javascript.cursorrules

# JavaScript Rules

## Modern Syntax
- Use const by default, let when reassignment is needed, never var
- Use arrow functions for callbacks and short functions
- Use template literals over string concatenation
- Destructure objects and arrays at point of use
- Use optional chaining (?.) and nullish coalescing (??) over manual checks

## Async
- Use async/await over .then() chains
- Always handle errors with try/catch in async functions
- Use Promise.all() for independent concurrent operations
- Never mix callbacks and promises in the same flow

## Functions
- Keep functions pure when possible — same input, same output
- Use default parameters over manual defaults
- Return early to reduce nesting
- Prefer named exports for better tree-shaking

## Arrays & Objects
- Use map/filter/reduce over for loops for transformations
- Use structuredClone() or spread for shallow copies
- Use Object.entries/Object.keys/Object.values over for...in

## Error Handling
- Throw Error objects, not strings
- Use custom error classes for domain-specific errors
- Handle promise rejections — never leave them unhandled