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CHAPTER 4 ■ CONTROLLING FLOW 128 "LunaRacer": 6.6, "Air Max": 13, "LunarGlide": 10.2, "Zoom Streak XC": 7, "Free": 8.6, "Mayfly": 4, "Zoom Vomero": 11.6, "LunarElite": 9.7 } var myOptions = []; for (var shoe in shoes) { if (shoes[shoe] >= 10) { myOptions.push(shoe); } } myOptions; // ["Air Max", "LunarGlide", "Zoom Vomero"] Figure 4–14. Enumerating members with a for in loop Our for in loop eliminated the drudgery of having to write the following eight if conditions: if (shoes["LunaRacer"] >= 10) { myOptions.push("LunaRacer"); } if (shoes["Air Max"] >= 10) { myOptions.push("Air Max"); } if (shoes["LunarGlide"] >= 10) { myOptions.push("LunarGlide"); } if (shoes["Zoom Streak XC"] >= 10) { myOptions.push(”Zoom Streak XC"); }
if (shoes["Free"] >= 10) { myOptions.push("Free"); } if (shoes["Mayfly"] >= 10) { myOptions.push("Mayfly"); } if (shoes["Zoom Vomero"] >= 10) { myOptions.push("Zoom Vomero"); } if (shoes["LunarElite"] >= 10) { myOptions.push("LunarElite"); } Snappier Conditionals CHAPTER 4 ■ CONTROLLING FLOW Now that we have explored conditionals and loops, let’s work through some techniques to make them run snappier—not just a little but two to seven times snappier, if not more. Insofar as the bulk of any JavaScript behavior is comprised of conditionals and loops, doing so will in turn make your behaviors feel that much more responsive to visitors. Yup, I like the sound of that, too! Let’s begin with a few ways to code speedier switch and if else conditionals. One simple thing you can do is favor switch over if else whenever coding five or more paths for JavaScript to conditionally take. Past four, incrementally adding conditions to a switch decrements speed much less than doing so for an if else. OK, that was painless. And so is the second way to code snappier conditionals—just order their paths from the one JavaScript is most likely to take to the one JavaScript is least likely to take. Doing so minimizes the number of boolean expressions JavaScript has to evaluate. Now for the third way to code snappier conditionals: don’t code them in the first place. No, that’s not a typo. In the event that you have written an if else or switch for variations of a string or number and every path does the same thing (just with a different value of that string or number), replacing the if else or switch with an array or object member query will result in an extraordinary speed gain, to put it mildly. There are a couple of reasons why doing so is preferable to conditional statements. For one thing, JavaScript does not have to drill down through conditions, just one [] operation. For another, whereas adding conditions to a switch or if else decrements evaluation speed, adding members to an object or elements to an array does not decrement query speed. Ideally, you are sold on replacing conditionals with an object or array query whenever possible. Now let’s give the technique a try. Double-clear Firebug, and then enter the following function, which contains a switch mapping jersey numbers to player names for Opening Day, 2010 as my Pittsburgh Pirates try to avert their eighteenth losing season in a row. Insofar as 17 of 25 Pirates were not in the dugout last April, namePirate() will come in handy! function namePirate(jersey) { var name; switch(jersey) { case 77: name = "D.J. Carrasco"; break; case 53: name = "Brendan Donnelly"; break; case 29: name = "Octavio Dotel"; 129
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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 76 } }; pur
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CHAPTER 5 ■ MEMBER INHERITANCE 18
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CHAPTER 6 ■ FUNCTIONS AND ARRAYS
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CHAPTER 7 ■ TRAVERSING AND MODIFY
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CHAPTER 8 ■ SCRIPTING CSS 308 Rem
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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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