () => and writing onClick={alert('click')} is a common mistake, and would fire the alert every time the component re-renders.

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

React components can have state by calling the useState function. The useState function returns a “state” value and a setter function, both of which should be considered as private to a React component that they’re defined in. Let’s store the current value of the Square and change it when the Square is clicked.

First, we’ll add a useState call to the to initialize the state:

import React, { useState } from 'react'
function Square(props) {
  // `value` is the state and `setValue` is the setter function
  const [value, setValue] = useState(null);
  return (
    <button className="square" onClick={() => alert('click')}>
      {props.value}
    </button>
  );
}

Now we’ll change the Square’s return value to display the current state’s value when clicked:

• Replace props.value with value inside the <button> tag.
• Replace the onClick={...} event handler with onClick={() => setValue('X')}.
• Put the className and onClick props on separate lines for better readability.

After these changes, the <button> tag that is returned by the Square component looks like this:

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

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

By calling setValue from an onClick handler, we tell React to re-render that Square whenever its <button> is clicked. After the update, the Square’s value will be 'X', so we’ll see the X on the game board. If you click on any Square, an X should show up.

When you call a setter function like setValue in a component, React automatically updates the child components inside of it too.

View the full code at this point

Developer Tools

The React Devtools extension for Chrome and Firefox lets you inspect a React component tree with your browser’s developer tools.

The React DevTools let you check the props and the state of your React components.

After installing React DevTools, you can right-click on any element on the page, click “Inspect” to open the developer tools, and the React tabs (“⚛️ Components” and “⚛️ Profiler”) will appear as the last tabs to the right. Use “⚛️ Components” to inspect the component tree.

However, note there are a few extra steps to get it working with CodePen:

1. Log in or register and confirm your email (required to prevent spam).
2. Click the “Fork” button.
3. Click “Change View” and then choose “Debug mode”.
4. In the new tab that opens, the devtools should now have a React tab.

Completing the Game

We now have the basic building blocks for our tic-tac-toe game. To have a complete game, we now need to alternate placing “X”s and “O”s on the board, and we need a way to determine a winner.

Lifting State Up

Currently, each Square component maintains the game’s state. To check for a winner, we’ll maintain the value of each of the 9 squares in one location.

We may think that Board should just ask each Square for the Square’s state. Although this approach is 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, just like we did when we passed a number to each Square.

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

Lifting state into a parent component is common when React components are refactored — let’s take this opportunity to try it out.

Add a useState call to the Board and set a state value called squares to start out with an array of 9 nulls corresponding to the 9 squares:

function Board() {
  const [squares, setSquares] = useState(Array(9).fill(null));
  renderSquare(i) {
    return <Square value={i} />;
  }

When we fill the board in later, the squares array will look something like this:

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

The Board’s renderSquare function currently looks like this:

  function renderSquare(i) {
    return <Square value={i} />;
  }

In the beginning, we passed the value prop down from the Board to show numbers from 0 to 8 in every Square. In a different previous step, we replaced the numbers with an “X” mark determined by Square’s own state. This is why Square currently ignores the value prop passed to it by the Board.

We will now use the prop passing mechanism again. We will modify the Board to instruct each individual Square about its current value ('X', 'O', or null). We have already defined the squares array in the Board’s constructor, and we will modify the Board’s renderSquareunction to read from it:

  function renderSquare(i) {
    return <Square value={squares[i]} />;  }

View the full code at this point

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

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

Instead, we’ll pass down a function from the Board to the Square, and we’ll have Square call that function when a square is clicked. We’ll change renderSquare in Board to:

  function renderSquare(i) {
    return (
      <Square
        value={squares[i]}
        onClick={() => handleClick(i)}      />
    );
  }

Note

We split the returned element into multiple lines for readability, and added parentheses so that JavaScript doesn’t insert a semicolon after return and break our code.

Now we’re passing down two props from Board to Square: value and onClick. The onClick prop is a function that Square can call when clicked. We’ll make the following changes to Square:

• Replace value with props.value in Square’s JSX
• Replace setValue with props.onClick in Square’s JSX
• Delete the useState call from Square because Square no longer keeps track of the game’s state

After these changes, the Square component looks like this:

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

When a Square is clicked, the onClick function provided by the Board is called. Here’s a review of how this is achieved:

1. The onClick prop on the built-in DOM <button> component tells React to set up a click event listener.
2. When the button is clicked, React will call the onClick event handler that is defined in Square.
3. This event handler calls props.onClick(). The Square’s onClick prop was specified by the Board.
4. Since the Board passed onClick={() => handleClick(i)} to Square, the Square calls handleClick(i) when clicked.
5. We have not defined the handleClick() function yet, so our code crashes. If you click a square now, you should see a red error screen saying something like “handleClick is not a function”.

