TypeScript Code Review Best Practices: Catching Any Types, Unsafe Casts & Mutability Bugs

Conducting code reviews for TypeScript codebases requires looking beyond basic syntax validity. Reviewers must evaluate type safety guarantees, API boundaries, and runtime safety.

TypeScript Code Review Checklist

1. Avoid as any Assertions: Flag any or blanket as type assertions; replace with unknown and explicit type guards.
2. Enforce Strict Null Checks: Ensure optional fields (string | undefined) are checked before dereferencing.
3. Prefer Readonly Data: Mark array and object parameters as readonly to prevent accidental mutations.
4. Use Discriminated Unions for Async State: Avoid boolean flag clutter (isLoading, isError, data) in favor of unified state objects.

// ❌ Flawed Review Candidate
function processData(input: any) {
  const data = input as UserData;
  return data.name.toUpperCase(); // Unsafe! Will crash if input is null or missing name.
}

// ✅ Corrected Implementation
function processDataSafe(input: unknown): string {
  if (typeof input === "object" && input !== null && "name" in input && typeof (input as any).name === "string") {
    return (input as { name: string }).name.toUpperCase();
  }
  throw new TypeError("Invalid input payload");
}