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1．UnderstandingAjaxPerformance 1

Trade-offs 1

Principles of Optimization 1

Ajax 4

Browser 4

Wow! 5

JavaScript 6

Summary 6

2．CreatingResponsiveWebApplications 7

What Is Fast Enough? 9

Measuring Latency 10

When Latency Goes Bad 12

Threading 12

Ensuring Responsiveness 13

Web Workers 14

Gears 14

Timers 16

Effects of Memory Use on Response Time 17

Virtual Memory 18

Troubleshooting Memory Issues 18

Summary 19

3．SplittingtheInitial Payload 21

Kitchen Sink 21

Savings from Splitting 22

Finding the Split 23

Undefined Symbols and Race Conditions 24

Case Study:Google Calendar 25

4．Loading Scripts Without Blocking 27

Scripts Block 27

Making Scripts Play Nice 29

XHR Eval 29

XHR Injection 31

Script in Iframe 31

Script DOM Element 32

Script Defer 32

document.write Script Tag 33

Browser Busy Indicators 33

Ensuring(or Avoiding)Ordered Execution 35

Summarizing the Results 36

And the Winner Is 38

5．Coupling Asynchronous Scripts 41

Code Example:menu.js 42

Race Conditions 44

Preserving Order Asynchronously 45

Technique 1:Hardcoded Callback 46

Technique 2:Window Onload 47

Technique 3:Timer 48

Technique 4:Script Onload 49

Technique 5:Degrading Script Tags 50

Multiple External Scripts 52

Managed XHR 52

DOM Element and Doc Write 56

General Solution 59

Single Script 59

Multiple Scripts 60

Asynchronicity in the Real World 63

Google Analytics and Dojo 63

YUI Loader Utility 65

6．Positioning Inline Scripts 69

Inline Scripts Block 69

Move Inline Scripts to the Bottom 70

Initiate Execution Asynchronously 71

Use Script Defer 73

Preserving CSS and JavaScript Order 73

Danger:Stylesheet Followed by Inline Script 74

Inline Scripts Aren't Blocked by Most Downloads 74

Inline Scripts Are Blocked by Stylesheets 75

This Does Happen 77

7．Writing Efficient JavaScript 79

Managing Scope 79

Use Local Variables 81

Scope Chain Augmentation 83

Efficient Data Access 85

Flow Control 88

Fast Conditionals 89

Fast Loops 93

String Optimization 99

String Concatenation 99

Trimming Strings 100

Avoid Long-Running Scripts 102

Yielding Using Timers 103

Timer Patterns for Yielding 105

Summary 107

8．Scaling with Comet 109

How Comet Works 109

Transport Techniques 111

Polling 111

Long Polling 112

Forever Frame 113

XHR Streaming 115

Future Transports 116

Cross-Domain 116

Effects of Implementation on Applications 118

Managing Connections 118

Measuring Performance 119

Protocols 119

Summary 120

9．Going Beyond Gzipping 121

Why Does This Matter? 121

What Causes This? 123

Quick Review 123

The Culprit 123

Examples of Popular Turtle Tappers 124

How to Help These Users? 124

Design to Minimize Uncompressed Size 125

Educate Users 129

Direct Detection of Gzip Support 130

10．Optimizing Images 133

Two Steps to Simplify Image Optimization 134

Image Formats 135

Background 135

Characteristics of the Different Formats 137

More About PNG 139

Automated Lossless Image Optimization 141

Crushing PNGs 142

Stripping JPEG Metadata 143

Converting GIF to PNG 144

Optimizing GIF Animations 144

Smush.it 145

Progressive JPEGs for Large Images 145

Alpha Transparency:Avoid AlphaImageLoader 146

Effects of Alpha Transparency 147

AlphaImageLoader 149

Problems with AlphaImageLoader 150

Progressively Enhanced PNG8 Alpha Transparency 151

Optimizing Sprites 153

？ber-Sprite Versus Modular Sprite 154

Highly Optimized CSS Sprites 155

Other Image Optimizations 155

Avoid Scaling Images 155

Crush Generated Images 156

Favicons 157

Apple Touch Icon 158

Summary 159

11．Sharding Dominant Domains 161

Critical Path 161

Who's Sharding? 163

Downgrading to HTTP/1.0 165

Rolling Out Sharding 168

IP Address or Hostname 168

How Many Domains 168

How to Split Resources 168

Newer Browsers 169

12．Flushing the Document Early 171

Flush the Head 171

Output Buffering 173

Chunked Encoding 175

Flushing and Gzip 176

Other Intermediaries 177

Domain Blocking During Flushing 178

Browsers:The Last Hurdle 178

Flushing Beyond PHP 179

The Flush Checklist 180

13．Using Iframes Sparingly 181

The Most Expensive DOM Element 181

Iframes Block Onload 182

Parallel Downloads with Iframes 184

Script Before Iframe 184

Stylesheet Before Iframe 185

Stylesheet After Iframe 186

Connections per Hostname 187

Connection Sharing in Iframes 187

Connection Sharing Across Tabs and Windows 188

Summarizing the Cost of Iframes 190

14．Simplifying CSS Selectors 191

Types of Selectors 191

ID Selectors 192

Class Selectors 193

Type Selectors 193

Adjacent Sibling Selectors 193

Child Selectors 193

Descendant Selectors 193

Universal Selectors 194

Attribute Selectors 194

Pseudo-Classes and Pseudo-Elements 194

The Key to Efficient CSS Selectors 194

Rightmost First 195

Writing Efficient CSS Selectors 195

CSS Selector Performance 197

Complex Selectors Impact Performance(Sometimes) 197

CSS Selectors to Avoid 200

Reflow Time 201

Measuring CSS Selectors in the Real World 202

Appendix:Performance Tools 205

Index 221