Client-Side ECMAScript/JavaScript Programming

Client-Side ECMAScript/JavaScript Programming
An Introduction with Examples in HTML

Prof. David Bernstein
James Madison University

Computer Science Department

bernstdh@jmu.edu

Overview

Accessing the Document from a Program:
- A Document Object Model (DOM) is a description of the parts of document (and their attributes) and an Application Programming Interface (API) that defines how to access them (e.g., the W3C DOM for HTML)
- A language binding for a DOM is a set of objects/classes (in that language) that provides the appropriate functionality (e.g., the ECMASCript/JavaScript bindings for the HTML DOM)
Including Programs in a Document:
- Typically, the document has an element that can contain statements in a programming language (e.g., the script element in HTML can contain ECMAScript/JavaScript statements or can refer to a URL that contains such statements)

Important Aspects of the ECMAScript/JavaScript Bindings

The Document Object:
- Represented by the document object
- Is a node in the "document tree"
Elements in the Document:
- Are all nodes in the "document tree"
Attributes of Elements:
- Are, somewhat surprisingly, also nodes in the "document tree" but needn't be treated as such

Accessing Nodes

The document Object:
- Has global scope
Elements:
- getElementsByTagName() returns a list of descendants with the given tag name
- getElementById() returns the object with the given ID (i.e., with an id attribute that matches the given unique ID)
Attributes:
- Element objects have properties that correspond to the attributes of the element that can be accessed using the . operator
- You can also use the getAttribute() and setAttribute() methods

Accessing Nodes

A Simple Example

ecmascriptexamples/clientsidebasics/access.html

        <!DOCTYPE html>
<html>
  <body>
    <label>Weight:<input type="text" id="weight" value="100" /></label>

    <script type="text/javascript">
      var weightField = document.getElementById("weight");

      alert("value in the weight input: " + weightField.value);
    </script>
  </body>
</html>

Event-Driven Programming

Your Background:
- Most introductory programming courses consider software products that "start at the top" and then execute "step-by-step"
Event-Driven Programmers Think About:
- The events that can occur (e.g., mouse clicks, timing signals, key presses)
- The objects that can generate events of different kinds (often called event generators)
- The objects/functions that need to respond to events of different kinds (often called event receivers or event handlers)

Important Events in Client-Side ECMAScript

Some Document/Window Events:
- load is fired after the document has been parsed and all content (e.g., images) has been retrieved
- unload is fired when the user leaves the document
Some Mouse Events:
- mousemove, mouseover, mouseout
- mousedown, mouseup, click, dblclick
Some Keyboard Events:
- keydown, keyup, keypress
Some Form Events (Generated by Input Elements):
- focus, blur
- change, forminput, input, invalid

Event Handlers

Defined:
- A function that is invoked when an event is generated
Naming:
- onevent

Event Handlers (cont.)

Specification as an Attribute:
- Approach: Use the HTML element's onevent attribute
- Example: <button id="calculate" onclick="calculateBMI();">Calculate</button>
- Major Drawback: Mixes code and markup (which can be particularly problematic when different members of a team have different responsibilities)
Specification as a Property:
- Approach: Assign a function to the ECMAScript object's onevent property
- Example: document.getElementById("calculate").onclick = calculateBMI;
- Minor Drawback: Requires the invocation of a function (at the end of the body) that assigns the handlers

An Example of Event Handling - The Functions

ecmascriptexamples/clientsidebasics/bmi.js

        /**
 * A function to calculate and display the body mass index (BMI)
 *
 * @param {number} weight - The weight in pounds
 * @param {number} height - The height in pounds
 */
function calculateBMI(weight, height)
{
   "use strict"; // ECMAScript 5
   var bmi;
   
   bmi = weight / (height * height) * 703;

   if (bmi < 15){
      bmi = 15;
   }else if (bmi > 60){
      bmi = 60;
   }

   return bmi;
}

An Example of Event Handling - The Functions (cont.)