Note

The DOM <button> element’s onClick attribute has a special meaning to React because it is a built-in component. For custom components like Square, the naming is up to you. We could give any name to the Square’s onClick prop or Board’s handleClick function, and the code would work the same. In React, it’s conventional to use on[Event] names for props which represent events and handle[Event] for the functions which handle the events.

When we try to click a Square, we should get an error because we haven’t defined handleClick yet. We’ll now add handleClick to the Board class:

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

  function handleClick(i) {    const newSquares = squares.slice();    newSquares[i] = 'X';    setSquares(newSquares);  }
  function renderSquare(i) {
    return (
      <Square
        value={squares[i]}
        onClick={() => handleClick(i)}
      />
    );
  }

  const status = 'Next player: X';

  return (
    <div>
      <div className="status">{status}</div>
      <div className="board-row">
        {renderSquare(0)}
        {renderSquare(1)}
        {renderSquare(2)}
      </div>
      <div className="board-row">
        {renderSquare(3)}
        {renderSquare(4)}
        {renderSquare(5)}
      </div>
      <div className="board-row">
        {renderSquare(6)}
        {renderSquare(7)}
        {renderSquare(8)}
      </div>
    </div>
  );
}

View the full code at this point

After these changes, we’re again able to click on the Squares to fill them, the same as we had before. However, now the state is stored in the Board component instead of the individual Square components. When the Board’s state changes, the Square components re-render automatically. Keeping the state of all squares in the Board component will allow it to determine the winner in the future.

Since the Square components no longer maintain state, the Square components receive values from the Board component and inform the Board component when they’re clicked. In React terms, the Square components are now controlled components. The Board has full control over them.

Note how in handleClick, we call .slice() to create a copy of the squares array to modify instead of modifying the existing array. We will explain why we create a copy of the squares array in the next section.

Why Immutability Is Important

In the previous code example, we suggested that you use the .slice() method to create a copy of the squares array to modify instead of modifying the existing array. We’ll now 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.

Data Change with Mutation

var player = {score: 1, name: 'Jeff'};
player.score = 2;
// Now player is {score: 2, name: 'Jeff'}

Data Change without Mutation

var player = {score: 1, name: 'Jeff'};

var newPlayer = Object.assign({}, player, {score: 2});
// Now player is unchanged, but newPlayer is {score: 2, name: 'Jeff'}

// Or if you are using object spread syntax proposal, you can write:
// var newPlayer = {...player, score: 2};

The end result is the same but by not mutating (or changing the underlying data) directly, we gain several benefits described below.

Complex Features Become Simple

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

Detecting Changes

Detecting changes in mutable objects is difficult because they are modified directly. This detection requires the mutable object to be compared to previous copies of itself and the entire object tree to be traversed.

Detecting changes in immutable objects is considerably easier. If the immutable object that is being referenced is different than the previous one, then the object has changed.

Determining When to Re-Render in React

The main benefit of immutability is that it helps you build pure components in React. Immutable data can easily determine if changes have been made which helps to determine when a component requires re-rendering.

You can learn more about how you can build pure components by reading Optimizing Performance.

Taking Turns

We now need to fix an obvious defect in our tic-tac-toe game: the “O”s cannot be marked on the board.

We’ll set the first move to be “X” by default. We can set this default by adding a new useState call:

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

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. We’ll update the Board’s handleClick function to flip the value of xIsNext:

  function handleClick(i) {
    const newSquares = squares.slice();
    newSquares[i] = xIsNext ? 'X' : 'O';    setSquares(newSquares);
    setXIsNext(!xIsNext);  }

With this change, “X”s and “O”s can take turns. Try it!

Let’s also change the “status” text in Board so that it displays which player has the next turn:

  const status = 'Next player: ' + (xIsNext ? 'X' : 'O');
  return (
    // the rest has not changed

After applying these changes, you should have this Board component:

function Board() {
  const [squares, setSquares] = useState(Array(9).fill(null));
  const [xIsNext, setXIsNext] = useState(true);
  function handleClick(i) {
    const newSquares = squares.slice();    newSquares[i] = xIsNext ? 'X' : 'O';    setSquares(newSquares);    setXIsNext(!xIsNext);  }

  function renderSquare(i) {
    return (
      <Square
        value={squares[i]}
        onClick={() => handleClick(i)}
      />
    );
  }

  const status = 'Next player: ' + (xIsNext ? 'X' : 'O');
  return (
    <div>
      <div className="status">{status}</div>
      <div className="board-row">
        {renderSquare(0)}
        {renderSquare(1)}
        {renderSquare(2)}
      </div>
      <div className="board-row">
        {renderSquare(3)}
        {renderSquare(4)}
        {renderSquare(5)}
      </div>
      <div className="board-row">
        {renderSquare(6)}
        {renderSquare(7)}
        {renderSquare(8)}
      </div>
    </div>
  );
}

