Sams Teach Yourself JavaScript in 24 Hours

By Michael Moncur

• Table of Contents
  • Copyright
  • About the Author
  • Acknowledgments
  • We Want to Hear from You!
  • Reader Services
  • Introduction
  • Part I: Getting Started
  • Hour 1. Understanding JavaScript
  • Hour 2. Creating a Simple Script
  • Hour 3. How JavaScript Programs Work
  • Part II: Learning JavaScript Basics
  • Hour 4. Using Functions and Variables
  • Hour 5. Using Strings and Arrays
    • Using String Objects
    • Working with Substrings
    • Using Numeric Arrays
    • Using String Arrays
    • Sorting a Numeric Array
    • Workshop: Displaying a Scrolling Message
    • Summary
    • Q&A
    • Quiz
    • Exercises
  • Hour 6. Testing and Comparing Values
  • Hour 7. Repeating Yourself: Using Loops
  • Hour 8. Using Math and Date Functions
  • Part III: The Document Object Model (DOM)
  • Hour 9. Working with the Document Object Model
  • Hour 10. Responding to Events
  • Hour 11. Using Windows and Frames
  • Hour 12. Getting Data with Forms
  • Hour 13. Using Graphics and Animation
  • Part IV: Moving on to Advanced JavaScript Features
  • Hour 14. Creating Cross-Browser Scripts
  • Hour 15. Creating Custom Objects
  • Hour 16. Working with Sounds and Plug-Ins
  • Hour 17. Debugging JavaScript Applications
  • Part V: Working with Dynamic HTML (DHTML)
  • Hour 18. Working with Style Sheets
  • Hour 19. Using Dynamic HTML (DHTML)
  • Hour 20. Using Advanced DOM Features
  • Part VI: Putting It All Together
  • Hour 21. Improving a Web Page with JavaScript
  • Hour 22. Creating a JavaScript Game
  • Hour 23. Creating DHTML Applications
  • Hour 24. JavaScript Tips and Tricks
  • Part VII: Appendices
  • Appendix A. Other JavaScript Resources
  • Appendix B. Tools for JavaScript Developers
  • Appendix C. Glossary
  • Appendix D. JavaScript Quick Reference
  • Appendix E. DOM Quick Reference

Using Numeric Arrays

An array is a numbered group of data items that you can treat as a single unit. For example, you might use an array called scores to store several scores for a game. Arrays can contain strings, numbers, objects, or other types of data. Each item in an array is called an element of the array.

Creating a Numeric Array

Unlike most other types of JavaScript variables, you usually need to declare an array before you use it. The following example creates an array with four elements:

scores = new Array(4);

To assign a value to the array, you use an index in brackets. Indexes begin with 0, so the elements of the array in this example would be numbered 0 to 3. These statements assign values to the four elements of the array:

scores[0] = 39;
scores[1] = 40;
scores[2] = 100;
scores[3] = 49;

You can also declare an array and specify values for elements at the same time. This statement creates the same scores array in a single line:

scores = new Array(39,40,100,49);

In JavaScript 1.2 and later, you can also use a shorthand syntax to declare an array and specify its contents. The following statement is an alternate way to create the scores array:

scores = [39,40,100,49];

Understanding Array Length

Like strings, arrays have a length property. This tells you the number of elements in the array. If you specified the length when creating the array, this value becomes the length property's value. For example, these statements would print the number 30:

scores = new Array(30);
document.write(scores.length);

You can declare an array without a specific length, and change the length later by assigning values to elements or changing the length property. For example, these statements create a new array and assign values to two elements:

test = new Array();
test[0]=21;
test[5]=22;

In this example, since the largest index number assigned so far is 5, the array has a length property of 6—remember, elements are numbered starting at zero.

Accessing Array Elements

You can read the contents of an array using the same notation you used when assigning values. For example, the following statements would display the values of the first three elements of the scores array:

scoredisp = "Scores: " + scores[0] + "," + scores[1] + "," + scores[2];
document.write(scoredisp);

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