TypeScript / JavaScript Language Idioms

Work with the language. Embrace its asynchronous nature, its structural type system, and its functional roots. Fight the urge to write Java-in-TypeScript.

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Core Philosophy

• Immutability by default — const everything, spread to copy, never mutate in place
• Type safety without ceremony — let inference work, annotate at boundaries
• Composition over inheritance — functions, modules, and interfaces beat class hierarchies
• Explicit over implicit — no coercion tricks, no hidden state, no magic

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References and Variables

The Rules

Keyword	Usage	Rationale
const	Default for all bindings	Prevents reassignment, signals intent
let	Only when reassignment is unavoidable	Loops, accumulators, state machines
var	Prohibited	Function-scoped, hoists, leaks out of blocks

Destructuring and Spread

Use destructuring to extract what you need. Use spread to copy without mutation:

// ✅ Destructure at the call site
const { name, age } = getUser();

// ✅ Shallow-copy with spread — never mutate the original
const updated = { ...config, timeout: 5000 };
const extended = [...items, newItem];

// ❌ Mutating the original
config.timeout = 5000;
items.push(newItem);

Use Object.hasOwn(obj, key) instead of obj.hasOwnProperty(key) — the latter can be shadowed.

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Functions

Arrow Functions

Use arrow functions for callbacks and short expressions. They preserve lexical this:

// ✅ Arrow for callbacks
const sorted = items.sort((a, b) => a.name.localeCompare(b.name));

// ✅ Arrow for inline transforms
const names = users.map((u) => u.name);

// ❌ Legacy — don't use `function` for callbacks
const names = users.map(function (u) {
  return u.name;
});

Use function declarations for top-level named functions (they hoist and have clear stack traces).

Options Objects

When a function's parameter list becomes unwieldy, use a single options object:

// ❌ Positional parameter overload
function createServer(
  port: number,
  host: string,
  ssl: boolean,
  timeout: number,
);

// ✅ Options object — extensible, self-documenting
interface ServerOptions {
  port: number;
  host?: string;
  ssl?: boolean;
  timeout?: number;
}
function createServer(options: ServerOptions);

Rest Parameters

Use rest syntax. The arguments object is prohibited:

// ✅ Rest gives you a real Array
function log(level: string, ...messages: string[]) {}

// ❌ arguments is not a real Array and has no type safety
function log() {
  console.log(arguments);
}

Pure Functions

Prefer functions that return values based solely on inputs. Reserve side effects for explicit boundaries (I/O, event handlers, logging).

Functional Iterators

Use map, filter, reduce to build declarative pipelines that produce new data:

// ✅ Declarative pipeline — new array, no mutation
const active = users.filter((u) => u.isActive).map((u) => u.email);

// ❌ Imperative mutation
const active: string[] = [];
users.forEach((u) => {
  if (u.isActive) active.push(u.email);
});

forEach is acceptable for genuine side effects (logging, DOM, events) — not for data transformation.

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Async Patterns

JavaScript is async-first. Treat asynchrony as the normal case, not the exception.

async/await

Prefer async/await over .then() chains:

// ✅ Linear, readable
async function fetchUser(id: string): Promise<User> {
  const res = await fetch(`/api/users/${id}`);
  if (!res.ok) throw new HttpError(res.status);
  return res.json();
}

// ❌ Nested chains obscure flow
function fetchUser(id: string): Promise<User> {
  return fetch(`/api/users/${id}`).then((res) => {
    if (!res.ok) throw new HttpError(res.status);
    return res.json();
  });
}

Concurrent Operations

Pattern	Behavior	Use When
Promise.all	Fails fast on first rejection	All must succeed
Promise.allSettled	Waits for all, reports each	Partial failure is acceptable
Promise.race	Resolves/rejects with first	Timeouts, fastest-wins

Cancellation

Use AbortController for cancellable operations:

const controller = new AbortController();
const res = await fetch(url, { signal: controller.signal });

// Cancel from elsewhere
controller.abort();

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Modules

ES Modules Only

Use import/export. CommonJS (require) is legacy.

Named Exports

Prefer named exports. They enable compile-time reference checks and explicit dependency graphs:

// ✅ Named — IDE catches broken imports immediately
export function parse(input: string): Token[] {
  /* ... */
}
export interface Token {
  type: string;
  value: string;
}

// ⚠️ Default — permits arbitrary rename at import site
export default function parse(input: string): Token[] {
  /* ... */
}

Default exports are acceptable for framework conventions (e.g., Vue SFCs, Next.js pages) but never preferred.

Barrel Files

Use barrel files (index.ts) sparingly. They defeat tree-shaking and create circular dependency traps. Export directly from source modules when possible.

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Type System

Strict Mode

Enable strict: true in tsconfig.json. This is non-negotiable — it activates strictNullChecks, noImplicitAny, and other critical checks.