View the full code at this point

Declaring a Winner

Now that we show which player’s turn is next, we should also show when the game is won and there are no more turns to make. Copy this helper function and paste it at the end of the file:

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;
}

Given an array of 9 squares, this function will check for a winner and return 'X', 'O', or null as appropriate.

We will call calculateWinner(squares) in the Board component to check if a player has won. If a player has won, we can display text such as “Winner: X” or “Winner: O”. We’ll replace the status declaration in Board’s render function with this code:

  const winner = calculateWinner(squares);  let status;  if (winner) {    status = 'Winner: ' + winner;  } else {    status = 'Next player: ' + (xIsNext ? 'X' : 'O');  }
  return (
    // the rest has not changed

We can now change the Board’s handleClick function to return early by ignoring a click if someone has won the game or if a Square is already filled:

  function handleClick(i) {
    if (calculateWinner(squares) || squares[i]) {
      return;    }    const newSquares = squares.slice();    newSquares[i] = xIsNext ? 'X' : 'O';
    setSquares(newSquares);
    setXIsNext(!xIsNext);
  }

View the full code at this point

Congratulations! You now have a working tic-tac-toe game. And you’ve just learned the basics of React too. So you’re probably the real winner here.

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

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

We’ll store the past squares arrays in another array called history. The history array represents all board states, from the first to the last move, and has a shape like this:

history = [
  // Before first move
  {
    squares: [
      null, null, null,
      null, null, null,
      null, null, null,
    ]
  },
  // After first move
  {
    squares: [
      null, null, null,
      null, 'X', null,
      null, null, null,
    ]
  },
  // After second move
  {
    squares: [
      null, null, null,
      null, 'X', null,
      null, null, 'O',
    ]
  },
  // ...
]

Now we need to decide which component should own the history state.

Lifting State Up, Again

We’ll want the top-level Game component to display a list of past moves. It will need access to the history to do that, so we will place the history state in the top-level Game component.

Placing the history state into the Game component lets us remove the squares state from its child Board component. Just like we “lifted state up” from the Square component into the Board component, we are now lifting 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, we’ll set up the initial state for the Game component with useState calls:

function Game() {
  const [history, setHistory] = useState([{ squares: Array(9).fill(null) }]);  const [xIsNext, setXIsNext] = useState(true);
  return (
    <div className="game">
      <div className="game-board">
        <Board />
      </div>
      <div className="game-info">
        <div>{/* status */}</div>
        <ol>{/* TODO */}</ol>
      </div>
    </div>
  );
}

Next, we’ll have the Board component receive squares and onClick props from the Game component. Since we now have a single click handler in Board for many Squares, we’ll need to pass the location of each Square into the onClick handler to indicate which Square was clicked. Here are the required steps to transform the Board component:

• Delete the useState calls in Board.
• Replace squares[i] with props.squares[i] in Board’s renderSquare.
• Replace handleClick(i) with props.onClick(i) in Board’s renderSquare.

The Board component now looks like this:

function Board(props) {
  function handleClick(i) {
    if (calculateWinner(squares) || squares[i]) {
      return;
    }
    const newSquares = squares.slice();
    newSquares[i] = xIsNext ? 'X' : 'O';
    setSquares(newSquares);
    setXIsNext(!xIsNext);
  }

  function renderSquare(i) {
    return (
      <Square
        value={props.squares[i]}        onClick={() => props.onClick(i)}      />
    );
  }

  const status = 'Next player: ' + (xIsNext ? 'X' : 'O');

  return (
    <div>
      <div className="status">{status}</div>
      <div className="board-row">
        {renderSquare(0)}
        {renderSquare(1)}
        {renderSquare(2)}
      </div>
      <div className="board-row">
        {renderSquare(3)}
        {renderSquare(4)}
        {renderSquare(5)}
      </div>
      <div className="board-row">
        {renderSquare(6)}
        {renderSquare(7)}
        {renderSquare(8)}
      </div>
    </div>
  );
}

