Typescript Strict

TypeScript Strict Mode

Modern TypeScript configuration with strict type checking for maximum safety and developer experience. This guide focuses on TypeScript 5.x best practices for 2025.

Core Expertise

What is Strict Mode?

• Type safety: Catch more bugs at compile time
• Better IDE experience: Improved autocomplete and refactoring
• Maintainability: Self-documenting code with explicit types
• Modern defaults: Align with current TypeScript best practices

Key Capabilities

• Strict null checking
• Strict function types
• No implicit any
• No unchecked indexed access
• Proper module resolution
• Modern import/export syntax enforcement

Recommended tsconfig.json (2025)

Minimal Production Setup

{
  "compilerOptions": {
    // Type Checking
    "strict": true,
    "noUncheckedIndexedAccess": true,
    "noImplicitOverride": true,
    "noPropertyAccessFromIndexSignature": true,
    "noFallthroughCasesInSwitch": true,
    "noUnusedLocals": true,
    "noUnusedParameters": true,

    // Modules
    "module": "ESNext",
    "moduleResolution": "Bundler",
    "resolveJsonModule": true,
    "allowSyntheticDefaultImports": true,
    "esModuleInterop": false,
    "verbatimModuleSyntax": true,

    // Emit
    "target": "ES2022",
    "lib": ["ES2023", "DOM", "DOM.Iterable"],
    "outDir": "dist",
    "declaration": true,
    "declarationMap": true,
    "sourceMap": true,
    "removeComments": false,
    "noEmit": false,

    // Interop
    "isolatedModules": true,
    "allowJs": false,
    "checkJs": false,
    "skipLibCheck": true,
    "forceConsistentCasingInFileNames": true
  },
  "include": ["src/**/*"],
  "exclude": ["node_modules", "dist", "build"]
}

Strict Flags Explained

strict: true (Umbrella Flag)

Enables all strict type-checking options:

{
  "strict": true
  // Equivalent to:
  // "noImplicitAny": true,
  // "strictNullChecks": true,
  // "strictFunctionTypes": true,
  // "strictBindCallApply": true,
  // "strictPropertyInitialization": true,
  // "noImplicitThis": true,
  // "alwaysStrict": true
}

Always enable strict: true for new projects.

Additional Strict Flags (Recommended for 2025)

noUncheckedIndexedAccess (Essential)

// With noUncheckedIndexedAccess
const users: Record<string, User> = {};
const user = users['john'];
// Type: User | undefined (correct - must check before use)

if (user) {
  console.log(user.name); // Type narrowed to User
}

Always enable this flag to prevent runtime errors.

Module Resolution

Choosing a Strategy

moduleResolution: "Bundler" (Vite/Bun)

{
  "compilerOptions": {
    "module": "ESNext",
    "moduleResolution": "Bundler"
  }
}

• No file extensions required in imports
• JSON imports without assertions
• Package.json exports field support

moduleResolution: "NodeNext" (Node.js)

{
  "compilerOptions": {
    "module": "NodeNext",
    "moduleResolution": "NodeNext"
  }
}

• Respects package.json type: "module"
• Requires explicit .js extensions (even for .ts files)
• Supports conditional exports

verbatimModuleSyntax (Recommended for 2025)

Prevents TypeScript from rewriting imports/exports.

{
  "compilerOptions": {
    "verbatimModuleSyntax": true
  }
}

// Error: 'User' is a type and must be imported with 'import type'
import { User } from './types';

// Correct
import type { User } from './types';

// Correct (mixed import)
import { fetchUser, type User } from './api';

Replaces deprecated importsNotUsedAsValues and preserveValueImports.

Agentic Optimizations

Quick Reference

Recommended Flags

For detailed examples, advanced patterns, and best practices, see REFERENCE.md.

References

• TypeScript Handbook: https://www.typescriptlang.org/docs/handbook/intro.html
• TSConfig Reference: https://www.typescriptlang.org/tsconfig
• Strict Mode Guide: https://www.typescriptlang.org/tsconfig#strict
• Module Resolution: https://www.typescriptlang.org/docs/handbook/module-resolution.html
• Best Practices: https://www.typescriptlang.org/docs/handbook/declaration-files/do-s-and-don-ts.html