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CHAPTER 8 ■ SCRIPTING CSS 334 Figure 8–12. Deleting the two newly inserted rules with deleteRule() swaps the sprite from green to fuchsia to blue. Querying Overall Styles from the Cascade Now then, what if you want to query the overall styles from the CSS cascade for an element? In other words, you want to know the declarations that have the highest precedence. Those form one humongous declaration block, which, like the declaration block for a rule, JavaScript represents with a CSSStyleDeclaration object. You already know how to query those cumulative declarations—the. operator and a CSS2Properties member. Remember they are camel case, not dash case. On the other hand, CSSStyleDeclaration.cssText contains the declarations block, minus the curly braces, as a string. However, this string is oftentimes too lengthy to bother with. The cumulative declaration block from the cascade differs from that of a rule in a style sheet in a few ways: p
• It is read-only. Try to assign a new value to a property from the cascade, and JavaScript calls you a dummy by way of an error. • Any relative values are converted to absolute values. Typically this means converting the value to pixels. CHAPTER 8 ■ SCRIPTING CSS • Any property that sets several properties in one fell swoop, such as a margin or border, may contain undefined, while their corresponding fully expanded properties, such as marginLeft or borderRightStyle, will always contain a value. As a rule of thumb, do not query rollup values from the cascade. Query their fully expanded equivalents instead. Refresh Firefox, clear Firebug, and let’s cobble together a helper function named queryCascade() that works with two parameters: • element will contain an Element node for the DOM tree. • property will contain the name of the CSS property as a camel case string. That is to say, don’t pass a dash case string or a camel case identifier. Now let’s send Firefox, Safari, and Opera down one path and Internet Explorer down another by way of an if statement and the else if idiom. First let’s test for the DOM function getComputedStyle() by way of the typeof operator. If typeof returns "function" rather than "undefined", invoke with two parameters, element and null. getComputedStyle() will then return a CSSStyleDeclaration object containing the cumulative declarations from the cascade, as well as those implicitly calculated by the browser, for element. Then query property by way of the [] operator. function queryCascade(element, property) { if (typeof getComputedStyle === "function") { return getComputedStyle(element, null)[property]; } } Do you remember why doing so with the . operator will not work? Come on, you know this one. I’m not telling you the answer. Maybe I’ll go reorganize the Penske file while you think it through. ■ Note George reorganized the Penske file in the Seinfeld episode “The Barber.” What do you think? Right, the . operator works with a hard-coded identifier. On the other hand, [] works with a string or any expression that evaluates to a string, most notably a variable or parameter. ■ Note The if statement and else if idiom were covered in Chapter 4, while the typeof and refinement operators were covered in Chapter 3. So if you have forgotten how those work, take a moment to flip back and refresh your memory. I’ll wait for you to return before continuing. Promise! 335
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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 2 ■ TYPE CONVERSION 26 Fi
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CHAPTER 2 ■ TYPE CONVERSION 54 To
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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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