TypeScript Strict Mode Configuration

Intermediatev1.0.0

Configure TypeScript for maximum type safety — strict mode flags, essential tsconfig.json settings, path aliases, and project references for monorepos.

Content

Overview

A properly configured tsconfig.json is the foundation of type-safe TypeScript. Strict mode catches entire categories of bugs at compile time. This skill covers the essential settings every project needs.

Why This Matters

-Bug prevention — strict mode catches ~40% more errors than default config
-Null safety — strictNullChecks prevents the "billion dollar mistake"
-Consistency — team-wide settings prevent configuration drift
-IDE support — proper config enables full autocomplete and refactoring

The Essential tsconfig.json

json

{
  "compilerOptions": {
    // Strict mode — enables all strict checks
    "strict": true,

    // Additional safety checks beyond strict
    "noUncheckedIndexedAccess": true,
    "exactOptionalPropertyTypes": true,
    "noFallthroughCasesInSwitch": true,

    // Module configuration
    "target": "ES2022",
    "module": "Node16",
    "moduleResolution": "Node16",
    "esModuleInterop": true,
    "resolveJsonModule": true,

    // Output
    "outDir": "./dist",
    "declaration": true,
    "declarationMap": true,
    "sourceMap": true,

    // Path aliases
    "baseUrl": ".",
    "paths": {
      "@/*": ["./src/*"],
      "@/types/*": ["./src/types/*"]
    },

    // Quality
    "skipLibCheck": true,
    "forceConsistentCasingInFileNames": true,
    "isolatedModules": true
  },
  "include": ["src/**/*"],
  "exclude": ["node_modules", "dist", "**/*.test.ts"]
}

What `strict: true` Enables

Flag	What It Does
`strictNullChecks`	null/undefined are distinct types — must handle explicitly
`noImplicitAny`	Variables must have types — no implicit any
`strictFunctionTypes`	Function parameter types checked contravariantly
`strictBindCallApply`	bind/call/apply check argument types
`strictPropertyInitialization`	Class properties must be initialized
`noImplicitThis`	this must have an explicit type
`alwaysStrict`	Emit "use strict" in output
`useUnknownInCatchVariables`	catch variables are unknown, not any

Additional Recommended Flags

json

{
  // Array/object indexing returns T | undefined (not just T)
  "noUncheckedIndexedAccess": true,

  // Optional properties must be explicitly set to undefined
  "exactOptionalPropertyTypes": true,

  // Unused locals and parameters are errors
  "noUnusedLocals": true,
  "noUnusedParameters": true,

  // Ensure every code path returns a value
  "noImplicitReturns": true
}

Path Aliases with @

json

{
  "compilerOptions": {
    "baseUrl": ".",
    "paths": {
      "@/*": ["./src/*"]
    }
  }
}

typescript

// Before
import { User } from '../../../types/user';
// After
import { User } from '@/types/user';

Best Practices

-Start with strict: true on new projects — non-negotiable
-Add noUncheckedIndexedAccess for safer array/object access
-Use path aliases to avoid deep relative imports
-Keep skipLibCheck: true for faster compilation
-Set isolatedModules: true for compatibility with esbuild/SWC/Vite

Common Mistakes

-Starting without strict mode and trying to add it later (painful)
-Using any to fix strict mode errors instead of proper types
-Not matching module/moduleResolution to your runtime (Node16 for Node.js)
-Forgetting to configure path aliases in bundler/test runner too

FAQ

Discussion

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The Essential tsconfig.json

json

{
  "compilerOptions": {
    // Strict mode — enables all strict checks
    "strict": true,

    // Additional safety checks beyond strict
    "noUncheckedIndexedAccess": true,
    "exactOptionalPropertyTypes": true,
    "noFallthroughCasesInSwitch": true,

    // Module configuration
    "target": "ES2022",
    "module": "Node16",
    "moduleResolution": "Node16",
    "esModuleInterop": true,
    "resolveJsonModule": true,

    // Output
    "outDir": "./dist",
    "declaration": true,
    "declarationMap": true,
    "sourceMap": true,

    // Path aliases
    "baseUrl": ".",
    "paths": {
      "@/*": ["./src/*"],
      "@/types/*": ["./src/types/*"]
    },

    // Quality
    "skipLibCheck": true,
    "forceConsistentCasingInFileNames": true,
    "isolatedModules": true
  },
  "include": ["src/**/*"],
  "exclude": ["node_modules", "dist", "**/*.test.ts"]
}

What strict: true Enables

Flag	What It Does
`strictNullChecks`	null/undefined are distinct types — must handle explicitly
`noImplicitAny`	Variables must have types — no implicit any
`strictFunctionTypes`	Function parameter types checked contravariantly
`strictBindCallApply`	bind/call/apply check argument types
`strictPropertyInitialization`	Class properties must be initialized
`noImplicitThis`	this must have an explicit type
`alwaysStrict`	Emit "use strict" in output
`useUnknownInCatchVariables`	catch variables are unknown, not any

Additional Recommended Flags

json

{
  // Array/object indexing returns T | undefined (not just T)
  "noUncheckedIndexedAccess": true,

  // Optional properties must be explicitly set to undefined
  "exactOptionalPropertyTypes": true,

  // Unused locals and parameters are errors
  "noUnusedLocals": true,
  "noUnusedParameters": true,

  // Ensure every code path returns a value
  "noImplicitReturns": true
}

TypeScript Strict Mode Configuration

Content

Overview

Why This Matters

The Essential tsconfig.json

What strict: true Enables

Additional Recommended Flags

Path Aliases with @

Best Practices

Common Mistakes

FAQ

Discussion

TypeScript Strict Mode Configuration

Content

Overview

Why This Matters

The Essential tsconfig.json

What strict: true Enables

Additional Recommended Flags

Path Aliases with @

Best Practices

Common Mistakes

FAQ

Discussion

What `strict: true` Enables

What `strict: true` Enables