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CHAPTER 8 ■ SCRIPTING CSS 336 Why do we use null for the second parameter? This can be a pseudo-element as a string, for example, ":before" or ":after". However, Internet Explorer’s proprietary alternative to getComputedStyle() does not support pseudo-elements. Therefore, pass null to make things work crossbrowser. Now for our DOM dummy, Internet Explorer, which does not implement the ViewCSS interface that provides getComputedStyle(). Internet Explorer’s way is simple: an Element node has not only a style member for inline styles but also a currentStyle member for cascade styles. Both style and currentStyle contain a CSSStyleDeclaration object. The Internet Explorer path is straightforward, and we’re done coding queryCascade(). function queryCascade(element, property) { if (typeof getComputedStyle === "function") { return getComputedStyle(element, null)[property]; } else if (element.currentStyle) { return element.currentStyle[property]; } } Now let’s query a CSS property from the cascade. Then we have a couple that were calculated behind the scenes by Firefox: function queryCascade(element, property) { if (typeof getComputedStyle === "function") { return getComputedStyle(element, null)[property]; } else if (element.currentStyle) { return element.currentStyle[property]; } } queryCascade(document.getElementById("adidas"), "width"); // "100px" queryCascade(document.getElementsByTagName("ul")[0], "width"); // "120px" queryCascade(document.getElementsByTagName("ul")[0], "height"); // "0px" For the Adidas element, queryCascade() returned the value from an explicit declaration in eight.css. On the other hand, the values for the element were calculated behind the scenes for us by Firefox. Note that insofar as the is a block element, its width is set to that of the containing , which we set to "120px". On the other hand, since the elements within the are absolutely positioned relative to the and the has a display value of inline, the collapses to a height of "0px". Here is the situation. Nowadays, CSS presentation and markup content are in separate files. Initially, the CSSStyleDeclaration object in the style member for an Element node will be irrelevant, with just scores of "" empty strings and other default values. On the other hand, the CSSStyleDeclaration object representing cascade declarations is totally relevant but read-only. So if you do not know what rule or style sheet contains the declaration with the highest precedence, then what do you do? Typically, scripters read the cascade and write the inline style attribute. That is to say, they query two separate CSSStyleDeclaration objects. Obviously, if you know where to find the rule, then you simply read and write the CSSStyleDeclaration object representing the rule’s declaration block, like we did earlier in the day. But more often than not, you won’t have that option. Making the computed styles from the cascade read-only is a DOM failing. But to be fair, the standard predates the notion of putting markup, CSS, and JavaScript in separate layers by five years. Maybe someday the flaw will be fixed. But that day is far off and may never come at all. So, let’s give the “read the cascade, write the attribute” thing a try.
CHAPTER 8 ■ SCRIPTING CSS Clear Firebug, but do not refresh Firefox because we want queryCascade() to remain available in memory. Then save the to a variable named myDIV. Pass myDIV and "left" to queryCascade(), which will then return "40px". Convert that string to the number 40 by way of parseInt(), and then add 10. Next, convert the sum of 50 to the string "50", and append "px". Finally, write that string, "50px", to the style attribute. Click Run repeatedly until left is set to "700px". JavaScript will move our running shoe menu to the right by 10 pixels each time you click Run. ■ Note If you have forgotten how parseInt() works, flip back to Chapter 2. Datatype conversion is covered there, too. function queryCascade(element, property) { if (typeof getComputedStyle === "function") { return getComputedStyle(element, null)[property]; } else if (element.currentStyle) { return element.currentStyle[property]; } } var myDIV = document.getElementById("running"); myDIV.style.left = parseInt(queryCascade(myDIV, "left")) + 10 + "px"; Verify your work with Figure 8–13. 337
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THE EXPERT’S VOICE ® IN WEB DEVE
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ii JavaScript for Absolute Beginner
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iv Contents at a Glance ■Contents
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■ CONTENTS vi ■Chapter 2: Type
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■ CONTENTS viii Sharing a Prototy
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■ CONTENTS x Querying Attributes
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■ CONTENTS xii Getting the User
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■ CONTENTS xiv About the Technica
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■ PREFACE xvi Preface In the 2005
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■ PREFACE xviii Opening Firebug L
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■ PREFACE xx Now press Return on
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■ PREFACE xxii editor reappear. N
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■ PREFACE xxiv
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CHAPTER 1 ■ REPRESENTING DATA WIT
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CHAPTER 3 ■ OPERATORS 58 Remember
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CHAPTER 3 ■ OPERATORS 60 Using Ja
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CHAPTER 3 ■ OPERATORS 62 dough.po
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CHAPTER 3 ■ OPERATORS 64 mincedLe
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CHAPTER 3 ■ OPERATORS 66 Figure 3
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CHAPTER 4 ■ CONTROLLING FLOW 98 s
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CHAPTER 4 ■ CONTROLLING FLOW 100
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CHAPTER 5 ■ MEMBER INHERITANCE 14
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CHAPTER 6 ■ FUNCTIONS AND ARRAYS
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CHAPTER 7 ■ TRAVERSING AND MODIFY
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CHAPTER 9 ■ LISTENING FOR EVENTS
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■ INDEX 462 parsing an XML respon
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■ INDEX 464 creating, 217 queryin
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■ INDEX 466 document.styleSheets,
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■ INDEX 468 listening for mousedo
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■ INDEX 470 chaining prototypes,
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