TypeScript Coding Standards

Basic Principles

One Function, One Responsibility

• If function name connects with "and" or "or", it's a signal to split
• If test cases are needed for each if branch, it's a signal to split

Conditional and Loop Depth Limited to 2 Levels

• Minimize depth using early return whenever possible
• If still heavy, extract into separate functions

Make Function Side Effects Explicit

• Example: If getUser also runs updateLastAccess(), specify it in the function name

Convert Magic Numbers/Strings to Constants When Possible

• Declare at the top of the file or class where used
• Consider separating into a constants file if there are many

Function Order by Call Order

• Follow class access modifier declaration order rules if clear
• Otherwise, order top-to-bottom for easy reading by call order

Review External Libraries for Complex Implementations

• When logic is complex and tests become bloated
• If industry-standard libraries exist, use them
• When security, accuracy, or performance optimization is critical
• When browser/platform compatibility or edge cases are numerous

Modularization (Prevent Code Duplication and Pattern Repetition)

• Absolutely forbid code repetition
• Modularize similar patterns into reusable forms
• Allow pre-modularization if reuse is confirmed
• Avoid excessive abstraction
• Modularization levels:
  • Same file: Extract into separate function
  • Multiple files: Separate into different file
  • Multiple projects/domains: Separate into package

Variable and Function Names

• Clear purpose while being concise
• Forbid abbreviations outside industry standards (id, api, db, err, etc.)
• Don't repeat context from the parent scope
• Boolean variables use is, has, should prefixes
• Function names are verbs or verb+noun forms
• Plural rules:
  • Pure arrays: "s" suffix (users)
  • Wrapped object: "list" suffix (userList)
  • Specific data structure: Explicit (userSet, userMap)
  • Already plural words: Use as-is

Field Order

• Alphabetically ascending by default
• Maintain consistency in usage
• Also alphabetically ordered in destructuring assignment

Error Handling

• Error handling level: Handle where meaningful response is possible
• Error messages: Technical details for logs, actionable guidance for users
• Error classification: Distinguish between expected and unexpected errors
• Error propagation: Add context when propagating up the call stack
• Recovery vs. fast fail: Recover from expected errors with fallback
• Error types: For domain-specific failures, create custom error classes extending Error. Never throw non-Error objects
• Async errors: Always handle Promise rejection. Use try-catch for async/await, .catch() for promise chains

Package Management

Package Manager

• Use pnpm as default package manager
• Forbid npm, yarn (prevent lock file conflicts)

File Structure

Common for All Files

1. Import statements (grouped)
2. Constant definitions (alphabetically ordered if multiple)
3. Type/Interface definitions (alphabetically ordered if multiple)
4. Main content (see below)

Inside Classes

• Decorators
• private readonly members
• readonly members
• constructor
• public methods (alphabetically ordered)
• protected methods (alphabetically ordered)
• private methods (alphabetically ordered)

Function Placement in Function-Based Files

• Main exported function
• Additional exported functions (alphabetically ordered, avoid many)
• Helper functions

Function Writing

Use Arrow Functions

• Always use arrow functions except for class methods
• Forbid function keyword entirely (exceptions: generator function*, function hoisting etc. technically impossible cases only)

Function Arguments: Flat vs Object

• Use flat if single argument or uncertain of future additions
• Use object form for 2+ arguments in most cases. Allow flat form when:
  • All required arguments without boolean arguments
  • All required arguments with clear order (e.g., (width,height), (start,end), (min,max), (from,to))

Type System

Type Safety

• Forbid unsafe type bypasses like any, as, !, @ts-ignore, @ts-expect-error
• Exceptions: Missing or incorrect external library types, rapid development needed (clarify reason in comments)
• Allow some unknown type when type guard is clear
• Allow as assertion when literal type (as const) needed
• Allow as assertion when widening literal/HTML types to broader types
• Allow "!" assertion when type narrowing impossible after type guard due to TypeScript limitation
• Allow @ts-ignore, @ts-expect-error in test code (absolutely forbid in production)

Interface vs Type

• Prioritize Type in all cases by default
• Use Interface only for these exceptions:
  • Public API provided to external users like library public API
  • Need to extend existing interface like external libraries
  • Designing OOP-style classes where implementation contract must be clearly defined

null/undefined Handling

• Actively use Optional Chaining (?.)
• Provide defaults with Nullish Coalescing (??)
• Distinguish between null and undefined by semantic meaning:
  • undefined: Uninitialized state, optional parameters, value not assigned yet
  • null: Intentional absence of value (similar to Go's nil)
• Examples:
  • Optional field: { name?: string } → can be undefined
  • Intentionally cleared value: user.profileImage = null
  • External API responses may use either convention

Code Style

Maintain Immutability

• Use const whenever possible, minimize let
• Create new values instead of directly modifying arrays/objects
• Use spread, filter, map instead of push, splice
• Exceptions: Extremely performance-critical cases

Recommended Libraries

• Testing: Vitest, Playwright
• Utilities: es-toolkit, dayjs
• HTTP: ky, @tanstack/query, @apollo/client
• Form: React Hook Form
• Type validation: zod
• UI: Tailwind + shadcn/ui
• ORM: Prisma (Drizzle if edge support important)
• State management: zustand
• Code formatting: prettier, eslint
• Build: tsup