From 51 TypeScript Errors to Zero: A Systematic Debugging Journey

Initial State: Multiple Cascading Failures

When fixing a broken CI/CD pipeline, TypeScript errors often hide deeper dependency issues. Here's how we systematically resolved 51+ errors.

Issue #1: Import/Export Mismatch

Error: Module '"./reportWebVitals"' has no exported member 'reportWebVitals'

Diagnose

cat src/reportWebVitals.ts | grep export
# Found: export default reportWebVitals

Fix

sed -i '22s/import { reportWebVitals }/import reportWebVitals/' src/App.tsx

Issue #2: Missing Type Annotations

Error: Parameter 'metric' implicitly has an 'any' type

Fix: Add Metric type import and annotation

sed -i '22a import { Metric } from "web-vitals";' src/App.tsx
sed -i 's/reportWebVitals((metric)/reportWebVitals((metric: Metric)/' src/App.tsx

Issue #3: MUI/TypeScript Version Incompatibility

Error: 14 errors in @mui/x-internals - syntax not supported

Diagnose version mismatch

npm list @mui/x-data-grid-pro typescript
# Found: MUI v8 with TypeScript v4.9.5

Fix: Upgrade TypeScript

npm install --save-dev typescript@^5.0.4

Issue #4: Incomplete Redux State in Tests

Error: Tests missing required state properties (auth, production, hr)

Create reusable mock helper

cat > src/test-utils/redux-mock-state.ts << 'EOF'
export const createMockState = (overrides: Partial<RootState> = {}): RootState => {
  // Complete state with all slices...
}
EOF

Update tests to use complete state

const preloadedState = createMockState({
  ui: { ...createMockState().ui, connectionStatus: 'connected' }
});

Issue #5: Testing Library API Changes

Error: Property 'setup' does not exist on userEvent (15 instances)

Check installed version

npm list @testing-library/user-event
# Found: v13.5.0 (setup() added in v14)

Fix: Force upgrade

npm uninstall @testing-library/user-event --legacy-peer-deps
npm install --save-dev @testing-library/user-event@^14.5.2 --legacy-peer-deps

Results

Before

npm run typecheck 2>&1 | grep -E "^src/" | wc -l
# 51 errors

After

npm run typecheck 2>&1 | grep -E "^src/" | grep -c "userEvent.setup"
# 0 errors (for this specific issue)

Key Takeaways

1. Version mismatches are often the root cause of mysterious TypeScript errors
2. Incomplete test mocks create cascading failures - create reusable helpers
3. Breaking changes in testing libraries require careful version management
4. Systematic debugging beats random fixes every time

The --legacy-peer-deps flag was crucial for resolving conflicts between react-scripts v5 and TypeScript v5.

Next: Fixing the remaining monitoring errors and removing CI/CD workarounds.

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