tsconfig Strictness Flags

tsconfig Strictness Flags

The "strict": true shorthand in tsconfig.json is a family of individual flags. Knowing what each flag does helps you understand the errors you see — and lets you adopt them incrementally in a legacy codebase.

• strictNullChecks — null and undefined are not assignable to other types; you must handle them explicitly.
• noImplicitAny — raises an error when TypeScript would infer any for a parameter or variable with no annotation.
• strictFunctionTypes — checks function parameter types contravariantly (prevents unsafe callbacks).
• strictBindCallApply — ensures bind, call, and apply are typed correctly.
• strictPropertyInitialization — class properties must be assigned in the constructor or have a definite-assignment assertion (!).
• noImplicitThis — raises an error when this would be typed as any inside a function.
• alwaysStrict — emits "use strict" in every output file and parses files in strict mode.
• useUnknownInCatchVariables — catch clause variables are typed unknown instead of any (TypeScript 4.4+).

// tsconfig.json — enabling flags individually
// {
//   "compilerOptions": {
//     "strictNullChecks": true,
//     "noImplicitAny": true,
//     "strictFunctionTypes": true,
//     "useUnknownInCatchVariables": true
//   }
// }

// With strictNullChecks:
function getLength(s: string | null): number {
  // return s.length; // Error — s could be null
  return s?.length ?? 0; // OK
}

// With useUnknownInCatchVariables:
try {
  JSON.parse("{");
} catch (err) {
  // err is unknown — must narrow before use
  if (err instanceof Error) {
    console.error(err.message);
  }
}

Additional safety flags

• noUncheckedIndexedAccess — accessing arr[i] or obj[key] returns T | undefined, not just T.
• exactOptionalPropertyTypes — optional properties may not be explicitly set to undefined; they must be omitted.
• noImplicitReturns — every code path in a function must return a value if the return type is not void.
• noFallthroughCasesInSwitch — switch cases without break or return are flagged.
• noPropertyAccessFromIndexSignature — dot notation is disallowed on index-signature types; bracket notation is required.