typescript-strict-migrator

TypeScript Strict Migrator

Incrementally migrate to TypeScript strict mode for maximum type safety.

Core Workflow

1. Audit current state: Check existing type errors
2. Enable incrementally: One flag at a time
3. Fix errors: Systematic approach per flag
4. Add type guards: Runtime type checking
5. Use utility types: Proper type transformations
6. Document patterns: Team guidelines

Strict Mode Flags

// tsconfig.json - Full strict mode
{
  "compilerOptions": {
    // Master flag (enables all below)
    "strict": true,

    // Individual flags (enabled by strict)
    "noImplicitAny": true,
    "strictNullChecks": true,
    "strictFunctionTypes": true,
    "strictBindCallApply": true,
    "strictPropertyInitialization": true,
    "noImplicitThis": true,
    "useUnknownInCatchVariables": true,
    "alwaysStrict": true,

    // Additional strict-ish flags (not in strict)
    "noUncheckedIndexedAccess": true,
    "noImplicitReturns": true,
    "noFallthroughCasesInSwitch": true,
    "noImplicitOverride": true,
    "exactOptionalPropertyTypes": true,
    "noPropertyAccessFromIndexSignature": true
  }
}

Incremental Migration Strategy

Phase 1: Basic Strict Flags

// tsconfig.json - Phase 1
{
  "compilerOptions": {
    "strict": false,
    "noImplicitAny": true,
    "alwaysStrict": true
  }
}

// Before: implicit any
function processData(data) {
  return data.map(item => item.value);
}

// After: explicit types
function processData(data: DataItem[]): number[] {
  return data.map(item => item.value);
}

interface DataItem {
  value: number;
  label: string;
}

Phase 2: Strict Null Checks

// tsconfig.json - Phase 2
{
  "compilerOptions": {
    "noImplicitAny": true,
    "strictNullChecks": true
  }
}

// Before: potential null errors
function getUserName(user: User) {
  return user.profile.name;  // Error if profile is undefined
}

// After: proper null handling
function getUserName(user: User): string | undefined {
  return user.profile?.name;
}

// With non-null assertion (use sparingly)
function getUserNameOrThrow(user: User): string {
  if (!user.profile?.name) {
    throw new Error('User has no name');
  }
  return user.profile.name;
}