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CHAPTER 4 ■ CONTROLLING FLOW 140 Figure 4–17. Replacing a for in sloth with a for gazelle Now that you know how to replace a for in sloth with a for, while, or do while gazelle, let’s explore a couple of ways to make those three snappier. Let’s begin with our for loop from the previous sample. First, JavaScript can query local variables within a function or global variables outside of a function faster than object members such as length—like two times as fast in Explorer 7 and 8. So, rather than query length over and over in the boolean expression, i < rocketRichard.length, let’s do so one time in the initialization expression, replacing i = 0 with i = rocketRichard.length. Second, it’s faster to iterate over an array in reverse because doing so provides a way to combine the boolean expression with the increment or decrement expression. Therefore, omit the latter, and decrement the loop variable i in the boolean expression. In turn, since we are now iterating over the array in reverse, we need to tweak the function literal we pass to sort() so that rocketRichard is ordered from fewest to most goals and then from Z to A. Make the following changes, click Run, and then verify your work with Figure 4–18: var topTwenty = { "Crosby": 49, "Ovechkin": 48, "Stamkos": 48, "Marleau": 43, "Gaborik": 41, "Kovalchuk": 40, "Heatley": 39,
"Semin": 39, "Parise": 37, "Burrows": 35, "Kopitar": 34, "Ryan": 34, "Carter": 33, "Nash": 33, "Iginla": 32, "Penner": 32, "Backstrom": 31, "Hornqvist": 30, "Jokinen": 30, "Kane": 30 } var rocketRichard = ["Ovechkin", "Crosby", "Marleau", "Stamkos"], note = ""; rocketRichard.sort(function(p1, p2) { var d = topTwenty[p1] - topTwenty[p2]; if (d !== 0) { return d; } else { return (p2 < p1) ? -1 : 1; } }); for (var i = rocketRichard.length; i --; ) { note = note + rocketRichard[i] + ": " + topTwenty[rocketRichard[i]] + ", "; } note.slice(0, -2); // "Crosby: 49, Ovechkin: 48, Stamkos: 48, Marleau: 43" CHAPTER 4 ■ CONTROLLING FLOW Note that in i --, the -- operator is in the post-decrement position. Why does that matter? It matters for a couple of reasons. For one thing, if you wrote -- i instead of i --, JavaScript would never query the fourth element in rocketRichard. For another, if rocketRichard were empty, which is to say its length was 0, then our for loop would never stop iterating. So, be sure that -- is in the post-decrement position! 141
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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 30 St
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CHAPTER 2 ■ TYPE CONVERSION 32 De
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CHAPTER 2 ■ TYPE CONVERSION 38
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CHAPTER 2 ■ TYPE CONVERSION 50 Co
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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 3 ■ OPERATORS 68 // 1.25
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CHAPTER 3 ■ OPERATORS 70 • If b
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CHAPTER 3 ■ OPERATORS 72 The impo
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CHAPTER 3 ■ OPERATORS 76 } }; pur
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CHAPTER 3 ■ OPERATORS 78 saigonCi
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CHAPTER 3 ■ OPERATORS 82 loganber
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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 8 ■ SCRIPTING CSS 312 tex
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CHAPTER 8 ■ SCRIPTING CSS 314 Fig
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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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■ INDEX 474 ■ P padJSON(), 445
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