typescript-best-practices

TypeScript Best Practices

Guide AI agents in writing high-quality TypeScript code. This skill provides coding standards, architecture patterns, and tools for analysis and scaffolding.

When to Use This Skill

Use this skill when:

• Generating new TypeScript code
• Reviewing TypeScript files for quality issues
• Creating new modules, services, or components
• Refactoring JavaScript to TypeScript
• Answering questions about TypeScript patterns or types
• Designing APIs or interfaces

Do NOT use this skill when:

• Working with pure JavaScript (no TypeScript)
• Debugging runtime errors (use debugging tools)
• Framework-specific patterns (React, Vue, etc. - use framework skills)

Core Principles

1. Type Safety First

Maximize compile-time error detection:

// Prefer unknown over any for unknown types
function processInput(data: unknown): string {
  if (typeof data === "string") return data;
  if (typeof data === "number") return String(data);
  throw new Error("Unsupported type");
}

// Explicit return types for public APIs
export function calculateTotal(items: ReadonlyArray<Item>): number {
  return items.reduce((sum, item) => sum + item.price, 0);
}

// Use const assertions for literal types
const CONFIG = {
  mode: "production",
  version: 1,
} as const;

2. Immutability by Default

Prevent accidental mutations:

// Use readonly for object properties
interface User {
  readonly id: string;
  readonly email: string;
  name: string; // Only mutable if intentional
}

// Use ReadonlyArray for collections
function processItems(items: ReadonlyArray<Item>): ReadonlyArray<Result> {
  return items.map(transform);
}

// Prefer spreading over mutation
function updateUser(user: User, name: string): User {
  return { ...user, name };
}

3. Error Handling with Types

Use the type system for error handling:

// Result type for recoverable errors
type Result<T, E = Error> =
  | { success: true; value: T }
  | { success: false; error: E };

// Typed error classes
class ValidationError extends Error {
  constructor(
    message: string,
    readonly field: string,
    readonly code: string
  ) {
    super(message);
    this.name = "ValidationError";
  }
}

// Function with Result return type
function parseConfig(input: string): Result<Config, ValidationError> {
  try {
    const data = JSON.parse(input);
    if (!isValidConfig(data)) {
      return {
        success: false,
        error: new ValidationError("Invalid config", "root", "INVALID_FORMAT"),
      };
    }
    return { success: true, value: data };
  } catch {
    return {
      success: false,
      error: new ValidationError("Parse failed", "root", "PARSE_ERROR"),
    };
  }
}

4. Code Organization

Structure code for maintainability:

// One concept per file
// user.ts - User type and related utilities
export interface User {
  readonly id: string;
  readonly email: string;
  readonly createdAt: Date;
}

export function createUser(email: string): User {
  return {
    id: crypto.randomUUID(),
    email,
    createdAt: new Date(),
  };
}

// Explicit exports (no barrel file wildcards)
// index.ts
export { User, createUser } from "./user.ts";
export { validateEmail } from "./validation.ts";

Quick Reference

Category	Prefer	Avoid
Unknown types	unknown	any
Collections	ReadonlyArray<T>	T[] for inputs
Objects	Readonly<T>	Mutable by default
Null checks	Optional chaining ?.	!= null
Type narrowing	Type guards	as assertions
Return types	Explicit on exports	Inferred on exports
Enums	String literal unions	Numeric enums
Imports	Named imports	Default imports
Errors	Result types	Throwing for flow control
Loops	for...of, .map()	for...in on arrays

Code Generation Guidelines

When generating TypeScript code, follow these patterns:

Module Structure

/**
 * Module description
 * @module module-name
 */

// === Types ===
export interface ModuleOptions {
  readonly setting: string;
}

export interface ModuleResult {
  readonly data: unknown;
}

// === Constants ===
const DEFAULT_OPTIONS: ModuleOptions = {
  setting: "default",
};

// === Implementation ===
export function processData(
  input: unknown,
  options: Partial<ModuleOptions> = {}
): ModuleResult {
  const opts = { ...DEFAULT_OPTIONS, ...options };
  // Implementation
  return { data: input };
}

