Bits & Bytes

Posts Tagged ‘image’

Using Arrays in JavaScript

Arrays are containers that hold a sequence of objects that can be accessed via the bracket operator [] and an integer index. Since JavaScript is not a strongly-typed language, JavaScript arrays are very versatile and can hold objects of different types. In this post, I will focus on the basic syntax and usage.

Below, we have the code for an HTML file and a JavaScript file. The HTML file is essentially blank; it is simply used to call the JavaScript file, “Arrays.js,” and execute the code. The rest is boilerplate code that I reuse for all of my JavaScript posts.

The JavaScript code file, “Arrays.js,” contains the entire JavaScript program. In it, I first declare the variable, qaPaintings, and assign it the value [], which makes the variable an Array object with zero elements in it. Then the first entry at index 0 is set to hold a new Image object and its source is set to be the Michelangelo’s painting of the creation of the Sun and the Moon from the Sistene Chapel that was painted in 1511 AD. The call to appendChild() adds the image to the document so that it is displayed.

The same thing is then done for the entries at 1 and 2 in the array. These are assigned the source images of the painting The Descent of the Holy Ghost by Titian circa 1545 AD and the painting of The Last Judgment from the Sistene Chapel by Michelangelo that was completed between 1536 AD and 1541 AD.

All of this shows how to create an array and assign values to its elements. Notice that when we first created the array, it had zero elements. By assigning values to the entries at 0, 1, and 2, we caused the array to be extended each time. Automatic array resizing is a convenient property of JavaScript arrays that differs from other languages like C++. In fact, JavaScript arrays


<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN"
<html xmlns="" xml:lang="en" lang="en">
  <meta http-equiv="Content-Type" content="text/html;charset=utf-8" />
  <script type="text/javascript" src="Arrays.js"></script>


var qaPaintings = [];

qaPaintings[0] = new Image();
qaPaintings[0].src = "SisteneChapel_Michelangelo_1511_1.jpg"

qaPaintings[1] = new Image();
qaPaintings[1].src = "TheDescentOfTheHolyGhost_Titian_2.jpg"

qaPaintings[2] = new Image();
qaPaintings[2].src = "TheLastJudgment_Michelangelo_3.jpg"

Clipping in JavaScript with Absolute Positioning

If you program games in JavaScript, you will often find it necessary to clip the region of an image that lies outside of the viewport. The solution is easy, but not obvious. The elements in a game are generally positioned using the “absolute” positioning designation: For example, the line = “absolute”; in the program below sets the positioning that the image of the Madonna, qrMadonna, refers to to absolute. (For an explanation of absolute positioning, see our post on positioning elements.)

Below, we have the code for an HTML file and a JavaScript file. The HTML file is essentially blank; it is simply used to call the JavaScript file, “ClipImage.js” and execute the code.

The file “ClipImage.js” contains four variables that refer to four nested HTML elements. The outermost element is the body; this element was created in the HTML file and is retrieved via a call to the getElementsByTagName() function, along with the array operator. After this, we create a div called qrOuterDiv to hold everything; this outer div is created to allow the code inside to flow normally, since it does not have absolute positioning (Otherwise, it is not needed.) The next element is qrInnerDiv, and it contains the image element that we are clipping; it is necessary that this element have its position as “absolute” and its overflow as “hidden”. Finally, the image element qrImage is 200×149 and is positioned at (125, 100) inside of the div, which is 300×200. So, the image hangs outside of the div by 25 and 49 pixels, respectively.

To illustrate this, we have an resulting image of what this clipping looks like below. The faint region that lies outside the dark gray rectangle is actually clipped. We show this region so that you can see what has been clipped. We also have the original image of the “Madonna and Child with Cherubs” that we used in the example, at the top of the article.

