TypeScript/JavaScript Development Skill

When to Invoke

• Writing new TypeScript or JavaScript files
• Refactoring existing TS/JS code
• Reviewing code for type safety and best practices
• Converting JavaScript to TypeScript
• Designing module APIs and type interfaces
• Fixing type errors or improving type coverage

Code Style

Naming Conventions

• camelCase for variables, functions, parameters
• PascalCase for types, interfaces, classes, enums, React components
• UPPER_SNAKE_CASE for constants and enum members
• Prefix interfaces with I only if project convention requires it - otherwise plain PascalCase
• Boolean variables: use is, has, should, can prefixes (isLoading, hasPermission)
• Event handlers: handleClick, onSubmit pattern

File Organization

• One primary export per file when possible
• Group related types with their implementation
• Barrel exports (index.ts) for public module APIs only - avoid deep barrel re-exports
• File naming: kebab-case.ts for utilities, PascalCase.tsx for React components

Import Ordering

1. Node built-in modules (node:fs, node:path)
2. External packages (react, lodash)
3. Internal aliases (@/utils, @/components)
4. Relative imports (./helpers, ../types)
5. Type-only imports (import type { Foo })

• Blank line between each group

TypeScript Patterns

Strict Mode

• Enable strict: true in tsconfig.json - never disable individual strict checks
• No // @ts-ignore or // @ts-expect-error without an explanatory comment
• Prefer unknown over any - narrow with type guards

Proper Typing

• Avoid any - use unknown and narrow, or define a proper type
• Prefer interface for object shapes that may be extended
• Prefer type for unions, intersections, mapped types, and utility types
• Use readonly for properties that should not be mutated
• Use as const for literal type inference on objects and arrays

Discriminated Unions

type Result<T> =
  | { success: true; data: T }
  | { success: false; error: Error };

function handle<T>(result: Result<T>) {
  if (result.success) {
    // result.data is T here
    return result.data;
  }
  // result.error is Error here
  throw result.error;
}

Type Guards

// User-defined type guard
function isString(value: unknown): value is string {
  return typeof value === "string";
}

// Assertion function
function assertDefined<T>(value: T | undefined, name: string): asserts value is T {
  if (value === undefined) {
    throw new Error(`Expected ${name} to be defined`);
  }
}

Generic Constraints

// Constrain generics to what you actually need
function getProperty<T, K extends keyof T>(obj: T, key: K): T[K] {
  return obj[key];
}

// Use defaults for common cases
type ApiResponse<T = unknown> = {
  data: T;
  status: number;
  timestamp: string;
};

Utility Types

• Partial<T> - all properties optional (use for update/patch operations)
• Required<T> - all properties required
• Pick<T, K> - select subset of properties
• Omit<T, K> - exclude properties
• Record<K, V> - typed key-value map
• Extract<T, U> / Exclude<T, U> - filter union members
• Prefer built-in utility types over manual type manipulation

Enums vs Union Types

• Prefer string union types for simple sets: type Status = "active" | "inactive"
• Use const enum only if you need numeric values and tree-shaking
• Use regular enum when you need runtime reverse mapping or iteration

React Patterns

Component Typing

// Function components - type props inline or with interface
interface ButtonProps {
  label: string;
  variant?: "primary" | "secondary";
  onClick: () => void;
  children?: React.ReactNode;
}

function Button({ label, variant = "primary", onClick, children }: ButtonProps) {
  return <button className={variant} onClick={onClick}>{children ?? label}</button>;
}

Hooks Rules

• Call hooks at the top level only - never inside conditions, loops, or nested functions
• Custom hooks must start with use prefix
• Specify dependency arrays accurately - never suppress exhaustive-deps lint
• Use useCallback for functions passed as props to memoized children
• Use useMemo for expensive computations - not for every variable

State Management

• Colocate state as close to where it is used as possible
• Lift state up only when siblings need to share it
• Use useReducer for complex state with multiple sub-values or transitions
• Context for truly global state (theme, auth, locale) - not for frequently changing data

Event Handling

// Type event handlers properly
function handleChange(e: React.ChangeEvent<HTMLInputElement>) {
  setValue(e.target.value);
}

function handleSubmit(e: React.FormEvent<HTMLFormElement>) {
  e.preventDefault();
  // ...
}

