frontend-js-best-practices

JavaScript Best Practices

Performance optimization and code style patterns for JavaScript and TypeScript code. Contains 17 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.