ecmascriptexamples/clientsidebasics/bmi-display.js

        /**
 * Display the BMI in an alert dialog
 *
 * This functions assumes that the document contains two text input
 * elements, one with an id of weight and the other with an id of height
 */
function displayBMI()
{
   "use strict"; // ECMAScript 5
   var bmi, height, heightField, weight, weightField;
   
   weightField = document.getElementById("weight");
   heightField = document.getElementById("height");
   
   weight = weightField.value;
   height = heightField.value;
   
   bmi = calculateBMI(weight, height);
   
   alert(bmi);
}

An Example of Event Handling (cont.)

Using Attributes

ecmascriptexamples/clientsidebasics/handler-attribute.html

        <!DOCTYPE html>
<html>
  <head>
  <script src="bmi.js"          type="text/javascript"></script>
  <script src="bmi-display.js"  type="text/javascript"></script>
  </head>

  <body>
    <label>Weight (pounds):<input type="text" id="weight" /></label><br/>
    <label>Height (inches):<input type="text" id="height" /></label><br/>
    <button onclick="displayBMI();">Calculate</button>
  </body>
</html>

An Example of Event Handling (cont.)

Using Properties

ecmascriptexamples/clientsidebasics/handler-property.html

        <!DOCTYPE html>
<html>
  <head>
  <script src="bmi.js"          type="text/javascript"></script>
  <script src="bmi-display.js"  type="text/javascript"></script>
  <script src="bmi-property.js" type="text/javascript"></script>
  </head>

  <body>
    <label>Weight (pounds):<input type="text" id="weight" /></label><br/>
    <label>Height (inches):<input type="text" id="height" /></label><br/>
    <button id="calculate">Calculate</button>

    <script type="text/javascript">assignHandlers();</script>
  </body>
</html>

An Example of Event Handling Using Properties (cont.)

ecmascriptexamples/clientsidebasics/bmi-property.js

        /**
 * The entry point of the BMI calculator
 *
 */
function assignHandlers()
{
   "use strict";
   var calculateButton;

   calculateButton = document.getElementById("calculate");
   calculateButton.onclick = displayBMI;
}

Event Handlers - Scope and Related Issues

this:
- When an event handler is invoked it is invoked as a property of the object that generated the event so this will refer to the generator
- For example, in button.onclick = calculator.start;, when start() is invoked, this will refer to button NOT to calculator
- You can, however, do the following: button.onclick = function(){calculator.start();};
The Scope Chain:
- The call/activation object is searched first
- The next object searched is not the global object it is the object that generated the event

Event Handlers - Avoiding the Extended Scope Chain

A Third Way to Specify Event Handlers:
- Use the addEventListener() method (defined in DOM 2)
Formal Parameters:
- The event
- The handler/listener
- shouldCapture (if true the handler captures the event; if false the event can bubble-up from descendants)
Multiple Handlers:
- If there is more than one they will all be invoked but in no particular order
Removing Handlers:
- Invoke removeEventListener() with the same parameters

An Example (cont.)

Using Registration

ecmascriptexamples/clientsidebasics/handler-register.html

        <!DOCTYPE html>
<html>
  <head>
  <script src="bmi.js"          type="text/javascript"></script>
  <script src="bmi-display.js"  type="text/javascript"></script>
  <script src="bmi-register.js" type="text/javascript"></script>
  </head>

  <body>
    <label>Weight (pounds):<input type="text" id="weight" /></label><br/>
    <label>Height (inches):<input type="text" id="height" /></label><br/>
    <button id="calculate">Calculate</button>

    <script type="text/javascript">registerHandlers();</script>
  </body>
</html>

An Example Using Registration (cont.)

ecmascriptexamples/clientsidebasics/bmi-register.js

        /**
 * Register the event handlers
 */
function registerHandlers()
{
   "use strict"; // ECMAScript 5   
   var calculateButton;

   calculateButton = document.getElementById("calculate");

   /* Since the last parameter is true, the event must be generated
      by the button (i.e., it will not be called if the event bubbles 
      up from a descendant) */
   calculateButton.addEventListener("click", displayBMI, true);   
}

Manipulating the Presentation/Style of the Document

The Idea:
- Have a program change the way the document is rendered
Approaches:
- Change the stylesheet
- Change the style attribute of individual elements
- Change the class attribute of individual elements (and have different pre-defined styles for different classes)
- Change an individual rule in the stylesheet.

