JavaScript Code Quality Detection Tools for Common Issues

When developers ask "why JavaScript is bad," they often point to specific code quality issues. These ready-to-use JavaScript utilities help detect and analyze the most commonly criticized JavaScript patterns, helping you understand and fix problematic code.

Type Coercion Detection Tool #

Detect potentially problematic type coercion that makes developers think JavaScript is bad:

## Global Variable Leak Detector

Identify accidental global variables that contribute to why JavaScript is bad:

::javascript-runner
---
code: |
  // Global leak detection utility
  function detectGlobalLeaks(functionCode) {
      const issues = [];
      const lines = functionCode.split('\n');
      
      lines.forEach((line, index) => {
          // Check for assignments without declarations
          const assignment = /^\s*([a-zA-Z_$][a-zA-Z0-9_$]*)\s*=/;
          const declaration = /^\s*(var|let|const|function)\s+/;
          
          if (assignment.test(line) && !declaration.test(line)) {
              const match = line.match(assignment);
              if (match) {
                  issues.push({
                      line: index + 1,
                      variable: match[1],
                      issue: 'Potential global variable leak'
                  });
              }
          }
          
          // Check for undeclared loop variables
          if (line.includes('for') && /for\s*\(\s*[a-zA-Z_$][a-zA-Z0-9_$]*\s*=/.test(line)) {
              if (!/for\s*\(\s*(var|let|const)\s+/.test(line)) {
                  issues.push({
                      line: index + 1,
                      issue: 'For loop variable not declared - becomes global'
                  });
              }
          }
      });
      
      return issues;
  }
  
  // Test with problematic code
  const badCode = `
  function example() {
      globalVar = "Oops!";
      for (i = 0; i < 10; i++) {
          console.log(i);
      }
      result = calculateSomething();
  }`;
  
  console.log('Global leak detection results:');
  detectGlobalLeaks(badCode).forEach(issue => {
      console.log(`Line ${issue.line}: ${issue.issue}`);
      if (issue.variable) console.log(`  Variable: ${issue.variable}`);
  });
---
::

## Context Binding Issue Detector

Find problematic `this` usage that contributes to JavaScript criticism:

```javascript
// Context binding analyzer
function analyzeThisUsage(code) {
    const issues = [];
    const lines = code.split('\n');
    
    lines.forEach((line, index) => {
        // Check for 'this' in callbacks without binding
        if (line.includes('this.') && 
            (line.includes('setTimeout') || 
             line.includes('setInterval') || 
             line.includes('addEventListener'))) {
            issues.push({
                line: index + 1,
                issue: 'Potential this context loss in callback',
                suggestion: 'Use arrow function or .bind(this)'
            });
        }
        
        // Check for function expressions with 'this'
        if (line.includes('function(') && line.includes('this.')) {
            issues.push({
                line: index + 1,
                issue: 'Function expression may lose this context',
                suggestion: 'Consider using arrow function'
            });
        }
    });
    
    return issues;
}

// Example usage
const contextProblems = `
obj.method = function() {
    setTimeout(function() {
        this.value = 'changed'; // Problem: 'this' is not obj
    }, 100);
};`;

console.log('Context binding issues:');
analyzeThisUsage(contextProblems).forEach(issue => {
    console.log(`Line ${issue.line}: ${issue.issue}`);
    console.log(`  Suggestion: ${issue.suggestion}`);
});

Callback Hell Detector #

Identify nested callback patterns that make JavaScript appear difficult:

Array Method Mutation Analyzer #

Detect array methods that mutate vs. return new arrays:

// Array mutation pattern analyzer
function analyzeArrayMutations(code) {
    const mutatingMethods = ['push', 'pop', 'shift', 'unshift', 'splice', 'sort', 'reverse'];
    const nonMutatingMethods = ['map', 'filter', 'reduce', 'forEach', 'find', 'some', 'every'];
    
    const analysis = {
        mutatingCalls: [],
        nonMutatingCalls: [],
        mixedUsage: false
    };
    
    const lines = code.split('\n');
    
    lines.forEach((line, index) => {
        mutatingMethods.forEach(method => {
            if (line.includes(`.${method}(`)) {
                analysis.mutatingCalls.push({
                    line: index + 1,
                    method: method,
                    code: line.trim()
                });
            }
        });
        
        nonMutatingMethods.forEach(method => {
            if (line.includes(`.${method}(`)) {
                analysis.nonMutatingCalls.push({
                    line: index + 1,
                    method: method,
                    code: line.trim()
                });
            }
        });
    });
    
    analysis.mixedUsage = analysis.mutatingCalls.length > 0 && analysis.nonMutatingCalls.length > 0;
    
    return analysis;
}

// Usage example
const arrayCode = `
const numbers = [1, 2, 3];
numbers.push(4);              // Mutates
const doubled = numbers.map(n => n * 2);  // Doesn't mutate
numbers.sort();              // Mutates
const filtered = numbers.filter(n => n > 2);  // Doesn't mutate
`;

const result = analyzeArrayMutations(arrayCode);
console.log('Array method analysis:');
console.log(`Mixed usage detected: ${result.mixedUsage}`);
console.log('Mutating methods:', result.mutatingCalls.map(call => call.method));
console.log('Non-mutating methods:', result.nonMutatingCalls.map(call => call.method));

Comprehensive Code Quality Checker #

Combine all detectors into a single utility:

Usage Tips #

These detection tools help identify why JavaScript gets criticized:

1. Run before code reviews to catch common issues
2. Integrate with build processes for automated quality checks
3. Use as learning tools to understand JavaScript pitfalls
4. Combine with ESLint for comprehensive analysis
5. Create custom rules for team-specific patterns

Summary #

While critics point to various reasons why JavaScript is bad, these detection tools help you:

• Identify problematic patterns before they cause issues
• Learn to recognize anti-patterns in existing code
• Improve overall code quality and maintainability
• Address the most common JavaScript criticisms proactively

Understanding and detecting these issues makes you a better JavaScript developer and helps counter the "why JavaScript is bad" narrative with concrete improvements.