name: frontend-js-best-practices
description: JavaScript performance optimization guidelines. Use when writing, reviewing, or refactoring JavaScript/TypeScript code to ensure optimal performance patterns. Triggers on tasks involving loops, data structures, DOM manipulation, or general JS optimization.

JavaScript Best Practices

Performance optimization and code style patterns for JavaScript and TypeScript code. Contains 18 rules focused on reducing unnecessary computation, optimizing data structures, and maintaining consistent conventions.

When to Apply

Reference these guidelines when:

• Writing loops or array operations
• Working with data structures (Map, Set, arrays)
• Manipulating the DOM directly
• Caching values or function results
• Optimizing hot code paths
• Declaring variables or functions

Rules Summary

const-let-usage (MEDIUM) — @rules/const-let-usage.md

Use const at module level, let inside functions.

// Module level: const with UPPER_SNAKE_CASE for primitives
const MAX_RETRIES = 3;
const userCache = new Map<string, User>();

// Inside functions: always let
function process(items: Item[]) {
  let total = 0;
  let result = [];
  for (let item of items) {
    total += item.price;
  }
  return { total, result };
}

function-declarations (MEDIUM) — @rules/function-declarations.md

Prefer function declarations over arrow functions for named functions.

// Good: function declaration
function calculateTotal(items: Item[]): number {
  let total = 0;
  for (let item of items) {
    total += item.price;
  }
  return total;
}

// Good: arrow for inline callbacks
let active = users.filter((u) => u.isActive);

// Good: arrow when type requires it
const handler: ActionFunction = async ({ request }) => {
  // ...
};

no-default-exports (MEDIUM) — @rules/no-default-exports.md

Use named exports. Avoid default exports (except Remix route components).

// Bad: default export
export default function formatCurrency(amount: number) { ... }

// Good: named export
export function formatCurrency(amount: number) { ... }

// Exception: Remix routes use default export named "Component"
export default function Component() { ... }

no-as-type-casts (HIGH) — @rules/no-as-type-casts.md

Avoid as Type casts. Use type guards or Zod validation instead.

// Bad: type assertion
let user = response.data as User;

// Good: Zod validation
let user = UserSchema.parse(response.data);

// Good: type guard
if (isUser(response.data)) {
  let user = response.data;
}

comments-meaningful-only (MEDIUM) — @rules/comments-meaningful-only.md

Only comment when adding info the code cannot express.

// Bad: restates the code
// Set the user's name
let userName = user.name;

// Good: explains business rule
// Transactions under $250 don't require written acknowledgment per policy
if (transaction.amount < 250) {
  return { requiresAcknowledgment: false };
}

set-map-lookups (LOW-MEDIUM) — @rules/set-map-lookups.md

Use Set/Map for O(1) lookups instead of Array methods.

// Bad: O(n) per check
const allowedIds = ["a", "b", "c"];
items.filter((item) => allowedIds.includes(item.id));

// Good: O(1) per check
const allowedIds = new Set(["a", "b", "c"]);
items.filter((item) => allowedIds.has(item.id));

index-maps (LOW-MEDIUM) — @rules/index-maps.md

Build Map once for repeated lookups.

// Bad: O(n) per lookup = O(n*m) total
orders.map((order) => ({
  ...order,
  user: users.find((u) => u.id === order.userId),
}));

// Good: O(1) per lookup = O(n+m) total
const userById = new Map(users.map((u) => [u.id, u]));
orders.map((order) => ({
  ...order,
  user: userById.get(order.userId),
}));

tosorted-immutable (MEDIUM-HIGH) — @rules/tosorted-immutable.md

Use toSorted() instead of sort() to avoid mutation.

// Bad: mutates original array
const sorted = users.sort((a, b) => a.name.localeCompare(b.name));

// Good: creates new sorted array
const sorted = users.toSorted((a, b) => a.name.localeCompare(b.name));

combine-iterations (LOW-MEDIUM) — @rules/combine-iterations.md

Combine multiple filter/map into one loop.

// Bad: 3 iterations
const admins = users.filter((u) => u.isAdmin);
const testers = users.filter((u) => u.isTester);
const inactive = users.filter((u) => !u.isActive);

// Good: 1 iteration
const admins: User[] = [],
  testers: User[] = [],
  inactive: User[] = [];
for (const user of users) {
  if (user.isAdmin) admins.push(user);
  if (user.isTester) testers.push(user);
  if (!user.isActive) inactive.push(user);
}

cache-property-access (LOW-MEDIUM) — @rules/cache-property-access.md

Cache object properties in loops.

// Bad: repeated lookups
for (let i = 0; i < arr.length; i++) {
  process(obj.config.settings.value);
}

// Good: cached lookup
const value = obj.config.settings.value;
const len = arr.length;
for (let i = 0; i < len; i++) {
  process(value);
}

cache-function-results (MEDIUM) — @rules/cache-function-results.md

Cache expensive function results in module-level Map.

const slugifyCache = new Map<string, string>();

function cachedSlugify(text: string): string {
  if (!slugifyCache.has(text)) {
    slugifyCache.set(text, slugify(text));
  }
  return slugifyCache.get(text)!;
}

cache-storage (LOW-MEDIUM) — @rules/cache-storage.md

Cache localStorage/sessionStorage reads in memory.

const storageCache = new Map<string, string | null>();

function getLocalStorage(key: string) {
  if (!storageCache.has(key)) {
    storageCache.set(key, localStorage.getItem(key));
  }
  return storageCache.get(key);
}

early-exit (LOW-MEDIUM) — @rules/early-exit.md

Return early when result is determined.

// Bad: continues after finding error
function validate(users: User[]) {
  let error = "";
  for (const user of users) {
    if (!user.email) error = "Email required";
  }
  return error ? { error } : { valid: true };
}

// Good: returns immediately
function validate(users: User[]) {
  for (const user of users) {
    if (!user.email) return { error: "Email required" };
  }
  return { valid: true };
}

length-check-first (MEDIUM-HIGH) — @rules/length-check-first.md

Check array length before expensive comparison.

// Bad: always sorts even when lengths differ
function hasChanges(a: string[], b: string[]) {
  return a.sort().join() !== b.sort().join();
}

// Good: early return if lengths differ
function hasChanges(a: string[], b: string[]) {
  if (a.length !== b.length) return true;
  let aSorted = a.toSorted();
  let bSorted = b.toSorted();
  return aSorted.some((v, i) => v !== bSorted[i]);
}

min-max-loop (LOW) — @rules/min-max-loop.md

Use loop for min/max instead of sort.

// Bad: O(n log n)
const latest = [...projects].sort((a, b) => b.updatedAt - a.updatedAt)[0];

// Good: O(n)
let latest = projects[0];
for (const p of projects) {
  if (p.updatedAt > latest.updatedAt) latest = p;
}

hoist-regexp (LOW-MEDIUM) — @rules/hoist-regexp.md

Hoist RegExp creation outside loops.

// Bad: creates regex every iteration
items.forEach(item => {
  if (/pattern/.test(item.text)) { ... }
})

// Good: create once
const PATTERN = /pattern/
items.forEach(item => {
  if (PATTERN.test(item.text)) { ... }
})

batch-dom-css (MEDIUM) — @rules/batch-dom-css.md

Batch DOM reads before writes to avoid layout thrashing.

// Bad: interleaved reads/writes force reflows
element.style.width = "100px";
const width = element.offsetWidth; // forces reflow
element.style.height = "200px";

// Good: batch writes, then read
element.style.width = "100px";
element.style.height = "200px";
const { width, height } = element.getBoundingClientRect();