TypeScript Strict Mode in Practice: Catching Bugs with Type Safety

This article is part of the and Design Advent Calendar 2025 (Day 18).

Yesterday I wrote about "Semantic Search." Today, I'll share my experience introducing TypeScript's strict mode to an existing project.

Why I Introduced Strict Mode

Even with TypeScript, runtime errors still occur.

Cannot read property 'name' of null
undefined is not a function

When these errors started piling up, I investigated and found a common pattern. They were happening in places where type checking was too lenient.

TypeScript uses lenient settings by default. Without enabling strict mode, many problems slip through undetected.

Options I Enabled

{
  "compilerOptions": {
    "strict": false,
    "noImplicitAny": true,
    "strictNullChecks": true,
    "noUnusedLocals": false,
    "noUnusedParameters": false
  }
}

noImplicitAny

Parameters without type annotations implicitly become any type. Since any allows any operation, type checking becomes ineffective.

// With noImplicitAny: false
function double(value) {  // value is any type
  return value * 2;
}

double("hello");  // Compiles fine, but result is NaN

With noImplicitAny: true, parameters without type annotations cause errors. You can immediately spot "places where you forgot to write types."

strictNullChecks

Without this option, all types implicitly include null and undefined.

// With strictNullChecks: false
const user: User = getUser();  // Might return null
console.log(user.name);  // Could crash at runtime

With strictNullChecks: true, functions that might return null must explicitly declare User | null. Since you get compile errors without null checks, you can prevent missed checks.

Effect of These Two Options

Option	Errors Prevented
noImplicitAny	Crashes from operating on unknown types
strictNullChecks	Crashes from null/undefined access

I delegated unused variable checking (noUnusedLocals) to Biome. TypeScript doesn't recognize _ prefixed variables, which causes issues when you only want to use part of a destructured assignment.

Division of Responsibilities: TypeScript and Biome

Biome is a tool that combines linting (code quality checking) and formatting (code styling). I chose it over ESLint because it's faster and has simpler configuration.

I divide checking responsibilities between TypeScript and Biome.

Check Item	Tool	Reason
Type consistency	TypeScript	Core strength of the type system
Null safety	TypeScript	Detectable at type level
Unused variables	Biome	Supports _ prefix convention
any type usage	Biome	Easier exception management with biome-ignore
Code style	Biome	Managed together with formatting

Here's an example Biome configuration.

{
  "linter": {
    "rules": {
      "correctness": {
        "noUnusedVariables": "warn",
        "noUnusedImports": "warn"
      },
      "suspicious": {
        "noExplicitAny": "error",
        "noDoubleEquals": "error"
      }
    }
  }
}

By setting noExplicitAny to error, I strictly limit the use of any types. When absolutely necessary, I leave a comment explaining why.

Bugs Prevented by Strict Mode

Here are typical bug patterns that strict mode can detect.

1. Missing Null Checks

The most common issue is missing null checks.

// Problematic code
function getUserName(user: User | null) {
  return user.name; // user might be null
}

With strictNullChecks enabled, this becomes a compile-time error.

// Fixed
function getUserName(user: User | null) {
  return user?.name ?? 'Anonymous';
}

This pattern is especially common with relational data. When a user is deleted, the creator field of related data becomes null. Code that doesn't account for this will crash at runtime.

2. Handling Optional Properties

Optional properties in API responses are another easy-to-miss pattern.

interface ApiResponse {
  data: {
    items: Item[];
    nextCursor?: string; // Optional
  };
}

// Problematic code
function getNextPage(response: ApiResponse) {
  return fetch(`/api?cursor=${response.data.nextCursor}`);
  // If nextCursor is undefined, this becomes "?cursor=undefined"
}

// Fixed
function getNextPage(response: ApiResponse) {
  if (!response.data.nextCursor) return null;
  return fetch(`/api?cursor=${response.data.nextCursor}`);
}

3. Type Definition and Schema Mismatch

During refactoring, it's easy to forget to update type definitions.

// Changed DB schema
// column_name → field_name

// Forgot to update type definition
interface Column {
  column_name: string; // Still using old name
}

With strict mode, everywhere using this type will show errors. You'll see red underlines in your IDE, so you won't miss any needed fixes.

IDE Benefits

When you enable strict mode, you immediately benefit in editors like VS Code.

Real-time Error Detection

When you write problematic code, red squiggly lines appear in the editor. You can catch issues before running the code, dramatically reducing debugging time.

Accurate Completions

When types are clear, property and method completion suggestions become accurate. When you type user., only properties that actually exist like name and email appear as suggestions.

Safer Refactoring

When you change a type definition, errors appear everywhere affected. You don't need to manually search for "what needs to be fixed."

// If you remove email from User type
interface User {
  id: string;
  name: string;
  // email: string;  removed
}

// Errors appear everywhere user.email is used

If there are any missed fixes, you get compile errors, so you can change code with confidence.

Preventing New Errors

Even if you fix existing errors, it's pointless if new code introduces more.

Husky is a tool for managing git hooks (scripts that run automatically on commit or push). I set up a pre-commit hook to run type checking before every commit.

#!/bin/sh
# .husky/pre-commit

echo "Running type check..."
bun run type-check

if [ $? -ne 0 ]; then
  echo "TypeScript errors found. Please fix before committing."
  exit 1
fi

{
  "scripts": {
    "type-check": "tsc --noEmit"
  }
}

Commits are blocked if there are TypeScript errors. Since "I'll fix it later" isn't allowed, errors don't accumulate.

Tips for Gradual Adoption

Here are tips for introducing strict mode to an existing project.

1. Progress Gradually

Trying to fix a large number of errors at once is discouraging. A more sustainable approach is to fix a few surrounding errors while working on features, or make only new files strict first and gradually convert the rest.

2. Document Reasons for any Types

When you absolutely need an any type, leave a comment explaining why. This allows for review when type definitions improve later.

// TODO: Type definitions will improve in library v3
// biome-ignore lint/suspicious/noExplicitAny: temporary workaround
const result = someLibrary.parse(data) as any;

3. Validate External Data with Zod

Zod is a library for runtime data validation. When you define a schema, TypeScript types are automatically generated.

For external data like API responses and form inputs, validate with Zod. This lets you manage type definitions and validation in one place.

import { z } from 'zod';

const UserSchema = z.object({
  id: z.string(),
  name: z.string(),
  email: z.string().email(),
});

type User = z.infer<typeof UserSchema>;

// Validate at runtime
const user = UserSchema.parse(apiResponse);

Summary

Here are the key points for introducing TypeScript strict mode to an existing project.

Point	Details
Enable selectively	Choose high-impact options rather than strict: true
Divide responsibilities	Split duties between TypeScript and Biome
Fix gradually	Progress bit by bit, not all at once
Prevent accumulation	Make type checking mandatory with pre-commit hooks

Ideally, these settings should be enabled from the project's start. Since retrofitting them incurs fixing costs, I recommend enabling strict mode from the beginning for new projects.

Tomorrow I'll discuss "Security Measures for Solo Development."

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Other Articles in This Series

• 12/17: Implementing "Search by Meaning": Introduction to pgvector + OpenAI Embeddings
• 12/19: Security Measures for Solo Development: The Minimum You Should Do