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
  • Hour 6. Testing and Comparing Values
  • Hour 7. Repeating Yourself: Using Loops
  • Hour 8. Using Math and Date Functions
    • What Is an Object?
    • The Math Object
    • Working with Dates
    • Workshop: Working with the Math Object
    • Summary
    • Q&A
    • Quiz
    • Exercises
  • 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

The Math Object

The Math object is a built-in JavaScript object that includes math constants and functions. You don't need to create a Math object; it exists automatically in any JavaScript program. The Math object's properties represent mathematical constants, and its methods are mathematical functions.

Rounding and Truncating

Three of the most useful methods of the Math object allow you to round decimal values up and down:

• Math.ceil() rounds a number up to the next integer.
• Math.floor()rounds a number down to the next integer.
• Math.round()rounds a number to the nearest integer.

All of these take the number to be rounded as their single parameter. You might notice one thing missing: the capability to round to a decimal place, such as for dollar amounts. Fortunately, you can easily simulate this. Here is a simple function that rounds numbers to two decimal places:

function round(num) {
   return Math.round(num * 100) / 100;
}

This function multiplies the value by 100 to move the decimal, and then rounds the number to the nearest integer. Finally, the value is divided by 100 to restore the decimal to its original position.

Generating Random Numbers

One of the most commonly used methods of the Math object is the Math.random() method, which generates a random number. This method doesn't require any parameters. The number it returns is a random decimal number between zero and one.

You'll usually want a random number between one and a value. You can do this with a general-purpose random number function. The following is a function that generates random numbers between one and the parameter you send it:

function rand(num) {
   return Math.floor(Math.random() * num) + 1;
}

This function multiplies a random number by the value specified in the num parameter, and then converts it to an integer between 1 and the number by using the Math.floor() method.

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