A simple JavaScript Game

In this tutorial we are going to create an interface to play Connect 4. Plain JavaScript and some basic CSS will be enough to make a nice and functional interface.

References for this tutorial are:

We refer to the book Eloquent JavaScript for more details.

The Q&A platform StackOverflow has good answers, but apply some critical thinking before using them. Many other tutorials you can find on the internet may refer to outdated versions of JavaScript, so be careful before copy-pasting.

Like you did for last tutorial, create a new project in Glitch.

The game board in pure CSS

For simplicity, let’s start with a 3×2 board.

  1. Create a valid HTML document containing a <div>, itself containing a table (<table>) of 3 lines by 2 columns, every cell being empty. Give a unique identifier to the <div>.

  2. Attach a style sheet to the HTML document. Give fixed width and height to the table cells. Using the border property, give blue borders to the cells.

    At this point you have square grid. Using the border-radius property, make the table cells round, like this:

    Last touch: fill the blanks between the cells. For that, use wisely the background-color property on some elements of the table to obtain this:

  3. It’s time to play. Add classes player1 and player2 to some table cells. Fill the cells with a red or yellow token, according to whether they have one class or the other (use background-color again).

Unobtrusive JavaScript

We are ready to write the real interactive application. We could create a 6×7 board right in the HTML sources, like in the previous section, but we are too lazy to type 42 times <td></td> by hand. Instead we will dynamically generate the board via JavaScript.

We are going to follow the popular mantra of Unobtrusive JavaScript, which says that an HTML page should be usable even without JavaScript, all dynamic elements being added only after JavaScript has been loaded.

In our specific instance, since our page only contains an interactive game, we will simply show an error message as long as JavaScript has not loaded.

  1. Remove the <table> from the HTML document. Replace the <div> by a simple

    <div id="...">Activez JavaScript pour jouer.</div>

    (replace ... with the identifier you chose previously).

  2. Attach a JavaScript file to the HTML page. To avoid DOM loading problems, use the defer attribute on the <script> tag like this:

        <script src="..." defer></script>
        <div id="...">Activez JavaScript pour jouer.</div>

    Perform a debug printing using console.log in your JavaScript file, test that your script has been loaded using the browser console (Shift+Ctrl+K in Firefox, F12 in Chrome).

  3. In our JavaScript application, we will separate the abstract internal representation of the game from its DOM rendering. We start with the abstract representation: define a global variable board and initialize it to a 6×7 array of integers, containing all zeros. Use console.log to check that the array is properly initialized.

    Note: contrary to C or Java, arrays (or, rather, lists) in JavaScript do not have a fixed dimension, and multi-dimensional arrays are simulated by list of lists (of lists of lists…).

    You can initialize an empty array of length n using Array(n). You can use the .fill() method to put the same value in all cells of an already initialized array.

    You can take some inspiration from the example below.

  4. Write a function set(row, column, player) that takes as input a row, a column, and an integer representing player 1 or 2, and changes the corresponding entry in board to 1 or 2.

    Test it in the browser console.

  5. We now move to the generation of the DOM representation. Write a function render() that:

    • Finds the DOM element corresponding to the <div> using document.querySelector.
    • Empties the element (you can use .innerHTML = '' to empty a DOM element).
    • Creates a <table> DOM node using document.createElement, and adds it to the <div> using the .appendChild method.
    • Using a double for loop (one for loop nested in another), create <tr>’s and <td>’s inside the <table> element, so as to obtain a 6×7 table.

    You can take some inspiration from the example below.

    Test your function by calling it on page load, and verifying that the game board is properly drawn on screen.

  6. Modify render() so that the <td> in position (i,j) has class player1 if board[i][j] is equal to 1, and class player2 if board[i][j] is equal to 2.

    You can set an element’s class with

    my_element.className = 'some_class_name';

    Test your functions in the browser console by successively calling set and render many times.

  7. Define a global variable turn, indicating whose turn it is to play. Initialize it to the value 1 (player 1 starts).

  8. Write a function play(column) that

    • takes as input a column number,
    • looks at board to find the first free row in the column (if any), starting from the bottom,
    • if no row is free in the column, it returns false,
    • if a row is free in the column, it changes the corresponding value in board to the value of turn, changes the value of turn, and returns the row number.

    Test your function in the console by repeatedly calling play() and render().

Handling events

It is now time to react to mouse clicks. We want to call the play function any time the user clicks on a column, regardless of the exact line that has been clicked.

We could define a event handler for each <td>, but this would not be elegant nor efficient: 42 event handlers take lots of resources, especially on a mobile phone… Instead, we are going to define a unique handler for the whole board, and we will use the Event object to know which <td> exactly has been clicked. It is import here to understand the difference between the properties .target and .currentTarget seen in the lesson on DOM. Also have a look at the example below:

But how shall we know which column corresponds to the clicked <td>? Among many solutions (we could, for example, navigate te DOM to count how many <td>’s are on the left of our clicked <td>), the HTML5 data attributes API is the most elegant one.

  1. Using .addEventListener(), define a handler for the click event on the whole board (e.g., on the <table> element). For the moment, just execute


    in it (event being the lone parameter to the handler). Test you handler by clicking on the board and reading in the console.

  2. Modify render() so that each <td> cell contains a data-column attribute equal to the column containing the <td>. These data-* attributes have a special API in Javascript: every DOM node has a dataset field, which gives read/write access to the data-* attributes. For example, if my_cell represent a <td> element, the following code

    // example using dataset
    ma_cell.dataset.column = 0;

    will insert a node

    <td data-column="0"></td>

    in the document. Warning, all dataset properties are automatically converted to strings by JavaScript.

    Modify the click event handler so that it prints in the console the value of the data-column attribute. Test in the console.

  3. Modify the click event handler to respond to clicks by playing a token in the corresponding column (calling play and render successively), or by not doing anything if the click happens to be outside of a cell (you can test the existence of dataset.column to know if the click happened on a <td> or somewhere else).

Final touches (optional)

You can tackle the following points in any order you like. Follow your inspiration!

  1. Write a function to test if a move is winning (4 same color tokens aligned vertically, horizontally ou diagonally). Note that it is enough to test 4 directions around the last clicked cell, and to go at most 3 cells away in each direction. The following example may serve as an inspiration (or maybe not).

  2. Modify your application to test at each move whether it is a winning one, or whether the board is completely full (a null match). When the game is over, the interface shows the winner. Clicking again starts a new match.

  3. Using the CSS animation property, make the 4 winning tokens flash. You can have a look at this guide on CSS animations (also in French).

  4. Add some style touches using the box-shadow property.

  5. Wrap your code in a JavaScript class, and remove all global variables, so as to allow having more than one board per page.

  6. Test your application on a smartphone, and adapt it to small screens. If you do not have a smartphone, you can test smartphone emulation in Chrome or Firefox (Shfit+Ctrl+M).

    You can read this guide on mobile development, in particular the part on using the viewport meta tag (in French).

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