Changing the Stylesheet

Get the Appropriate Element:
- By ID
- By tag name
Change the href Element:
- Be careful about the protocol and path

An Example of Changing the Stylesheet

An Example

ecmascriptexamples/clientsidebasics/stylesheet-change.html

        <!DOCTYPE html>
<html>
  <head>
    <title>Some Courses Taught by David Bernstein</title>
    <meta charset="UTF-8">
    <link rel="stylesheet" type="text/css" id="stylesheet" href="modern.css" />
    <script type="text/javascript" src="stylesheet-change.js"></script>
  </head>

  <body>

    <header>
      <img src="bernstdh.png" />
      <p>
      Some Courses Taught by David Bernstein
      </p>
    </header>

    <section>
      <h1>Core/Required Courses</h1>

      <section>
      <h2>CS345</h2> <h2>Software Engineering</h2>

      <p>
      Software engineering is the application of theories, methods and
      tools to the specification, design, creation,
      verification/validation, deployment, operation, and maintenance
      of software products. This course provides an overview of the
      engineering methods, processes, techniques and measurements used
      in the software industry. After completing this course, students
      will have gained some experience with these concepts should be
      able to explain their advantages and disadvantages, and should
      be able to apply them.
      </p>
      </section>
    </section>

    <section class="category">
      <h1>Electives</h1>

      <section>
      <h2>CS488</h2> <h2>Computer Graphics Applications</h2>

      <p>
      This course is an introduction to 2-D and 3-D computer graphics
      that covers the material from the bottom up. That is, it starts
      with the mathematical foundations, then explores the algorithmic
      issues that arise when implementing those mathematical concepts,
      and then considers the practice of developing graphical
      applications (using existing libraries).
      </p>
      </section>
    </section>


    <footer>
      <p>
      Copyright (c) 
      <a href="http://www.jmu.edu">James Madison University</a>.
      </p>
    </footer>

    <script type="text/javascript">registerHandlers();</script>
  </body>
</html>

An Example of Changing the Stylesheet (cont.)

ecmascriptexamples/clientsidebasics/stylesheet-change.js

        /**
 * Change the stylesheet
 */
function changeStyleSheet()
{
   "use strict"; // ECMAScript 5
   var current, file, path, pathEnd, sheet;
   

   
   sheet   = document.getElementById("stylesheet");   
   current = sheet.href;
   pathEnd = current.lastIndexOf("/");
   path    = current.substring(0, pathEnd+1);
   file    = current.substring(pathEnd+1);
   
   if (file === "traditional.css") {
      current = path+"modern.css";
   } else {
      current = path+"traditional.css"; 
   }

   sheet.href = current;
}

/**
 * Register the event handlers
 */
function registerHandlers()
{
   "use strict"; // ECMAScript 5

   document.body.addEventListener("click", changeStyleSheet, true);   
}

Changing the Style Attribute of an Individual Element

Steps:
- Get the element
- Get the style property
- Set the value of the appropriate rule
Naming Issues:
- Names in CSS rules can contain - characters but ECMAScript/JavaScript identifiers can't so identifiers omit the - characters and use "camel case" (e.g., font-size becomes fontSize)
- CSS rules can use ECMAScript/JavaScript reserved words (e.g., float) so the associated identifiers are prefixed with css (e.g., cssFloat)