HTML File: “ClipImage.html”

<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN"
<html xmlns="" xml:lang="en" lang="en">
    <meta http-equiv="Content-Type" content="text/html;charset=utf-8" />
    <title>'s Javascript Clipping Example</title>
    <script type="text/javascript" src="ClipImage.js"></script>

JavaScript File: “ClipImage.js”

var qrBody;
var qrOuterDiv;
var qrInnerDiv;
var qrImage;

function PageLoaded() {
    // Get the main body element, first/only element of array
    qrBody = document.getElementsByTagName("body")[0];

    qrOuterDiv = document.createElement("div");

    qrInnerDiv = document.createElement("div"); = "#444444"; = "300px"; = "200px"; = "absolute"; = "hidden";

    qrImage = document.createElement("img");
    qrImage.src = "MadonnaAndChildWithCherubs.jpg"; = "absolute"; = "125px"; = "100px";

window.onload = PageLoaded;

Creating a Walkable 2D Game Terrain in JavaScript

The Demonstration

This post demonstrates how to program the basic elements of a 2D tiled game with terrain images. This is an update to my prior post with terrain images added, along with an image for our game character. You can try out the demo, by left-clicking this link and then using the arrow keys to move around.

The Code

Most of this code was explained already in my prior post; so, I will explain the minor changes that have been made, beginning with the HTML file. In the HTML file, I have changed the 25 div elements to img elements so that they can hold images. I have also added an additional img element to hold the character image at the end of the background div.

Inside the JavaScript File, I have added variables to hold the terrain images and adapted the functions to use these images. At the top of the file, there are four new variables declared to hold the images for the terrain types: grass, forest, water, and bushes. In the Initialize() function, we allocate and set these terrain image variables. The function GetRandomTerrain() has been changed from the prior GetRandomColor() function to return an image object rather than a color. The FillBoard() function has been changed from the prior ColorBoard() function so that it now returns a null image for squares outside of the map; this has the effect of leaving these squares medium gray, as defined by the background element. The rest of the changes consist of calling these to functions instead of the prior functions. So, the changes are simple and straightforward.

Again, the HTML code can be put into any file with a .html extension, but the JavaScript file should be named “XoaXGameTerrain2D.js” in order to work with the code as is. Also, you will need 100 pixel by 100 pixel terrain images and a character image, which should be stored in the same folder with the HTML and JavaScript files.


<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN"

<html xmlns="" xml:lang="en" lang="en">

<title>'s Javascript 2D Game Board Example</title>

<script type="text/javascript" src="XoaXGameTerrain2D.js"></script>


<div id="gameboard" style="position:absolute; width:500px; height:500px; background:#aaaaaa; margin:30px;">
<img id="A11" style="left:0px; top:0px; position:absolute; width:100px; height:100px;" />
<img id="A12" style="left:100px; top:0px; position:absolute; width:100px; height:100px;" />
<img id="A13" style="left:200px; top:0px; position:absolute; width:100px; height:100px;" />
<img id="A14" style="left:300px; top:0px; position:absolute; width:100px; height:100px;" />
<img id="A15" style="left:400px; top:0px; position:absolute; width:100px; height:100px;" />

<img id="A21" style="left:0px; top:100px; position:absolute; width:100px; height:100px;" />
<img id="A22" style="left:100px; top:100px; position:absolute; width:100px; height:100px;" />
<img id="A23" style="left:200px; top:100px; position:absolute; width:100px; height:100px;" />
<img id="A24" style="left:300px; top:100px; position:absolute; width:100px; height:100px;" />
<img id="A25" style="left:400px; top:100px; position:absolute; width:100px; height:100px;" />

<img id="A31" style="left:0px; top:200px; position:absolute; width:100px; height:100px;" />
<img id="A32" style="left:100px; top:200px; position:absolute; width:100px; height:100px;" />
<img id="A33" style="left:200px; top:200px; position:absolute; width:100px; height:100px;" />
<img id="A34" style="left:300px; top:200px; position:absolute; width:100px; height:100px;" />
<img id="A35" style="left:400px; top:200px; position:absolute; width:100px; height:100px;" />

<img id="A41" style="left:0px; top:300px; position:absolute; width:100px; height:100px;" />
<img id="A42" style="left:100px; top:300px; position:absolute; width:100px; height:100px;" />
<img id="A43" style="left:200px; top:300px; position:absolute; width:100px; height:100px;" />
<img id="A44" style="left:300px; top:300px; position:absolute; width:100px; height:100px;" />
<img id="A45" style="left:400px; top:300px; position:absolute; width:100px; height:100px;" />

<img id="A51" style="left:0px; top:400px; position:absolute; width:100px; height:100px;" />
<img id="A52" style="left:100px; top:400px; position:absolute; width:100px; height:100px;" />
<img id="A53" style="left:200px; top:400px; position:absolute; width:100px; height:100px;" />
<img id="A54" style="left:300px; top:400px; position:absolute; width:100px; height:100px;" />
<img id="A55" style="left:400px; top:400px; position:absolute; width:100px; height:100px;" />

<img src="Man.png" style="left:200px; top:200px; position:absolute; width:100px; height:100px;" />


JavaScript File

var qpBkdg = null;
var qppBoard = null;
var qppMap = null;
var iLocX = 10;
var iLocY = 10;
var qpGrass = null;
var qpForest = null;
var qpWater = null;
var qpBushes = null;

function KeyHandler(qKeyEvent) {
    var iKeyDown = 0;
    var iLeftArrow = 37;
    var iUpArrow = 38;
    var iRightArrow = 39;
    var iDownArrow = 40;

    if (qKeyEvent) {
        iKeyDown = qKeyEvent.which;
    } else {
        iKeyDown = window.event.keyCode;

    if (iKeyDown === iLeftArrow) {
        if (iLocX > 0) {
            iLocX = iLocX - 1;
    } else if (iKeyDown === iRightArrow) {
        if (iLocX < 19) {
            iLocX = iLocX + 1;
    } else if (iKeyDown === iUpArrow) {
        if (iLocY > 0) {
            iLocY = iLocY - 1;
    } else if (iKeyDown === iDownArrow) {
        if (iLocY < 19) {
            iLocY = iLocY + 1;
    return false;

function GetRandomTerrain() {
    var iRnd = Math.floor(Math.random()*10);
    switch(iRnd) {
        case 0:
            return qpWater;
        case 1:
            return qpForest;
        case 2:
            return qpBushes;
            return qpGrass;

function FillBoard() {
    for (var iY = iLocY - 2; iY < iLocY + 3; iY++) {
        for (var iX = iLocX - 2; iX < iLocX + 3; iX++) {
            if (iY < 0 || iX < 0 || iY > 19 || iX > 19) {
                qppBoard[iY - iLocY + 2][iX - iLocX + 2].src = null;
            } else {
                qppBoard[iY - iLocY + 2][iX - iLocX + 2].src = qppMap[iY][iX].src;

function Initialize() {
    qpGrass = new Image();
    qpGrass.src = "Grass.png"
    qpForest = new Image();
    qpForest.src = "Forest.png"
    qpWater = new Image();
    qpWater.src = "Water.png"
    qpBushes = new Image();
    qpBushes.src = "Bushes.png"

    // Generate map
    qppMap = new Array(20);
    for (var iY = 0; iY < 20; iY++) {
        qppMap[iY] = new Array(20);
        for (var iX = 0; iX < 20; iX++) {
            qppMap[iY][iX] = GetRandomTerrain();

    // Allocate the visible board
    qppBoard = new Array(5);
    for (var iY = 0; iY < 5; iY++) {
        qppBoard[iY] = new Array(5);
        for (var iX = 0; iX < 5; iX++) {
            qppBoard[iY][iX] = document.getElementById('A'+((iY+1)*10+(iX+1)));

    document.onkeydown = KeyHandler;
    qpBkdg = document.getElementById('gameboard');

window.onload = Initialize;