Bits & Bytes

Archive for the ‘JavaScript’ Category

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

Arrays.html

<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN"
   "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
<html xmlns="http://www.w3.org/1999/xhtml" xml:lang="en" lang="en">
<head>
  <meta http-equiv="Content-Type" content="text/html;charset=utf-8" />
  <title>XoaX.net</title>
</head>
<body>
  <script type="text/javascript" src="Arrays.js"></script>
</body>
</html>

Arrays.js

var qaPaintings = [];

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

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

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

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 qrMadonna.style.position = “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"
"http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
<html xmlns="http://www.w3.org/1999/xhtml" xml:lang="en" lang="en">
<head>
    <meta http-equiv="Content-Type" content="text/html;charset=utf-8" />
    <title>XoaX.net's Javascript Clipping Example</title>
</head>
<body>
    <script type="text/javascript" src="ClipImage.js"></script>
</body>
</html>

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");
    qrBody.appendChild(qrOuterDiv);

    qrInnerDiv = document.createElement("div");
    qrInnerDiv.style.backgroundColor = "#444444";
    qrInnerDiv.style.width = "300px";
    qrInnerDiv.style.height = "200px";
    qrInnerDiv.style.position = "absolute";
    qrInnerDiv.style.overflow = "hidden";
    qrOuterDiv.appendChild(qrInnerDiv);

    qrImage = document.createElement("img");
    qrImage.src = "MadonnaAndChildWithCherubs.jpg";
    qrImage.style.position = "absolute";
    qrImage.style.left = "125px";
    qrImage.style.top = "100px";
    qrInnerDiv.appendChild(qrImage);
}

window.onload = PageLoaded;

Using an External JavaScript File

Putting JavaScript into a separate file has many advantages over writing the code inline in an HTML file. First, it makes the JavaScript code much cleaner since the HTML code is in a separate file. Second, it helps to promote modularization, which is always a benefit in programming. Finally, it the eliminates the need for creating CDATA sections to encapsulate JavaScript code that contains XHTML characters; we will explain this more later

Beginning with the code from our earlier JavaScript post, we have a simple HTML file, which I have called “JavaScriptBasis.html” in this post. The original HTML code with the embedded JavaScript code looks like this:

Old JavaScriptBasis.html

<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN"
   "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
<html xmlns="http://www.w3.org/1999/xhtml" xml:lang="en" lang="en">
<head>
  <meta http-equiv="Content-Type" content="text/html;charset=utf-8" />
  <title>XoaX.net</title>
</head>
<body>

<script type="text/javascript">
    document.write("Welcome to XoaX.net!");
</script>

</body>
</html>

What we want to do now is remove the JavaScript code, put it into a separate .js file, and call it via the script tag from the HTML file. To do this, use your favorite text editor to create a .js file called “BasicWrite.js” in the same directory as your HTML file and save it with this code pasted into it:

BasicWrite.js

document.write("Welcome to XoaX.net!");

Then open the file “JavaScriptBasis.html” and change code in the script tags to this:

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

Now, your HTML file will call your JavaScript file. The code in the HTML file should look like this:

JavaScriptBasis.html

<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN"
   "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
<html xmlns="http://www.w3.org/1999/xhtml" xml:lang="en" lang="en">
<head>
  <meta http-equiv="Content-Type" content="text/html;charset=utf-8" />
  <title>XoaX.net</title>
</head>
<body>

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

</body>
</html>

After you have saved the new files, double-click the HTML file to open it and you should see the message “Welcome to XoaX.net!” printed just as you did before.

It may not feel like much has been accomplished yet. In fact, it might seem like we have made the code more complex. However, the advantages will become more apparent when we write bigger programs.

Finally, we remark that using .js files allows us to eliminate the need to use CDATA sections. When XHMTL characters, like <, are used inside of an embedded JavaScript section, they create validation problems. For example, in order to allow this code to validate, we need the CDATA section as shown here:

CDATA Section


<script type="text/javascript">
var iX = 10;
/*<![CDATA[*/
if (iX < 13) {
    document.write("Welcome to XoaX.net!");
}
]]>
/*]]>*/
</script>

When this code is put into an external JavaScript, this CDATA section is not needed for validation. Note that the CDATA section looks particularly horrific since it requires us to comment out both endpoints of the CDATA section so that it does not adversely affect the JavaScript code:
. . .
/*<![CDATA[*/
. . .
/*]]>*/
. . .