typescript-strict

TypeScript Strict

Strict TypeScript conventions: strict mode config, no-any rules, utility types, discriminated unions, branded types, and error handling patterns.

Strict Mode Configuration

Always enable strict mode. Use this tsconfig.json base:

{
  "compilerOptions": {
    "strict": true,
    "noUncheckedIndexedAccess": true,
    "noImplicitOverride": true,
    "noPropertyAccessFromIndexSignature": true,
    "noFallthroughCasesInSwitch": true,
    "forceConsistentCasingInFileNames": true,
    "exactOptionalPropertyTypes": true,
    "verbatimModuleSyntax": true
  }
}

What strict: true Enables

• strictNullChecks — null and undefined are their own types
• strictFunctionTypes — function parameter types are checked strictly
• strictBindCallApply — bind, call, apply are typed correctly
• strictPropertyInitialization — class properties must be initialized
• noImplicitAny — no implicit any types
• noImplicitThis — this must have an explicit type
• alwaysStrict — emit "use strict" in every file

No any Rules

1. Never use any — use unknown when the type is truly unknown
2. Use unknown and narrow — type guards, instanceof, typeof
3. Use generics for flexible types — function parse<T>(input: string): T
4. Use Record<string, unknown> over object — more explicit
5. Suppress with // @ts-expect-error (not // @ts-ignore) — @ts-expect-error fails if the error is fixed

// Bad
function parse(data: any): any {
  return JSON.parse(data);
}

// Good
function parse(data: string): unknown {
  return JSON.parse(data);
}

// Good — with type guard
function isUser(value: unknown): value is User {
  return (
    typeof value === "object" &&
    value !== null &&
    "id" in value &&
    "email" in value
  );
}

const data: unknown = parse(input);
if (isUser(data)) {
  console.log(data.email); // typed as User
}

Utility Types

Use built-in utility types instead of manual type construction: