Strict compiler options

TypeScript · Reference cheat sheet

📋 Overview

strict: true enables a suite of soundness checks. Turn it on for new projects; migrate incrementally with per-flag toggles. TS 5.x also adds related flags (exactOptionalPropertyTypes, noUncheckedIndexedAccess, verbatimModuleSyntax).

🔧 Core concepts

Extra (recommended):

• noUncheckedIndexedAccess — T[K] includes undefined.
• exactOptionalPropertyTypes — distinguish missing vs undefined.
• noImplicitReturns / noFallthroughCasesInSwitch.
• verbatimModuleSyntax — type-only imports must use import type.

💡 Examples

// tsconfig.json
{
  "compilerOptions": {
    "strict": true,
    "noUncheckedIndexedAccess": true,
    "exactOptionalPropertyTypes": true,
    "noImplicitOverride": true,
    "useUnknownInCatchVariables": true,
    "verbatimModuleSyntax": true
  }
}

// strictNullChecks
function len(s: string | null) {
  // return s.length; // error
  return s?.length ?? 0;
}

// strictPropertyInitialization
class User {
  name!: string; // definite assignment assertion — use sparingly
  constructor(name: string) {
    this.name = name;
  }
}

try {
  /* ... */
} catch (e) {
  if (e instanceof Error) console.error(e.message);
}

import type { UserId } from "./ids"; // verbatimModuleSyntax

⚠️ Pitfalls

• Enabling strict on a large JS codebase surfaces many errors — fix by folder / flag.
• noUncheckedIndexedAccess adds | undefined everywhere — handle explicitly.
• exactOptionalPropertyTypes breaks \{ prop?: T \} assignability with undefined.
• skipLibCheck: true hides issues in .d.ts (common, but masks problems).
• Don’t disable strict to “save time” — prefer local as / narrowing.

🔗 Related

• config.md
• any_unknown_never.md
• commandline.md
• type_guards.md
• this_types.md