An Example of Changing the Style Attribute

An Example

ecmascriptexamples/clientsidebasics/styleattribute-change.html

        <!DOCTYPE html>
<html>
  <head>
    <title>Some Courses Taught by David Bernstein</title>
    <meta charset="UTF-8">
    <link rel="stylesheet" type="text/css" href="modern.css" />
    <script type="text/javascript" src="styleattribute-change.js"></script>
  </head>

  <body>

    <header>
      <img src="bernstdh.png" />
      <p>
      Some Courses Taught by David Bernstein
      </p>
    </header>

    <section>
      <h1>Core/Required Courses</h1>

      <section>
      <h2>CS345</h2> <h2>Software Engineering</h2>

      <p>
      Software engineering is the application of theories, methods and
      tools to the specification, design, creation,
      verification/validation, deployment, operation, and maintenance
      of software products. This course provides an overview of the
      engineering methods, processes, techniques and measurements used
      in the software industry. After completing this course, students
      will have gained some experience with these concepts should be
      able to explain their advantages and disadvantages, and should
      be able to apply them.
      </p>
      </section>
    </section>

    <section class="category">
      <h1>Electives</h1>

      <section>
      <h2>CS488</h2> <h2>Computer Graphics Applications</h2>

      <p>
      This course is an introduction to 2-D and 3-D computer graphics
      that covers the material from the bottom up. That is, it starts
      with the mathematical foundations, then explores the algorithmic
      issues that arise when implementing those mathematical concepts,
      and then considers the practice of developing graphical
      applications (using existing libraries).
      </p>
      </section>
    </section>


    <footer>
      <p>
      Copyright (c) 
      <a href="http://www.jmu.edu">James Madison University</a>.
      </p>
    </footer>

    <script type="text/javascript">registerHandlers();</script>
  </body>
</html>

An Example of Changing the Style Attribute (cont.)

ecmascriptexamples/clientsidebasics/styleattribute-change.js

        /**
 * Change the stylesheet
 */
function increaseFontSize()
{
   "use strict";  // ECMAScript 5
   var size;
   
   size = parseFloat(document.body.style.fontSize);
   if ((size <= 0) || isNaN(size)){size = 12;}
   
   document.body.style.fontSize = (size * 1.1) + "pt";
}


/**
 * Register the event handlers
 */
function registerHandlers()
{
   "use strict";  // ECMAScript 5

   document.body.addEventListener("click", increaseFontSize, true);   
}

Changing the Class of an Individual Element

Getting Started:
- Pre-define rules for all of the classes you will need
In the Program:
- Get the element (e.g., by ID)
- Change the class property of the element

An Example of Changing the Class

An Example

ecmascriptexamples/clientsidebasics/class-change.html

        <!DOCTYPE html>
<html>
  <head>
    <meta charset="UTF-8"> 
    <link rel="stylesheet" type="text/css" href="class-change.css" />  
    <title>Changing the class of an Element</title>

    <script src="class-change.js" type="text/javascript"></script>
  </head>

  <body>
    <p>
      <label>Weight (pounds):</label>
      <input type="text" id="weight" class="valid" />
    </p>

    <script type="text/javascript">registerHandlers();</script>
  </body>
</html>

An Example of Changing the Class (cont.)

ecmascriptexamples/clientsidebasics/class-change.css

        .invalid {
  color: rgb(153,0,0);
}

.valid {
  color: rgb(0,153,0);
}

An Example of Changing the Class (cont.)

ecmascriptexamples/clientsidebasics/class-change.js

        /**
 * Register the event handlers
 */
function registerHandlers()
{
   "use strict"; // ECMAScript 5
   var weightField;

   weightField = document.getElementById("weight");
   weightField.addEventListener("input", validateWeight, true);   
}


