typescript-rules

TypeScript/JavaScript Coding Rules

Coding Style

Immutability (CRITICAL)

ALWAYS create new objects, NEVER mutate existing ones:

Use spread operator for immutable updates:

// WRONG: Mutation
function updateUser(user, name) {
  user.name = name  // MUTATION!
  return user
}

// CORRECT: Immutability
function updateUser(user, name) {
  return {
    ...user,
    name
  }
}

File Organization

MANY SMALL FILES > FEW LARGE FILES:

• High cohesion, low coupling
• 200-400 lines typical, 800 max
• Extract utilities from large modules
• Organize by feature/domain, not by type

Error Handling

ALWAYS handle errors comprehensively:

• Handle errors explicitly at every level
• Provide user-friendly error messages in UI-facing code
• Log detailed error context on the server side
• Never silently swallow errors

Use async/await with try-catch:

try {
  const result = await riskyOperation()
  return result
} catch (error) {
  console.error('Operation failed:', error)
  throw new Error('Detailed user-friendly message')
}

Input Validation

ALWAYS validate at system boundaries:

• Validate all user input before processing
• Use schema-based validation where available
• Fail fast with clear error messages
• Never trust external data (API responses, user input, file content)

Use Zod for schema-based validation:

import { z } from 'zod'

const schema = z.object({
  email: z.string().email(),
  age: z.number().int().min(0).max(150)
})

const validated = schema.parse(input)

Console.log

• No console.log statements in production code
• Use proper logging libraries instead

Code Quality Checklist

Before marking work complete:

• Code is readable and well-named
• Functions are small (<50 lines)
• Files are focused (<800 lines)
• No deep nesting (>4 levels)
• Proper error handling
• No hardcoded values (use constants or config)
• No mutation (immutable patterns used)

Patterns