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Maintainable JavaScript

Cover of Maintainable JavaScript by Nicholas C. Zakas Published by O'Reilly Media, Inc.
  1. Maintainable JavaScript
  2. SPECIAL OFFER: Upgrade this ebook with O’Reilly
  3. Introduction
  4. Preface
    1. Conventions Used in This Book
    2. Using Code Examples
    3. Safari® Books Online
    4. How to Contact Us
  5. I. Style Guidelines
    1. 1. Basic Formatting
      1. Indentation Levels
      2. Statement Termination
      3. Line Length
      4. Line Breaking
      5. Blank Lines
      6. Naming
      7. Literal Values
    2. 2. Comments
      1. Single-Line Comments
      2. Multiline Comments
      3. Using Comments
      4. Documentation Comments
    3. 3. Statements and Expressions
      1. Brace Alignment
      2. Block Statement Spacing
      3. The switch Statement
      4. The with Statement
      5. The for Loop
      6. The for-in Loop
    4. 4. Variables, Functions, and Operators
      1. Variable Declarations
      2. Function Declarations
      3. Function Call Spacing
      4. Immediate Function Invocation
      5. Equality
      6. eval()
      7. Primitive Wrapper Types
  6. II. Programming Practices
    1. 5. Loose Coupling of UI Layers
      1. What Is Loose Coupling?
      2. Keep JavaScript Out of CSS
      3. Keep CSS Out of JavaScript
      4. Keep JavaScript Out of HTML
      5. Keep HTML Out of JavaScript
    2. 6. Avoid Globals
      1. The Problems with Globals
      2. Accidental Globals
      3. The One-Global Approach
      4. The Zero-Global Approach
    3. 7. Event Handling
      1. Classic Usage
      2. Rule #1: Separate Application Logic
      3. Rule #2: Don’t Pass the Event Object Around
    4. 8. Avoid Null Comparisons
      1. Detecting Primitive Values
      2. Detecting Reference Values
      3. Detecting Properties
    5. 9. Separate Configuration Data from Code
      1. What Is Configuration Data?
      2. Externalizing Configuration Data
      3. Storing Configuration Data
    6. 10. Throw Your Own Errors
      1. The Nature of Errors
      2. Throwing Errors in JavaScript
      3. Advantages of Throwing Errors
      4. When to Throw Errors
      5. The try-catch Statement
      6. Error Types
    7. 11. Don’t Modify Objects You Don’t Own
      1. What Do You Own?
      2. The Rules
      3. Better Approaches
      4. A Note on Polyfills
      5. Preventing Modification
    8. 12. Browser Detection
      1. User-Agent Detection
      2. Feature Detection
      3. Avoid Feature Inference
      4. Avoid Browser Inference
      5. What Should You Use?
  7. III. Automation
    1. 13. File and Directory Structure
      1. Best Practices
      2. Basic Layout
    2. 14. Ant
      1. Installation
      2. The Build File
      3. Running the Build
      4. Target Dependencies
      5. Properties
      6. Buildr
    3. 15. Validation
      1. Finding Files
      2. The Task
      3. Improving the Target
      4. Other Improvements
      5. Buildr Task
    4. 16. Concatenation and Baking
      1. The Task
      2. Line Endings
      3. Headers and Footers
      4. Baking Files
    5. 17. Minification and Compression
      1. Minification
      2. Compression
    6. 18. Documentation
      1. JSDoc Toolkit
      2. YUI Doc
    7. 19. Automated Testing
      1. YUI Test Selenium Driver
      2. Yeti
      3. PhantomJS
      4. JsTestDriver
    8. 20. Putting It Together
      1. Missing Pieces
      2. Planning the Build
      3. Using a CI System
  8. A. JavaScript Style Guide
    1. Indentation
    2. Line Length
    3. Primitive Literals
    4. Operator Spacing
    5. Parentheses Spacing
    6. Object Literals
    7. Comments
      1. Single-Line Comments
      2. Multiline Comments
      3. Comment Annotations
    8. Variable Declarations
    9. Function Declarations
    10. Naming
    11. Strict Mode
    12. Assignments
    13. Equality Operators
    14. Ternary Operator
    15. Statements
      1. Simple Statements
      2. return Statement
      3. Compound Statements
      4. if Statement
      5. for Statement
      6. while Statement
      7. do Statement
      8. switch Statement
      9. try Statement
    16. White Space
    17. Things to Avoid
  9. B. JavaScript Tools
    1. Build Tools
    2. Documentation Generators
    3. Linting Tools
    4. Minification Tools
    5. Testing Tools
  10. Index
  11. About the Author
  12. Colophon
  13. SPECIAL OFFER: Upgrade this ebook with O’Reilly
  14. Copyright
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Chapter 7. Event Handling

Event handling is an important part of any JavaScript application. All JavaScript is tied to the UI through events, so most web developers spend much of their time coding and modifying event handlers. Unfortunately, this is also an area of JavaScript programming that hasn’t received much attention since the language was first introduced. Even as developers started to embrace more traditional concepts of architecture in JavaScript, event handling was one of those areas in which little has changed. Most event-handling code is very tightly coupled to the event environment (what is available to the developer at the time an event is fired) and therefore is not very maintainable.

Classic Usage

Most developers are familiar with the event object that is passed in to an event handler when the event is fired. The event object contains all of the information related to the event, including the event target as well as additional data based on the event type. Mouse events expose additional location information on the event object, keyboard events expose information about keys that have been pressed, and touch events expose information about the location and duration of touches. All of this information is provided so that the UI can react appropriately.

In many cases, however, you end up using a very small subset of the information present on event. Consider the following:

// Bad function handleClick(event) { var popup = document.getElementById("popup"); popup.style.left ...

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