/**
 * Validate the weight field
 */
function validateWeight()
{
   "use strict"; // ECMAScript 5
   var weight, weightField;

   weightField = document.getElementById("weight");
   weight      = parseInt(weightField.value, 10);
   
   if (isNaN(weight) || (weight < 0)){
      weightField.className = "invalid";      
   }else{
      weightField.className = "valid";
   }
}

Changing a Rule

Getting the Stylesheets:
- The document object has a stylesheets collection
Getting the Rules:
- Each member of stylesheets has a cssRules collection
Changing a Rule:
- Use the style property
Example:
- document.styleSheets[0].cssRules[i].style.color = "blue";

Changing the Document

During Loading:
- Use document.write() to insert HTML text into the document
After Loading:
- Manipulate the nodes in the document tree

Manipulating Nodes in the Document Tree

Get the Node of Interest:
- Directly: node = document.getElementById("result");
- First Child: node = parent.firstChild
- Another Child: node = parent.childNodes[i]
If Necessary, Construct a Node:
- For example newTextNode = document.createTextNode("...");
Make the Changes:
- For example parent.replaceChild(node, newTextNode); or parent.appendChild(newTextNode);
- For example node.data = "..."; or node.nodeValue = "...";
- For example, the non-standard node.innerHTML = "...";

An Example of Manipulating Nodes

An Example that Modifies Nodes

ecmascriptexamples/clientsidebasics/bmi-calculator.html

        <!DOCTYPE html>
<html>
  <head>
    <meta charset="UTF-8"> 
    <link rel="stylesheet" type="text/css" href="bmi-calculator.css" />  
    <title>BMI Calculator</title>

    <script src="bmi.js"            type="text/javascript"></script>
    <script src="bmi-calculator.js" type="text/javascript"></script>
  </head>

  <body>
    <section>
    <p class="input">
      <label>Weight (pounds):</label>
      <input type="text" id="weight" />
    </p>
    <p class="input">
      <label>Height (inches):</label>
      <input type="text" id="height" />
    </p>
    </section>

    <section class="output">
    <p class="output">
       <label>BMI:</label>
       <span id="bmi">&nbsp;</span>
    </p>
    <p class="output">
      <label>Category:</label>
      <span id="category">&nbsp;</span>
    </p>
    </section>

    <script type="text/javascript">registerHandlers();</script>
  </body>
</html>

An Example of Modifying Nodes (cont.)

ecmascriptexamples/clientsidebasics/bmi-calculator.css

        .normal {
  color: rgb(0, 101, 0);
  text-transform: capitalize;
}

.obese {
  color: rgb(204,0,0);
  text-transform: capitalize;
}

.overweight {
  color: rgb(255,153,0);
  text-transform: capitalize;
}

.underweight {
  color: rgb(255,153,0);
  text-transform: capitalize;
}

body {
  color: rgb(102,153,204);
  font-family: Arial;
}

input {
  text-align: right;
  border-color: rgb(102,153,204);
  border-radius: 5px;
  border-style: solid;
  border-width: 1px;
}

input:focus {
  background-color: rgb(255,255,204);
}

label {
  float: left;
  text-align: right;
  width: 50%;
}

p.input {
  margin: 0px;
  padding: 5px;
  text-align: right;
}

p.output {
  padding: 5px;
  text-align: right;
}

section {
  width: 400px;
}

section.output {
  background:    rgb(255,240,220);
  border-color:  rgb(110,110,100);
  border-radius: 5px;
  border-style:  solid;
  border-width:  1px;
}

span {
  float: right;
  text-align: right;
  width: 50%;
}

An Example of Modifying Nodes (cont.)

ecmascriptexamples/clientsidebasics/bmi-calculator.js

        /**
 * Note: This is a good example of a situation in which an object-oriented
 * approach is preferred. Such an approach would enable us to retrieve the
 * fields once and then store them as properties of the object.
 */

/**
 * Register the event handlers
 */
function registerHandlers()
{
   "use strict";   // ECMAScript 5
   var heightField, weightField;

   weightField = document.getElementById("weight");
   heightField = document.getElementById("height");

   weightField.addEventListener("input", displayResults, true);   
   heightField.addEventListener("input", displayResults, true);   
   
