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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 6. Avoid Globals

The JavaScript execution environment is unique in a lot of ways. One of those ways is in the use of global variables and functions. The default JavaScript execution environment is, in fact, defined by the various globals available to you at the start of script execution. All of these are said to be defined on the global object, a mysterious object that represents the outermost context for a script.

In browsers, the window object is typically overloaded to also be the global object, so any variable or function declared in the global scope becomes a property of the window object. For example:

var color = "red"

function sayColor() {
    alert(color);
}

console.log(window.color);              // "red"
console.log(typeof window.sayColor);    // "function"

In this code, the global variable color and the global function sayColor() are defined. Both are added to the window object as properties even though they weren’t explicitly set to do so.

The Problems with Globals

Creating globals is considered a bad practice in general and is specifically problematic in a team development environment. Globals create a number of nontrivial maintenance issues for code going forward. The more globals, the greater the possibility that errors will be introduced due to the increasing likelihood of a few common problems.

Naming Collisions

The potential for naming collisions increases as the number of global variables and functions increase in a script, as do the chances that you’ll use an already declared variable ...

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