Type Inference vs. Annotation

Let TypeScript infer when the type is obvious. Annotate at boundaries — function parameters, return types, and public APIs:

// ✅ Inferred — obvious from the literal
const count = 0;
const name = "alice";

// ✅ Annotated — boundary, not obvious
function parseConfig(raw: string): Config {
  /* ... */
}

unknown over any

any disables type checking. Use unknown and narrow explicitly:

// ✅ Forces narrowing before use
function handle(input: unknown) {
  if (typeof input === "string") {
    console.log(input.toUpperCase());
  }
}

// ❌ Silently accepts anything — bugs hide here
function handle(input: any) {
  console.log(input.toUpperCase()); // runtime bomb
}

Discriminated Unions

Model variant state with a literal discriminant. This enables exhaustive switch checking:

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

function handle(result: Result<string>) {
  if (result.ok) {
    console.log(result.value); // narrowed to { ok: true; value: string }
  } else {
    console.error(result.error); // narrowed to { ok: false; error: Error }
  }
}

satisfies

Validate that a value conforms to a type without widening:

// ✅ Validates shape, preserves literal types
const routes = {
  home: "/",
  about: "/about",
  users: "/users",
} satisfies Record<string, string>;

// TypeScript still knows routes.home is "/" (literal), not just string

as const

Use as const for immutable literal types. Replaces enums in most cases:

// ✅ as const object — runtime value + narrow types
const Status = {
  Active: "active",
  Inactive: "inactive",
  Pending: "pending",
} as const;

type Status = (typeof Status)[keyof typeof Status];
// "active" | "inactive" | "pending"

Utility Types

Use built-in utility types to derive types from existing ones:

Type	Purpose	Example
Partial<T>	All properties optional	Patch/update payloads
Required<T>	All properties required	Validated config
Readonly<T>	All properties readonly	Frozen state
Pick<T, K>	Subset of properties	API response shaping
Omit<T, K>	Exclude properties	Remove internal fields
Record<K, V>	Map of key-value pairs	Lookup tables

Branded Types

Simulate nominal typing for domain safety:

type UserId = string & { readonly __brand: unique symbol };
type PostId = string & { readonly __brand: unique symbol };

function createUserId(id: string): UserId {
  return id as UserId; // validated assertion at construction
}

// Now UserId and PostId are incompatible at compile time

Type Erasure

TypeScript types are stripped at build time. They cannot guard runtime execution.

• Prefer erasable-only syntax — types, interfaces, type aliases all vanish cleanly
• Avoid enums — they emit runtime JavaScript; use as const objects or union types instead
• Use #field for private — not the private keyword, which is TypeScript-only
• Validate at boundaries — external data (API responses, user input) must be validated at runtime, not just typed

// ✅ Native private — survives type erasure
class User {
  #email: string;
  constructor(email: string) {
    this.#email = email;
  }
}

// ❌ TypeScript private — erased, accessible at runtime
class User {
  private email: string;
  constructor(email: string) {
    this.email = email;
  }
}

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Error Handling

Custom Error Classes

Extend Error with domain-specific types. Always set name:

class HttpError extends Error {
  constructor(
    public readonly status: number,
    message?: string,
  ) {
    super(message ?? `HTTP ${status}`);
    this.name = "HttpError";
  }
}

Async Error Handling

Catch at the appropriate level. Don't swallow errors silently:

// ✅ Catch and handle or rethrow with context
try {
  const user = await fetchUser(id);
} catch (err) {
  if (err instanceof HttpError && err.status === 404) {
    return null; // expected case
  }
  throw err; // unexpected — propagate
}

Error Narrowing

TypeScript's catch clause types as unknown. Narrow before accessing:

try {
  await riskyOperation();
} catch (err) {
  if (err instanceof Error) {
    console.error(err.message);
  } else {
    console.error("Unknown error:", err);
  }
}

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Naming Conventions

Scope	Style	Example
Classes, interfaces, type aliases	PascalCase	UserAccount, ServerOptions
Functions, variables, properties	camelCase	calculateTotal, isActive
Exported constants (true invariants)	SCREAMING_SNAKE	MAX_RETRIES, API_VERSION
Private class fields	#camelCase	#connectionPool
Generic type parameters	Single uppercase or T-prefix	T, K, TResult

Equality

Use strict equality (=== / !==). Never rely on abstract coercion (==).

Use shortcuts for booleans (if (isValid)) but explicit comparisons for strings and numbers (if (name !== ""), if (count > 0)) to prevent coercion surprises.

Nullish Handling

Prefer ?? over || for defaults — || coerces 0, "", and false to falsy:

// ✅ Only falls through on null/undefined
const timeout = options.timeout ?? 3000;

// ❌ Falls through on 0, "", false
const timeout = options.timeout || 3000;

Use optional chaining (?.) for safe property access.

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Anti-Patterns

Anti-Pattern	Description	Remedy
any leakage	Using any to silence the compiler	Use unknown and narrow
Enum abuse	TypeScript enums that emit runtime code	as const objects or union types
Class-heavy OOP	Porting Java patterns (abstract classes, deep hierarchies)	Composition, interfaces, plain functions
Barrel file sprawl	Re-exporting everything through index.ts	Direct imports from source modules
Swallowed errors	Empty catch {} blocks	Handle, log, or rethrow
Type assertions	as Type to override the compiler	Annotations (const x: Type) or narrowing
Mutation in map/filter	Side effects inside declarative pipelines	forEach for effects, map for transforms

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Tooling

tsc --noEmit              # Type-check without emitting
npx eslint .              # Lint
npx prettier --check .    # Format check
npx prettier --write .    # Auto-format
npx biome check .         # All-in-one (alternative to eslint + prettier)

Formatting is not a debate. Pick prettier or biome and enforce it in CI.

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Quick Reference

• const by default — let only when unavoidable, var never
• strict: true — always, no exceptions
• unknown over any — narrow explicitly
• async/await — not .then() chains
• Named exports — not default exports
• as const — not enums
• #field — not private keyword
• === — not ==
• ?? — not || for defaults
• Spread to copy — never mutate the original
• Annotate boundaries — infer the rest

These idioms refine but are subordinate to the Code-Edit Constraints.