---

name: typescript-strict

prettier-ignore

description: Use when writing TypeScript with strict mode - covers type definitions, generics, and declaration files

TypeScript Strict Mode Best Practices

Quick Start

Copy// Enable strict in tsconfig.json
{
  "compilerOptions": {
    "strict": true,
    "noUncheckedIndexedAccess": true,
    "exactOptionalPropertyTypes": true
  }
}

Core Principles

• Strict null checks: Handle undefined and null explicitly
• No implicit any: Always declare types, avoid any
• Readonly by default: Use readonly for immutable data
• Discriminated unions: Use type guards with tagged unions

Type Definition Patterns

AST Node Types

Copy// Discriminated union for AST nodes
type Expr =
  | { type: "NumberLiteral"; value: number }
  | { type: "BinaryExpr"; op: string; left: Expr; right: Expr }
  | { type: "Identifier"; name: string };

// Type guard
function isNumberLiteral(expr: Expr): expr is Extract<Expr, { type: "NumberLiteral" }> {
  return expr.type === "NumberLiteral";
}

Generic Types

Copy// Constrained generics
function map<T, U>(arr: readonly T[], fn: (item: T, index: number) => U): U[] {
  return arr.map(fn);
}

// Conditional types
type Unwrap<T> = T extends Promise<infer U> ? U : T;

Declaration Files

Copy// module.d.ts
declare module "lea-lang" {
  export function run(code: string): unknown;
  export function parse(code: string): Program;
}

Common Patterns

• Use unknown over any for safer type narrowing
• Prefer interface for extendable types, type for unions
• Use as const for literal types
• Leverage satisfies for type checking without widening

Reference Files

• references/generics.md - Advanced generic patterns
• references/type-guards.md - Type narrowing techniques