We’ll update the Game component to use the most recent history entry to determine and display the game’s status:

function Game() {
  const [history, setHistory] = useState([{ squares: Array(9).fill(null) }]);
  const [xIsNext, setXIsNext] = useState(true);

  const current = history[history.length - 1];  const winner = calculateWinner(current.squares);  let status;  if (winner) {    status = 'Winner: ' + winner;  } else {    status = 'Next player: ' + (xIsNext ? 'X' : 'O');  }
  return (
    <div className="game">
      <div className="game-board">
        <Board          squares={current.squares}          onClick={(i) => handleClick(i)}
        />
      </div>
      <div className="game-info">
        <div>{status}</div>        <ol>{/* TODO */}</ol>
      </div>
    </div>
  );
}

Since the Game component is now rendering the game’s status, we can remove the corresponding code from the Board. After refactoring, the Board’s return looks like this:

  return (    <div>      <div className="board-row">        {renderSquare(0)}        {renderSquare(1)}
        {renderSquare(2)}
      </div>
      <div className="board-row">
        {renderSquare(3)}
        {renderSquare(4)}
        {renderSquare(5)}
      </div>
      <div className="board-row">
        {renderSquare(6)}
        {renderSquare(7)}
        {renderSquare(8)}
      </div>
    </div>
  );

Finally, we need to move the handleClick function from the Board component to the Game component. We also need to modify handleClick because the Game component’s state is structured differently. Within the Game’s handleClick function, we concatenate new history entries onto history.

  function handleClick(i) {
    const current = history[history.length - 1];    const squares = current.squares.slice();    if (calculateWinner(squares) || squares[i]) {      return;
    }
    squares[i] = xIsNext ? 'X' : 'O';

    setHistory(history.concat([{ squares: squares }]));
    setXIsNext(!xIsNext);  }

Note

Unlike the array push() method you might be more familiar with, the concat() method doesn’t mutate the original array, so we prefer it.

At this point, the Board component only needs the renderSquare function. The game’s state and the handleClick function should be in the Game component.

View the full code at this point

Showing the Past Moves

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

We learned earlier that React elements are first-class JavaScript objects; we can pass them around in our applications. To render multiple items in React, we can use an array of React elements.

In JavaScript, arrays have a map() method that is commonly used for mapping data to other data, for example:

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

Using the map method, we can map our history of moves to 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:

  const current = history[history.length - 1];
  const winner = calculateWinner(current.squares);

  const moves = history.map((step, move) => {    const desc = move ?      'Go to move #' + move :      'Go to game start';    return (      <li>        <button onClick={() => jumpTo(move)}>{desc}</button>      </li>    );  });
  let status;
  if (winner) {
    status = 'Winner: ' + winner;
  } else {
    status = 'Next player: ' + (xIsNext ? 'X' : 'O');
  }

  return (
    <div className="game">
      <div className="game-board">
        <Board
          squares={current.squares}
          onClick={(i) => handleClick(i)}
        />
      </div>
      <div className="game-info">
        <div>{status}</div>
        <ol>{moves}</ol>      </div>
    </div>
  );

View the full code at this point

For each move in the tic-tac-toe game’s history, we create a list item <li> which contains a button <button>. The button has a onClick handler which calls a function called jumpTo(). We haven’t implemented the jumpTo() function yet. For now, we should see a list of the moves that have occurred in the game and a warning in the developer tools console that says:

Warning: Each child in an array or iterator should have a unique “key” prop. Check the JSX in the “Game” component.

Let’s discuss what the above warning means.

Picking a Key

When we render a list, React stores some information about each rendered list item. When we update a list, React needs to determine what has changed. We could have added, removed, re-arranged, or updated the list’s items.

Imagine transitioning from

<li>Alexa: 7 tasks left</li>
<li>Ben: 5 tasks left</li>

to

<li>Ben: 9 tasks left</li>
<li>Claudia: 8 tasks left</li>
<li>Alexa: 5 tasks left</li>

In addition to the updated counts, a human reading this would probably say that we swapped Alexa and Ben’s ordering and inserted Claudia between Alexa and Ben. However, React is a computer program and does not know what we intended. Because React cannot know our intentions, we need to specify a key property for each list item to differentiate each list item from its siblings. One option would be to use the strings alexa, ben, claudia. If we were displaying data from a database, Alexa, Ben, and Claudia’s database IDs could be used as keys.

<li key={user.id}>{user.name}: {user.taskCount} tasks left</li>

When a list is re-rendered, React takes each list item’s key and searches the previous list’s items for a matching key. If the current list has a key that didn’t exist before, React creates a component. If the current list is missing a key that existed in the previous list, React destroys the previous component. If two keys match, the corresponding component is moved. Keys tell React about the identity of each component which allows React to maintain state between re-renders. If a component’s key changes, the component will be destroyed and re-created with a new state.

key is a special and reserved property in React (along with ref, a more advanced feature). When an element is created, React extracts the key property and stores the key directly on the returned element. Even though key may look like it belongs in props, key cannot be referenced using this.props.key. React automatically uses key to decide which components to update. A component cannot inquire about its key.

It’s strongly recommended that you assign proper keys whenever you build dynamic lists. If you don’t have an appropriate key, you may want to consider restructuring your data so that you do.

If no key is specified, React will present a warning and use the array index as a key by default. Using the array index as a key is problematic when trying to re-order a list’s items or inserting/removing list items. Explicitly passing key={i} silences the warning but has the same problems as array indices and is not recommended in most cases.

Keys do not need to be globally unique; they only need to be unique between components and their siblings.

Implementing Time Travel

In the tic-tac-toe game’s history, each past move has a unique ID associated with it: it’s the sequential number of the move. The moves are never re-ordered, deleted, or inserted in the middle, so it’s safe to use the move index as a key.

In the Game component, we can add the key as <li key={move}> and React’s warning about keys should disappear:

    const moves = history.map((step, move) => {
      const desc = move ?
        'Go to move #' + move :
        'Go to game start';
      return (
        <li key={move}>          <button onClick={() => jumpTo(move)}>{desc}</button>
        </li>
      );
    });

View the full code at this point

Clicking any of the list item’s buttons throws an error because the jumpTo function is undefined. Before we implement jumpTo, we’ll add stepNumber to the Game component’s state to indicate which step we’re currently viewing.

First, add stepNumber: 0 to the initial state in Game:

function Game() {
  const [history, setHistory] = useState([{ squares: Array(9).fill(null) }]);
  const [stepNumber, setStepNumber] = useState(0);  const [xIsNext, setXIsNext] = useState(true);

Next, we’ll define the jumpTo function in Game to update that stepNumber. We also set xIsNext to true if the number that we’re changing stepNumber to is even:

  function handleClick(i) {
    // this function has not changed
  }

  function jumpTo(step) {    setStepNumber(step);    setXIsNext((step % 2) === 0);  }

We will now make a few changes to the Game’s handleClick function which fires when you click on a square.

The stepNumber state we’ve added reflects the move displayed to the user now. After we make a new move, we need to update stepNumber by calling setStepNumber(history.length). This ensures we don’t get stuck showing the same move after a new one has been made.

We will also replace reading history with history.slice(0, stepNumber + 1). This ensures that if we “go back in time” and then make a new move from that point, we throw away all the “future” history that would now become incorrect.

  function handleClick(i) {
    const currentHistory = history.slice(0, stepNumber + 1);    const currentStep = currentHistory[currentHistory.length - 1];
    const squares = currentStep.squares.slice();
    if (calculateWinner(squares) || squares[i]) {
      return;
    }
    squares[i] = xIsNext ? "X" : "O";

    setHistory(currentHistory.concat([{ squares: squares }]));
    setStepNumber(currentHistory.length);    setXIsNext(!xIsNext);
  }

Finally, we will modify the Game component from always rendering the last move to rendering the currently selected move according to stepNumber:

  const current = history[stepNumber];  const winner = calculateWinner(current.squares);

    // the rest has not changed

If we click on any step in the game’s history, the tic-tac-toe board should immediately update to show what the board looked like after that step occurred.

View the full code at this point

Wrapping Up

Congratulations! You’ve created a tic-tac-toe game that:

• Lets you play tic-tac-toe,
• Indicates when a player has won the game,
• Stores a game’s history as a game progresses,
• Allows players to review a game’s history and see previous versions of a game’s board.

Nice work! We hope you now feel like you have a decent grasp of how React works.

Check out the final result here: Final Result.

If you have extra time or want to practice your new React skills, here are some ideas for improvements that you could make to the tic-tac-toe game which are listed in order of increasing difficulty:

1. Display the location for each move in the format (col, row) in the move history list.
2. Bold the currently selected item in the move list.
3. Rewrite Board to use two loops to make the squares instead of hardcoding them.
4. Add a toggle button that lets you sort the moves in either ascending or descending order.
5. When someone wins, highlight the three squares that caused the win.
6. When no one wins, display a message about the result being a draw.

Throughout this tutorial, we touched on React concepts including elements, components, props, and state. For a more detailed explanation of each of these topics, check out the rest of the documentation. To learn more about defining components, check out the React.Component API reference.