}


/**
 * Display the results
 */
function displayResults()
{
   "use strict";   // ECMAScript 5
   var category, categoryDisplay, bmi, bmiDisplay, height, heightField, text, weight, weightField;
   

   weightField = document.getElementById("weight");
   heightField = document.getElementById("height");
   
   weight = parseFloat(weightField.value);
   height = parseFloat(heightField.value);

   // Clear the current results
   bmiDisplay      = document.getElementById("bmi");
   categoryDisplay = document.getElementById("category");

   text = bmiDisplay.firstChild;
   text.data = " ";
   text = categoryDisplay.firstChild;
   text.data = " ";

   // Calculate the BMI
   bmi = calculateBMI(weight, height);
   
   // Display the results
   if (!isNaN(bmi)){
      text = bmiDisplay.firstChild;
      text.data = bmi.toFixed(1);
      
      text = categoryDisplay.firstChild;
      category = "normal";
      if      (bmi <= 18.5 ){
         category = "underweight";
      }else if ((bmi >= 25) && (bmi < 30.0)){
         category = "overweight";
      }else if (bmi > 30){
         category = "obese";
      }
      
      text.data                 = category; // It will be capitalized
      categoryDisplay.className = category;
      
      
      /* Note: We could have changed the style property rather
         than the class, but this approach is better because it
         delegates all presentation decisions to the stylesheet. */
   }
}

Another Example of Manipulating Nodes

An Example that Adds Nodes to the Tree

ecmascriptexamples/clientsidebasics/addnodes.html

        <!DOCTYPE html>
<html>
  <head>
    <link rel="stylesheet" type="text/css" href="addnodes.css" />

    <script src="addnodes.js" type="text/javascript"></script>
    <title>Add Nodes to the Document Tree</title>
  </head>
  <body>
    <section id="prices">
      <h1>Prices</h1>
    </section>

    <script type="text/javascript">
    start();
    </script>
  </body>
</html>

An Example of Adding Nodes (cont.)

ecmascriptexamples/clientsidebasics/addnodes.css

        h1 {
    background: rgb(102,153,204);
    border-color: rgb(102,153,204);
    border-style: solid;
    border-width: 1px;
    border-top-left-radius: 5px;
    border-top-right-radius: 5px;
    color: white;
    font-size: 14pt;
    text-align: center;
    margin: 0px;
    padding: 0px;  
}

input:focus {
    background-color: rgb(255,255,204);
}

input {
    border-color: rgb(102,153,204);
    border-style: solid;
    border-width: 1px;
    border-radius: 5px;
    display: block;
    margin: 5px;
    margin-left: auto;
    margin-right: auto;
    width: 90%;
}

section {
    border-color: rgb(102,153,204);
    border-radius: 5px;
    border-style: solid;
    border-width: 1px;
    float: left;
    margin: 5px;
}

An Example of Adding Nodes in the Tree (cont.)

ecmascriptexamples/clientsidebasics/addnodes.js

        function start()
{
   "use strict"; // ECMAScript 5
   var child, i, prices, size;

   size   = window.prompt("Size?");
   prices = document.getElementById("prices");

   for (i=0; i<size; ++i){
      child = document.createElement("input");
      child.type = "text";
      child.value = "";
      prices.appendChild(child);
   }
}

Other Important Properties of the Document Object

baseURI and URL:
- Contain the base URI (which can be modified by a base element and full URL of the document,respectively
- Read-only
domain:
- Contains the domain name of the device that served-up the document
- Read-only
lastModified:
- Contains the date and time that the document was last modified
- Read-only

Other Important Properties (cont.)

referrer:
- Contains the URL of the document that loaded the current document (or the empty string if the document was not opened through a link)
- Read-only
cookie:
- Contains the name/value pairs of all cookies

Some Useful Functions

Value Checking:
- isFinite() and isNaN()
Type Checking/Conversion/Enforcement:
- parseFloat() and parseInt()
- Date.parse()
Encoding Functions:
- escape()() and unescape()
Tokenizing/Parsing:
- Location objects have methods for parsing URLs

Timers

The Issue:
- One sometimes needs to have something happen after a certain amount of time has elapsed (either once or repeatedly)
The Solution:
- Use a timer

Timers (cont.)

