Add investigation and verification for issue #523 - util._extend deprecation

ISSUE-523-INVESTIGATION.md

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+# Investigation: Issue #523 - util._extend Deprecation Warning
+
+## Issue Summary
+**Issue:** [#523 - util._extend deprecated](https://github.com/plaid/quickstart/issues/523)  
+**Reported:** June 29, 2025  
+**Status:** Appears to be resolved
+
+## Deprecation Warning
+```
+(node:1964) [DEP0060] DeprecationWarning: The util._extend API is deprecated. 
+Please use Object.assign() instead.
+```
+
+## Investigation Results
+
+### Source Code Analysis
+✅ Searched all source files for `util._extend`  
+✅ No instances found in the quickstart application code
+
+### Dependency Analysis
+✅ Installed all dependencies (`npm install`)  
+✅ Searched all files in `node_modules` for `util._extend` pattern  
+✅ **No matches found in current dependency tree**
+
+### Current Status
+The deprecated `util._extend` API does not appear in:
+- Application source code
+- Any installed dependencies (as of current package.json versions)
+
+## Verification
+
+A verification script has been added: `node/verify-issue-523.js`
+
+To verify yourself:
+```bash
+cd node
+npm install
+node verify-issue-523.js
+npm start  # Check if deprecation warning appears
+```
+
+## Conclusion
+
+The issue appears to be **resolved** through natural dependency updates. The deprecated code is no longer present in the current version of dependencies specified in `package.json`.
+
+### Possible Resolution Paths
+1. **Dependency Updates**: One or more dependencies were updated and no longer use `util._extend`
+2. **Node.js Version**: Newer Node.js versions may have stricter deprecation handling
+3. **Transitive Dependency Updates**: A sub-dependency was updated by maintainers
+
+## Recommendation
+
+If testing confirms no deprecation warning appears with current dependencies, this issue can be **closed as resolved**.
+
+---
+
+**Verification Date:** December 2025  
+**Node.js Version Tested:** See verification script output  
+**Dependencies:** See `package.json` and `package-lock.json`

node/verify-issue-523.js

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+#!/usr/bin/env node
+
+/**
+ * Verification script for Issue #523
+ * Checks if util._extend deprecation warning appears
+ * 
+ * This script helps verify whether the deprecated util._extend API
+ * is still being used in the application or its dependencies.
+ */
+
+const util = require('util');
+
+console.log('=== Issue #523 Verification Script ===\n');
+console.log('Checking for util._extend usage...\n');
+
+// Check if util._extend exists
+if (util._extend) {
+  console.log('⚠️  util._extend is available in this Node.js version');
+  console.log(`   Node version: ${process.version}`);
+
+  // Test if using it triggers the warning
+  console.log('\nTesting if util._extend triggers deprecation warning:');
+  const testObj = { a: 1 };
+  const extended = util._extend({}, testObj);
+  console.log('   Result:', extended);
+} else {
+  console.log('✅ util._extend is not available (removed in newer Node.js)');
+  console.log(`   Node version: ${process.version}`);
+}
+
+console.log('\n=== Dependency Check ===');
+console.log('To manually check dependencies, run:');
+console.log('  Get-ChildItem -Path node_modules -Recurse -Include *.js | Select-String -Pattern "util\\._extend" -List\n');
+
+console.log('=== Recommendation ===');
+if (util._extend) {
+  console.log('If you see a deprecation warning above, it means:');
+  console.log('  1. Some dependency is using util._extend, OR');
+  console.log('  2. This test script triggered it (expected)');
+  console.log('\nTo identify the source:');
+  console.log('  - Run the main application (npm start)');
+  console.log('  - If no warning appears, the issue is likely resolved');
+} else {
+  console.log('✅ This Node.js version has removed util._extend entirely.');
+  console.log('   The deprecation issue cannot occur.');
+}
+
+console.log('\n=== End of Verification ===\n');