TypeScript Strict Mode Skill

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.

When to Use This Skill

| Use this skill when... | Use another approach when... | |------------------------|------------------------------| | Setting up a new TypeScript project | Debugging runtime errors (use debugger) | | Migrating to stricter type safety | Configuring build tools (use bundler docs) | | Choosing moduleResolution strategy | Writing application logic | | Enabling noUncheckedIndexedAccess | Optimizing bundle size (use bundler skills) |

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)

| Flag | Purpose | |------|---------| | noUncheckedIndexedAccess | Index signatures return T \| undefined instead of T | | noImplicitOverride | Require override keyword for overridden methods | | noPropertyAccessFromIndexSignature | Force bracket notation for index signatures | | noFallthroughCasesInSwitch | Prevent fallthrough in switch statements | | exactOptionalPropertyTypes | Optional properties cannot be set to undefined explicitly |

`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

| Strategy | Use For | Extensions Required | |----------|---------|---------------------| | Bundler | Vite, Webpack, Bun projects | No | | NodeNext | Node.js libraries and servers | Yes (.js) |

`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

| Context | Command | |---------|---------| | Check errors | bunx tsc --noEmit | | Check single file | bunx tsc --noEmit src/utils.ts | | Show config | bunx tsc --showConfig | | Check module resolution | bunx tsc --showConfig \| grep moduleResolution |

Quick Reference

Recommended Flags

| Flag | Value | Category | |------|-------|----------| | strict | true | Type Checking | | noUncheckedIndexedAccess | true | Type Checking | | noImplicitOverride | true | Type Checking | | noPropertyAccessFromIndexSignature | true | Type Checking | | noFallthroughCasesInSwitch | true | Type Checking | | module | ESNext / NodeNext | Modules | | moduleResolution | Bundler / NodeNext | Modules | | verbatimModuleSyntax | true | Modules | | target | ES2022 | Emit | | isolatedModules | true | Interop | | skipLibCheck | true | Interop |

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

Agent Skills: TypeScript Strict Mode

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