Testing

File Naming

• Test files: *.test.ts or *.spec.ts alongside the source file
• Or in __tests__/ directory mirroring the source structure
• Test utilities: test-utils.ts or testing/ directory

Test Structure

describe("calculateTotal", () => {
  it("returns 0 for empty cart", () => {
    expect(calculateTotal([])).toBe(0);
  });

  it("sums item prices with quantities", () => {
    const items = [
      { price: 10, quantity: 2 },
      { price: 5, quantity: 1 },
    ];
    expect(calculateTotal(items)).toBe(25);
  });

  it("throws for negative quantities", () => {
    expect(() => calculateTotal([{ price: 10, quantity: -1 }])).toThrow();
  });
});

Assertion Patterns

• Use toBe for primitives, toEqual for objects/arrays
• Use toThrow for error cases - wrap in arrow function
• Use toHaveBeenCalledWith for spy/mock assertions
• Prefer specific matchers over generic toBeTruthy/toBeFalsy
• Type test fixtures and mocks - avoid as any in tests

Mocking

• Mock external dependencies, not internal implementation
• Use vi.fn() (Vitest) or jest.fn() for function mocks
• Use vi.spyOn / jest.spyOn to mock methods while preserving type safety
• Reset mocks in beforeEach or use afterEach(() => vi.restoreAllMocks())

Common Anti-Patterns

Using any Instead of Proper Types

// BAD
function parse(data: any) { return data.name; }

// GOOD
function parse(data: unknown): string {
  if (typeof data === "object" && data !== null && "name" in data) {
    return String((data as { name: unknown }).name);
  }
  throw new Error("Invalid data");
}

Non-Null Assertion Overuse

// BAD
const name = user!.name!;

// GOOD
if (!user?.name) throw new Error("User name required");
const name = user.name;

Barrel File Performance Issues

// BAD - importing everything through deep barrel
import { Button } from "@/components";  // pulls entire component tree

// GOOD - direct import
import { Button } from "@/components/Button";

Ignoring Return Types

// BAD - return type inferred as complex union
function getData(id: string) {
  if (!id) return null;
  return fetch(`/api/${id}`).then(r => r.json());
}

// GOOD - explicit return type
async function getData(id: string): Promise<ApiResponse | null> {
  if (!id) return null;
  const r = await fetch(`/api/${id}`);
  return r.json() as Promise<ApiResponse>;
}

Mutating Function Parameters

// BAD
function addItem(items: Item[], item: Item) {
  items.push(item);  // mutates input
  return items;
}

// GOOD
function addItem(items: readonly Item[], item: Item): Item[] {
  return [...items, item];
}

Error Handling

Result Pattern

type Result<T, E = Error> =
  | { ok: true; value: T }
  | { ok: false; error: E };

async function fetchUser(id: string): Promise<Result<User>> {
  try {
    const res = await fetch(`/api/users/${id}`);
    if (!res.ok) return { ok: false, error: new Error(`HTTP ${res.status}`) };
    const user = await res.json();
    return { ok: true, value: user };
  } catch (e) {
    return { ok: false, error: e instanceof Error ? e : new Error(String(e)) };
  }
}

Async Error Handling

• Always try/catch around await calls that can fail
• Never use .catch() and await on the same promise chain
• Prefer returning error results over throwing in library code
• Throw only for programmer errors (assertion failures, invariant violations)
• Log errors at the boundary, not at every level

Error Boundaries (React)

• Wrap major UI sections in error boundaries
• Provide meaningful fallback UI - not blank screens
• Log errors to monitoring service in componentDidCatch
• Reset error boundary state on navigation changes

Typed Errors

class NotFoundError extends Error {
  readonly code = "NOT_FOUND" as const;
  constructor(resource: string, id: string) {
    super(`${resource} ${id} not found`);
    this.name = "NotFoundError";
  }
}

class ValidationError extends Error {
  readonly code = "VALIDATION" as const;
  constructor(public readonly fields: Record<string, string>) {
    super("Validation failed");
    this.name = "ValidationError";
  }
}