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CHAPTER 7 ■ TRAVERSING AND MODIFYING THE DOM TREE 284 myElement.setAttribute("class", "sprout"); Then verify the change with Element.getAttribute(): myElement.getAttribute("class"); // "sprout" That worked. Note that, in the event the element does not have the attribute passed in the first parameter, JavaScript will add a new attribute to the element. It doesn’t even have to be a standard one: var myElement = document.getElementById("twitter"); myElement.setAttribute("verb", "tweet"); Then verify the addition with Element.getAttribute(): myElement.getAttribute("verb"); // "tweet" Let’s try to query myElement.verb as a member with the . operator: typeof myElement.verb; // "undefined" Where did the value for our custom attribute verb go? Nowhere, it is still there. myElement.getAttribute("verb"); // "tweet" In a nutshell, if you add a custom attribute to an element with Element.setAttribute() or directly in your XHTML markup, then you must query the attribute with Element.getAttribute(). Conversely, if you add a member with the . or [] operators, it will become an attribute only if the DTD for your markup defines an attribute by that name. To illustrate this, try the following sample, verifying your work with Figure 7–14. myElement.slogan = "What's happening?"; myElement.getAttribute("slogan"); // null To JavaScript, predefined attributes like href or id are both attributes and members, while custom attributes like verb are attributes but not members. Conversely, predefined members like tagName and custom members like slogan are members but not attributes. At least, that’s true in all browsers except for the one making mischief in its wolf suit. For Internet Explorer, custom attributes like verb are members, and custom members like slogan are attributes. That’s just one more reason to query attributes like members with the . or [] operator. Now for a couple more reasons. First, the style member, which I cover in Chapter 8, contains a CSSStyleDeclaration object, but the style attribute contains a string of text. Second, event listener members, which I cover in Chapter 9, contain a function, but event listener attributes contain a string of text—except in Internet Explorer. Prior to version 8, Internet Explorer returns a CSSStyleDeclaration object for both the style member and attribute and a function for event listener members and attributes. With this mess in mind, only query custom attributes with DOM methods.
Figure 7–14. Custom members are not attributes. CHAPTER 7 ■ TRAVERSING AND MODIFYING THE DOM TREE ■ Note If you are wondering whether an attribute is distinct from the member it maps to, it is. An attribute is represented by an Attr node, but its corresponding member is not. Querying Attr Nodes DOM provides a way to represent both HTML and XML markup. Whereas HTML attribute values may be fully represented with strings, this is not so for XML attributes. For this reason, DOM provides an Attr interface for representing attributes as nodes. Those are not part of the DOM tree. So, you will not bump into them while traversing the DOM. Anyway, for HTML markup, Attr nodes have a single Text node child that you may query by way of the value member. On the other hand, XML Attr nodes may contain both a Text and EntityReference node, which is why XML attributes cannot be conveyed with just a string. Click Clear in both Firebug panels, and let’s try querying the members of the Attr node representing the class attribute for the Twitter . First save the Attr node in a variable named myAttrNode. Note that you can query the Attr node with Element.getAttributeNode(). So, append Node to Element.getAttribute(), and you’re good to go: var myAttrNode = document.getElementById("twitter").getAttributeNode("class"); 285
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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 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 3 ■ OPERATORS 68 // 1.25
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CHAPTER 3 ■ OPERATORS 72 The impo
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CHAPTER 3 ■ OPERATORS 78 saigonCi
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CHAPTER 3 ■ OPERATORS 94 fridge.y
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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 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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