Ultracite

Modern JavaScript/TypeScript

• Use arrow functions for callbacks and short functions
• Prefer for...of loops over .forEach() and indexed for loops
• Use optional chaining (?.) and nullish coalescing (??) for safer property access
• Prefer template literals over string concatenation
• Use destructuring for object and array assignments
• Use const by default, let only when reassignment is needed, never var

Async & Promises

• Always await promises in async functions - don't forget to use the return value
• Use async/await syntax instead of promise chains for better readability
• Handle errors appropriately in async code with try-catch blocks
• Don't use async functions as Promise executors

React & JSX

• Use function components over class components
• Call hooks at the top level only, never conditionally
• Specify all dependencies in hook dependency arrays correctly
• Use the key prop for elements in iterables (prefer unique IDs over array indices)
• Nest children between opening and closing tags instead of passing as props
• Don't define components inside other components
• Use semantic HTML and ARIA attributes for accessibility:
  • Provide meaningful alt text for images
  • Use proper heading hierarchy
  • Add labels for form inputs
  • Include keyboard event handlers alongside mouse events
  • Use semantic elements (<button>, <nav>, etc.) instead of divs with roles

Error Handling & Debugging

• Remove console.log, debugger, and alert statements from production code
• Throw Error objects with descriptive messages, not strings or other values
• Use try-catch blocks meaningfully - don't catch errors just to rethrow them
• Prefer early returns over nested conditionals for error cases

Code Organization

• Keep functions focused and under reasonable cognitive complexity limits
• Extract complex conditions into well-named boolean variables
• Use early returns to reduce nesting
• Prefer simple conditionals over nested ternary operators
• Group related code together and separate concerns

Security

• Add rel="noopener" when using target="_blank" on links
• Avoid dangerouslySetInnerHTML unless absolutely necessary
• Don't use eval() or assign directly to document.cookie
• Validate and sanitize user input

Performance

• Avoid spread syntax in accumulators within loops
• Use top-level regex literals instead of creating them in loops
• Prefer specific imports over namespace imports
• Avoid barrel files (index files that re-export everything)
• Use proper image components (e.g., Next.js <Image>) over <img> tags

Framework-Specific Guidance

Next.js:

• Use Next.js <Image> component for images
• Use next/head or App Router metadata API for head elements
• Use Server Components for async data fetching instead of async Client Components

React 19+:

• Use ref as a prop instead of React.forwardRef

Solid/Svelte/Vue/Qwik:

• Use class and for attributes (not className or htmlFor)

Testing

• Write assertions inside it() or test() blocks
• Avoid done callbacks in async tests - use async/await instead
• Don't use .only or .skip in committed code
• Keep test suites reasonably flat - avoid excessive describe nesting

When Biome Can't Help

Biome's linter will catch most issues automatically. Focus your attention on:

1. Business logic correctness - Biome can't validate your algorithms
2. Meaningful naming - Use descriptive names for functions, variables, and types
3. Architecture decisions - Component structure, data flow, and API design
4. Edge cases - Handle boundary conditions and error states
5. User experience - Accessibility, performance, and usability considerations
6. Documentation - Add comments for complex logic, but prefer self-documenting code

Most formatting and common issues are automatically fixed by Biome. Run npx ultracite fix before committing to ensure compliance.