How I Fixed 50 TypeScript Errors in 10 Minutes with Claude Code

The Situation

It was Friday afternoon. I'd just upgraded our project from TypeScript 4.9 to 5.3. The terminal exploded with 50 type errors across 23 files. The old me would have sighed, grabbed more coffee, and settled in for a long evening. But I had Claude Code.

$ tsc --noEmit

Found 50 errors in 23 files.

src/services/UserService.ts(34,5): error TS2345...

src/utils/validation.ts(12,20): error TS2322...

src/components/DataTable.tsx(89,12): error TS2739...

... 47 more errors

The Traditional Approach

Normally, I'd click through each error, understand the context, research the TypeScript change that caused it, and manually fix each one. For 50 errors, that's easily 2-3 hours of tedious work. And that's if none of the fixes cascade into new errors.

The Claude Code Approach

Step 1: Set the Context (30 seconds)

"I just upgraded TypeScript from 4.9 to 5.3. Run tsc and fix all the type errors. Maintain the existing code patterns and don't change any actual functionality."

Step 2: Let Claude Work (5 minutes)

Claude immediately ran `tsc --noEmit` to see all the errors, then started methodically working through them. It understood the patterns in our codebase and fixed each error consistently.

The errors fell into categories:

Type narrowing changes: 15 errors
Stricter generic constraints: 12 errors
Updated module resolution: 8 errors
Deprecated API usage: 10 errors
Null safety improvements: 5 errors

Step 3: Review the Fixes (4 minutes)

I scanned through the changes. Each fix was clean, minimal, and followed our existing patterns. A few examples:

// Before: Type narrowing issue

if (user) {
  return user.name; // Error: 'user' could be undefined
}

// After: Proper type guard

if (user !== undefined && user !== null) {
  return user.name; // OK
}

Step 4: Run Tests (30 seconds)

"Run the test suite to make sure nothing broke"

$ npm test

All 234 tests passed.

Why This Worked So Well

Pattern Recognition

Claude understood that similar errors should be fixed the same way across files. Consistency that would be hard to maintain manually.

Full Codebase Context

Claude read the surrounding code and understood our conventions. Fixes matched our style, not generic solutions.

TypeScript Knowledge

Claude knew exactly what changed between TS versions and applied the correct fix for each error type.

No Copy-Paste Errors

Every fix was fresh. No accidentally carrying over mistakes from one file to another.

The Results

10 min

Total time

Errors fixed

Files modified

100%

Tests passing

Key Takeaways

Be specific about what not to change: Saying "don't change functionality" prevented over-eager refactoring.
Let AI categorize the errors: It naturally grouped similar issues and fixed them consistently.
Trust but verify: The review step caught one edge case that needed manual adjustment.
Run tests: Always verify with your test suite before committing.

That Friday evening? I went home on time.

How I Fixed 50 TypeScript Errors in 10 Minutes