Fix TypeScript Strict Mode Errors with Claude Code

The Problem

You enable "strict": true in tsconfig.json and suddenly your project has hundreds of errors:

src/api/users.ts:23:5 - error TS2322: Type 'string | undefined' is not assignable to type 'string'.
src/utils/format.ts:8:22 - error TS7006: Parameter 'item' implicitly has an 'any' type.
src/models/User.ts:12:3 - error TS2564: Property 'email' has no initializer.

Quick Fix

Ask Claude Code to fix strict mode errors file by file:

Run tsc --noEmit and show me the error count per file. Then fix each file
starting with the one that has the most errors. Use proper types, not any.

What strict: true Enables

Step-by-Step Fix

Step 1: Enable strict mode incrementally

{
  "compilerOptions": {
    "strict": false,
    "strictNullChecks": true
  }
}

Step 2: Fix strictNullChecks errors

// Error: Object is possibly 'undefined'
function getUserName(user?: User): string {
  return user.name; // TS2532
}

// Fix with a guard
function getUserName(user?: User): string {
  if (!user) throw new Error('User is required');
  return user.name;
}

// Or with a default
function getUserName(user?: User): string {
  return user?.name ?? 'Anonymous';
}

Step 3: Fix noImplicitAny errors

// Error: Parameter 'item' implicitly has an 'any' type
function processItems(items) {
  return items.map(item => item.name);
}

// Fix: add type annotations
interface Item { name: string; value: number; }
function processItems(items: Item[]): string[] {
  return items.map(item => item.name);
}

Step 4: Fix strictPropertyInitialization

// Error: Property 'email' has no initializer
class User {
  name: string;  // TS2564
  email: string; // TS2564
}

// Fix: Initialize in constructor
class User {
  name: string;
  email: string;
  constructor(name: string, email: string) {
    this.name = name;
    this.email = email;
  }
}

Step 5: Fix useUnknownInCatchVariables

// Error: Object is of type 'unknown'
try { await fetchData(); }
catch (error) { console.log(error.message); } // TS18046

// Fix: type guard
try { await fetchData(); }
catch (error) {
  if (error instanceof Error) {
    console.log(error.message);
  } else {
    console.log('Unknown error:', String(error));
  }
}

Prevention

Always start new projects with strict mode enabled. Add to your CLAUDE.md: strict mode is enabled, do not add @ts-ignore or type assertions. Use proper null guards, not non-null assertions. Every function parameter must have an explicit type. Prefer unknown over any for dynamic values.