Execute a Statement in the Future:
- window.setTimeout(statement,millis)
Repeatedly Execute a Statement:
- To start: identifier = window.setInterval(statement,millis);
- To stop: window.clearInterval(identifier);

A Timer Example

A Simple Example of Delayed Execution

ecmascriptexamples/clientsidebasics/splashscreen.html

        <!DOCTYPE html>
<html>
  <head>
    <title>An Example of a Splash Screen</title>
    <meta charset="UTF-8">
    <link rel="stylesheet" type="text/css" href="splashscreen.css"/>

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


  <body>

    <header id="splashscreen" class="centered">
     <img src="MultimediaBookCover.png" />
    </header>

    <section>
      <h1>Sampled Static Visual Content</h1>
      <p>
      The sampling of static visual content involves the sampling of
      both the color spectrum and space (usually in that order).  The
      two are similar in that they both involve the discretization of
      continuous information.  They are different in their
      dimensionality.
      </p>

      <dl>
      <dt>Color Sampling</dt>
      <dd>
      A process for converting from a continuous (infinite) set of 
      colors to a discrete (finite) set of colors.
      </dd>
      </dl>

      <p>
      Such processes are sometimes called
      quantization schemes.  The result of color sampling (i.e., the
      discrete set of colors) is often referred to as
      a <em>palette</em>, though one has to be somewhat careful
      because this term is sometimes used to describe the set of
      actual samples and other times is used to describe the set of
      all possible samples.
      </p>
    
      <dl>
      <dt>Spatial Sampling</dt>
      <dd>
      A process for discretizing space.
      </dd>
      </dl>

      <p>
      Though there are many ways to perform spatial sampling, this
      book limits its attention to the plane and, in particular, with
      spatial sampling schemes that involve the use of a finite grid
      with cells of equal size.  In essence, one places a regular grid
      on a planar surface and assigns a single color to each cell in
      the grid.
      </p>

      <p> 
      The result of color sampling and spatial sampling is a
      matrix/table of picture elements (or <em>pixels</em>), each of
      which contains a single color in the palette.  Such a
      matrix/table is often called a <em>raster
      representation</em>. One example is
      illustrated in the following Figure.  In practice, sampled
      static visual content is usually created in one of two ways.  In
      some cases, it is created using a <em>scanner</em> that samples
      from a source of some kind (e.g., a drawing or painting).  In
      other cases, it is created on a computer by a person (who
      selects colors from a discrete set) using a
      pointing device of some kind (e.g., a mouse or a pen on a
      graphics tablet) that performs the spatial sampling.  Two common
      examples of sampled static visual content are <em>bitmapped
      images</em> and <em>bitmapped fonts</em>.
      </p>

      <figure>
        <img src="raster.png"/>
        <figcaption>
        A Raster Representation
        </figcaption>
      </figure>

    </section>
    <script type="text/javascript">splash();</script>
  </body>
</html>

A Delayed Execution Example (cont.)

ecmascriptexamples/clientsidebasics/splashscreen.css

        .centered {
    text-align: center;
}

.excluded {
    display: none;
}

section {
    display: none;
}

A Delayed Execution Example (cont.)

ecmascriptexamples/clientsidebasics/splashscreen.js

        /**
 * Setup the timer
 */
function splash()
{
   "use strict"; // ECMAScript 5
   
   window.setTimeout(removeSplashScreen, 2000);
}


/**
 * Remove the splash screen
 */
function removeSplashScreen()
{
   "use strict"; // ECMAScript 5
   var i, section, splashScreen;

   splashScreen = document.getElementById("splashscreen");

   /* Append to the className property in case the element
      already has a class for other purposes */
   splashScreen.className += " excluded";

   /* Make the sections "visible" */
   section = document.getElementsByTagName("section");
   for (i=0; i<section.length; ++i)
   {
      section[i].style.display = "block";
   }
}

Another Timer Example

A Simple Example of Repeated Execution

ecmascriptexamples/clientsidebasics/fishtank.html

        <!DOCTYPE html>
<html>
  <head>
    <meta charset="UTF-8"> 
    <link rel="stylesheet" type="text/css" href="fishtank.css" />  
    <title>Fishtank</title>

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

  <body>
    <section id="tank">
    <img src="fish.png" id="fish" />
    </section>

    <script type="text/javascript">setup();</script>
  </body>
</html>

A Repeated Execution Example (cont.)

ecmascriptexamples/clientsidebasics/fishtank.css

        img#fish {
  position: absolute;
  left:     0px;
  top:      200px;
}

section#tank {   
  background: blue;
  background-image: url('ocean.png');
  height:     480px;
  position:   absolute;
  width:      640px;
}

A Repeated Execution Example (cont.)

ecmascriptexamples/clientsidebasics/fishtank.js

        var fishTimer;

/**
 * Setup the timer
 */
function setup()
{
   "use strict"; // ECMAScript 5
   
   fishTimer = window.setInterval(move, 40);
}


/**
 * Move the fish
 */
function move()
{
   "use strict"; // ECMAScript 5
   var fish, left;

   fish = document.getElementById("fish");

   if ((fish.style.left === undefined) || (fish.style.left === "")){
      left = 0;
   }else{
      left = parseInt(fish.style.left, 10);
   }
   
   left += 1;
   if (left > 400){window.clearInterval(fishTimer);}
   
   fish.style.left = left+"px";
}

CSS Transitions instead of Timers

The Issue:
- It sometimes makes sense to think of a document (or a component of a document) as having a form/presentation that changes over time
The Solution:
- Use a CSS transition (i.e., a change in a property that takes effect over time)
Syntax:
- transition: property duration [timing-function] [delay]

An Example of CSS Transitions

A Simple Example

ecmascriptexamples/clientsidebasics/ghost.html

        <!DOCTYPE html>
<html>
  <head>
    <meta charset="UTF-8"> 
    <link rel="stylesheet" type="text/css" href="ghost.css" />  
    <title>A Ghost (Using CSS Transitions)</title>

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

  <body>
    <section id="campus">
      <img src="ghost.png"  id="ghost"/>
    </section>

    <script type="text/javascript">setup();</script>
  </body>
</html>

An Example of CSS Transitions (cont.)

ecmascriptexamples/clientsidebasics/ghost.css

        img#ghost {
  left:       0px;
  position:   absolute;
  top:        200px;
  transition: all 4s;
}

section#campus {   
  background-image: url('wilson-hall.png');
  height:     375px;
  position:   absolute;
  width:      525px;
}

An Example of CSS Transitions (cont.)

ecmascriptexamples/clientsidebasics/ghost.js

        /**
 * Setup the event listener
 */
function setup()
{
   "use strict"; // ECMAScript 5
   var    campus;
   
   campus = document.getElementById("campus");
   campus.addEventListener("click", moveGhost, true); 
}


/**
 * Move the ghost to its final position.
 *
 * Not: The CSS transition will handle the tweening (i.e., the
 * interpolation)
 */
function moveGhost(event)
{
   "use strict"; // ECMAScript 5
   var     ghost;

   if (!event){event = window.event;}
   
   ghost = document.getElementById("ghost");
   ghost.style.left = event.clientX + "px";
   ghost.style.top  = event.clientY + "px";
}