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