JavaScript核心技术：打造专业网页

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"Essential Javascript For Web Professionals 是一本面向web专业人员的基础JavaScript教程，由Dan Barrett、Dan Livingston和Micah Brown合著，于1999年8月首次出版，出版社为Prentice Hall PTR。本书旨在帮助读者理解和掌握JavaScript在网页设计中的应用，包括动态内容生成、图像 rollovers、导航、表单错误处理和JavaScript窗口等关键概念。" 在本书中，作者们通过一系列项目实践来讲解JavaScript的核心概念和技巧： 1. **动态性和检测**：这部分介绍如何根据用户平台生成特定内容，以及如何获取并显示版权信息和文件的最后修改日期。这些基础技能是构建响应式网站和提供个性化用户体验的关键。 2. **图像Rollovers**：Rollover技术是网页交互设计中常见的元素，项目I介绍了基本的rollover脚本，项目II则进一步增加了高级功能，如动态效果和多状态交互。此外，书中还讨论了不同浏览器可能存在的兼容性问题。 3. **JavaScript导航**：这部分内容涉及使用JavaScript创建下拉菜单，项目I和II逐步引导读者实现单个和多个下拉菜单的交互功能。项目III展示了如何在登录页面上利用JavaScript进行用户输入验证，提高了用户界面的互动性。 4. **表单错误处理**：有效的表单验证是提高用户体验和数据准确性的重要环节。项目I教读者如何检查表单字段是否为空，项目II则涉及对输入数据的正确性进行验证。项目III通过JavaScript向用户提供错误提示，使得用户能即时了解输入的问题所在。 5. **JavaScript窗口**：这部分探讨了如何利用JavaScript控制弹出窗口，包括打开新窗口、调整大小和位置，以及与主窗口的交互，提供了更丰富的用户体验。这本书适合有一定HTML和网页设计基础，希望通过JavaScript提升网页交互性的读者。每个项目都包含基础和高级两个部分，逐步引导读者从基础概念到复杂应用的过渡，从而深入理解JavaScript在实际web开发中的运用。通过这本书，读者可以掌握创建动态、响应式的web页面所需的JavaScript技能。

Introduction of the Last-Modified Property

After a little research into the different properties of the DOCUMENT object, you

come across a property that is going to make our script much easier. It seems

that we're not the first people to think a last-modified date would be a good idea.

Those nice people who created JavaScript included the lastModified property to

the DOCUMENT object. When a page is loaded and the JavaScript hierarchy is

created, the engine goes to the HTTP header and gets the date of the last

modification to the page and assigns it to the property. There is one drawback,

however: Not all Web servers include this information in the header, so we are

going to have to use an if statement to make sure we only print it if the value is

given. But, beggars can't be choosers, and an extra if statement will give us

some more practice.

If the Web server doesn't provide a date, then the last-Modified property will

have a value of 0—we will use this value as our test in our if statement. First,

let's take a look at the section of HTML code in which we will be putting our script.

<TR>

**** OUR SCRIPT WILL GO HERE ****

</FONT>

</TD>

</TR>

</TABLE>

</FORM>

</BODY>

</HTML>

Now that we know where to put our code, let's start by inserting our <SCRIPT>

tags and the HTML comments that will hide our script from older browsers.

<!-- Code after this will be ignored by older browsers

…

// Stop hiding the code here -->

</SCRIPT>

The first step in our script is to create and assign a variable the value of the

lastModified property.

<!-- Code after this will be ignored by older browsers

• How to combine strings and nonstring values with the use of the

concatenation operator (+.)

Recap

Well, we have gotten off to a great start with the Shelley Biotech Web site. In a

relatively short time we have added two very useful new features to the

homepage. As time passes and there are new browsers on the market that

handle HTML a little differently, being able to get user information and customize

your pages for site visitors is becoming ever more important. The other area that

we touched on in this chapter, dynamic creation of content, is an area of

JavaScript that has major potential for making the up-keep and maintenance of a

Web site more efficient. As we get further into this book, we will see many

examples of what we can accomplish using these concepts.

Advanced Projects

The scripts in this chapter are fairly simple examples of what you can accomplish

with platform/browser detection and dynamic HTML generation. Try using these

techniques in the following projects to get a better understanding of the great

results you can achieve:

1. Use browser detection to dynamically create pages that are customized to

work with specific browser versions.

2. Create a page that will gather all of the information contained in the

NAVIGATOR object, and dynamically generate a table that will display that

information to the user. (The information gathered from the user's

browser and placed in the table is useful as a development tool.)

3. If you have a site where specific content appears on all pages, use the

documentwriteln() method to print the code, and then save the script in

an external JavaScript file. In each page, simply call the external file using

the SRC attribute of the <SCRIPT> tag. This will let you update only one file

to affect site-wide changes.

Chapter 2. Image Rollovers

In This Chapter

• Project I: Rollover Script

• Project II: Adding Advanced Functionality to Our Rollovers

• Rollover Quirks

• Recap

• Advanced Projects

You've gotten the ball rolling on the homepage and shown that you've got what it

takes to get the job done—now your boss wants you to wow him. He is looking to

make the site more dynamic and give it a little bit of flash. He has been checking

out your competitors' sites and they all have image rollovers—JavaScript rollovers

are used to swap an image with a different version of that image when the user

moves the cursor over the image. Conversely, when the user moves the cursor

off the image, the new image is replaced with the original. This technique is

widely used on Web pages, and he feels this is just what your site needs.

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