Function Design

// Pure functions preferred
function transform(input: Input): Output {
  // No side effects, same input = same output
  return { ...input, processed: true };
}

// Explicit parameter types
function fetchUser(id: string, options?: FetchOptions): Promise<User> {
  // Implementation
}

// Use function overloads for complex signatures
function parse(input: string): ParsedData;
function parse(input: Buffer): ParsedData;
function parse(input: string | Buffer): ParsedData {
  // Implementation
}

Interface Design

// Prefer interfaces for object shapes
interface UserData {
  readonly id: string;
  readonly email: string;
}

// Use type for unions and intersections
type UserRole = "admin" | "user" | "guest";
type AdminUser = UserData & { readonly role: "admin" };

// Document with JSDoc
/**
 * Configuration for the API client
 * @property baseUrl - The base URL for API requests
 * @property timeout - Request timeout in milliseconds
 */
interface ApiConfig {
  readonly baseUrl: string;
  readonly timeout?: number;
}

Common Anti-Patterns

Avoid these patterns when generating code:

Anti-Pattern	Problem	Solution
any type	Disables type checking	Use unknown and narrow
as assertions	Runtime errors	Use type guards
Non-null !	Null pointer errors	Optional chaining ?.
Mutable params	Unexpected mutations	Readonly<T>
Magic strings	Typos, no autocomplete	String literal types
God classes	Hard to test/maintain	Single responsibility
Circular deps	Build/runtime issues	Dependency inversion
Index signatures	Lose type info	Explicit properties

See references/anti-patterns/common-mistakes.md for detailed examples.

Scripts Reference

analyze.ts

Analyze TypeScript code for quality issues:

deno run --allow-read scripts/analyze.ts <path> [options]

Options:
  --strict        Enable all checks
  --json          Output JSON for programmatic use
  --fix-hints     Show suggested fixes

Examples:
  # Analyze a file
  deno run --allow-read scripts/analyze.ts ./src/utils.ts

  # Analyze directory with strict mode
  deno run --allow-read scripts/analyze.ts ./src --strict

  # JSON output for CI
  deno run --allow-read scripts/analyze.ts ./src --json

generate-types.ts

Generate TypeScript types from JSON data:

deno run --allow-read --allow-write scripts/generate-types.ts <input> [options]

Options:
  --name <name>   Root type name (default: inferred)
  --output <path> Output file path
  --readonly      Generate readonly types
  --interface     Use interface instead of type

Examples:
  # Generate from JSON file
  deno run --allow-read scripts/generate-types.ts ./data.json --name Config

  # Generate readonly interface
  deno run --allow-read --allow-write scripts/generate-types.ts ./api-response.json \
    --interface --readonly --output ./types/api.ts

scaffold-module.ts

Create properly structured TypeScript modules:

deno run --allow-read --allow-write scripts/scaffold-module.ts [options]

Options:
  --name <name>   Module name (required)
  --path <path>   Target directory (default: ./src)
  --type <type>   Type: service, util, component
  --with-tests    Include test file

Examples:
  # Create a utility module
  deno run --allow-read --allow-write scripts/scaffold-module.ts \
    --name "string-utils" --type util

  # Create a service with tests
  deno run --allow-read --allow-write scripts/scaffold-module.ts \
    --name "user-service" --type service --with-tests

Additional Resources

Type System Deep Dives

• references/type-system/advanced-types.md - Generics, conditional types, mapped types
• references/type-system/type-guards.md - Type narrowing techniques
• references/type-system/utility-types.md - Built-in utility types

Pattern Guides

• references/patterns/error-handling.md - Result types, typed errors
• references/patterns/async-patterns.md - Async/await best practices
• references/patterns/functional-patterns.md - Immutability, composition
• references/patterns/module-patterns.md - Exports, dependency injection

Architecture

• references/architecture/project-structure.md - Directory organization
• references/architecture/api-design.md - Interface design, versioning

Templates

• assets/templates/module-template.ts.md - Module starter template
• assets/templates/service-template.ts.md - Service class template
• assets/tsconfig-presets/strict.json - Maximum strictness config
• assets/tsconfig-presets/recommended.json - Balanced defaults