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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 9. Separate Configuration Data from Code

Code does nothing more than define a set of instructions for a computer to execute. Data is frequently passed around and modified by those instructions, ultimately producing a result. The problem comes when the data must change. There’s a risk of creating an error any time you edit source code, and editing code just to change some data value introduces unnecessary risk for something that shouldn’t affect the surrounding instructions. Well-designed applications keep vital data outside of the main source code to ensure worry-free editing.

What Is Configuration Data?

Configuration data is any hardcoded value in an application. Consider the following example:

// Configuration data embedded in code
function validate(value) {
    if (!value) {
        alert("Invalid value");
        location.href = "/errors/invalid.php";
    }
}

function toggleSelected(element) {
    if (hasClass(element, "selected")) {
        removeClass(element, "selected");
    } else {
        addClass(element, "selected");
    }
}

There are three pieces of configuration data in this code. The first is the string “Invalid value,” which is displayed to the user. As a UI string, there’s a good chance that it will change frequently. The second is the URL /errors/invalid.php. URLs tend to change as development progresses, due to architectural decisions. The third is the CSS class name “selected.” This class name is used three times, meaning that a class name change requires changes in three different places, increasing the ...

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