Add exitCode property as alias for code in error objects

[konard]

🤖 AI-Powered Solution

This pull request automatically solves issue #38 by adding error.exitCode as an alias for error.code to maintain compatibility with Node.js standard error handling patterns.

📋 Issue Reference

Fixes #38

🔧 What Changed

Core Implementation

• Added error.exitCode property: All error objects now include both error.code and error.exitCode with the same value
• Fixed virtual exit command: Now throws proper Error objects instead of plain objects
• Maintained backward compatibility: Existing code using error.code continues to work unchanged

Modified Files

• src/$.mjs: Added error.exitCode = <code> alongside every error.code = <code> assignment
• src/commands/$.exit.mjs: Fixed to throw proper Error objects with both properties
• tests/exitcode-compatibility.test.mjs: Comprehensive test suite for both patterns
• examples/test-exitcode-compatibility.mjs: Demonstration script showing compatibility

✅ Solution Details

The fix implements Option 1: Add Alias (Backward Compatible) from the issue description:

// Before (command-stream only)
catch (error) {
  if (error.code === 1) { /* handle */ }
  // error.exitCode was undefined ❌
}

// After (both patterns work)
catch (error) {
  if (error.code === 1) { /* handle */ }     // ✅ Still works
  if (error.exitCode === 1) { /* handle */ } // ✅ Now works too
}

🧪 Testing

All error scenarios now provide both properties:

• Command execution failures (errexit)
• Pipeline failures (pipefail)
• Virtual command failures (exit, etc.)
• File system errors

Test Coverage:

• ✅ Both properties exist and have same value
• ✅ Backward compatibility with existing error.code usage
• ✅ Forward compatibility with Node.js standard error.exitCode
• ✅ Pipeline error handling
• ✅ Multiple exit code values (1, 2, 127, 255)
• ✅ File system error handling

📊 Compatibility Matrix

Library	Exit Code Property	Status
Node.js child_process	error.exitCode	✅ Compatible
execa	error.exitCode	✅ Compatible
cross-spawn	error.exitCode	✅ Compatible
command-stream (before)	error.code	❌ Non-standard
command-stream (after)	error.code + error.exitCode	✅ Both supported

🚀 Impact

This change enables:

• Easy migration from other command execution libraries
• Standard Node.js patterns without modification
• Zero breaking changes to existing code
• Improved developer experience following principle of least surprise

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This PR was automatically generated and tested by the AI issue solver