jsdoc-best-practices

name	jsdoc-best-practices
description	Enforces JSDoc documentation standards for this TypeScript project. This skill should be used when writing or reviewing TypeScript code to ensure proper documentation with file preambles, function docs, interface docs, and the critical distinction between documenting "what" vs "why". Use this skill to understand the project's JSDoc ESLint rules and established patterns.

JSDoc Best Practices

Overview

This skill defines the JSDoc documentation standards for this project. The core principle is that documentation should explain "why", not just "what". Code already shows what it does—good documentation explains the reasoning, context, and non-obvious details that help developers understand and maintain the code.

Core Philosophy: Why Over What

The Problem with "What" Documentation

// Bad: Just restates the code
/**
 * Gets the user by ID
 * @param id - The ID
 * @returns The user
 */
function getUserById(id: string): User { ... }

This documentation adds no value—the function name already tells us it gets a user by ID.

The Solution: Document "Why"

// Good: Explains context, constraints, and non-obvious behavior
/**
 * Retrieves a user by their unique identifier
 * @param id - The user's UUID (not the legacy numeric ID)
 * @returns The user if found, null if not found or soft-deleted
 * @remarks Used by DataLoader for batching - maintains input order
 */
function getUserById(id: string): User | null { ... }

This documentation adds value by explaining:

• What kind of ID (UUID vs legacy)
• What happens when not found
• Why this specific implementation exists (DataLoader batching)

ESLint Enforcement

The project enforces JSDoc through eslint-plugin-jsdoc with these rules:

Required Documentation

Rule	Setting	What It Enforces
jsdoc/require-jsdoc	error	JSDoc on function declarations, interfaces, type aliases, and PascalCase arrow functions
jsdoc/require-param-description	error	All @param tags must have descriptions
jsdoc/require-returns-description	error	All @returns tags must have descriptions
jsdoc/require-property-description	error	All @property tags must have descriptions

Allowed Tags

Rule	Setting	Effect
jsdoc/check-tag-names	definedTags: ["remarks"]	Allows @remarks for "why" documentation
jsdoc/no-types	off	TypeScript types in JSDoc are optional
jsdoc/require-param-type	off	Types come from TypeScript, not JSDoc
jsdoc/require-returns-type	off	Types come from TypeScript, not JSDoc

What Requires Documentation

Per jsdoc/require-jsdoc configuration:

{
  require: {
    FunctionDeclaration: true,      // function foo() {}
    MethodDefinition: false,        // class methods (optional)
    ClassDeclaration: false,        // classes (optional but recommended)
    ArrowFunctionExpression: false, // const foo = () => {} (optional)
    FunctionExpression: false,      // const foo = function() {} (optional)
  },
  contexts: [
    "TSInterfaceDeclaration",       // interface Foo {}
    "TSTypeAliasDeclaration",       // type Foo = ...
    // PascalCase arrow functions (React components, factories):
    "VariableDeclaration[declarations.0.init.type='ArrowFunctionExpression']:has([id.name=/^[A-Z]/])"
  ]
}

Documentation Patterns

File Preambles

Every file should have a preamble comment at the top:

/**
 * @file complexity.plugin.ts
 * @description Apollo Server plugin for query complexity analysis and limiting
 * @module graphql
 */

Tag	Purpose
@file	The filename (for navigation and search)
@description	What this file provides
@module	The feature module this belongs to

Service Documentation

/**
 * Service for managing user accounts
 * @description Provides CRUD operations for user entities
 * @remarks
 * - All methods are idempotent
 * - Throws NotFoundException for missing resources
 * - Uses DataLoader batching for bulk operations
 */
@Injectable()
export class UserService { ... }

Method Documentation

/**
 * Batch loads entities by IDs (for DataLoader)
 * @param ids - Array of entity IDs to load
 * @returns Promise resolving to array of entities in same order as input
 * @remarks Used by DataLoader for batching - maintains input order
 */
async findByIds(ids: readonly string[]): Promise<Entity[]> { ... }

Interface Documentation

/**
 * Interface for authentication services
 * @description Defines the contract for both Cognito and Local auth implementations.
 * This interface ensures both AuthService (production) and LocalAuthService
 * (local development) provide the same public API for authentication operations.
 */
export interface IAuthService {
  /**
   * Initiates the sign-in flow by sending an OTP to the user
   * @param input - The sign-in input containing the user identifier (phone/email)
   * @returns A promise resolving to the sign-in result with session and challenge info
   */
  signIn(input: SignInInput): Promise<SignInResult>;
}

Type/Constant Documentation

/**
 * Default complexity configuration
 * @description Tune these values based on your server capacity
 */
const COMPLEXITY_CONFIG = {
  /** Maximum allowed query complexity */
  maxComplexity: 100,
  /** Default complexity for fields without explicit complexity */
  defaultComplexity: 1,
} as const;

The @remarks Tag

Use @remarks to document the "why" and important context:

When to Use @remarks

Use Case	Example
Design decisions	@remarks Uses closure pattern to cache between Lambda invocations
Usage constraints	@remarks Call getLoaders() once per GraphQL request in context factory
Non-obvious behavior	@remarks Maintains input order for DataLoader compatibility
Important caveats	@remarks All methods are idempotent - safe to retry
Integration details	@remarks Connects on module initialization, disconnects on destruction

@remarks Format

Use bullet points for multiple remarks:

/**
 * Apollo Server plugin that calculates and limits query complexity
 * @description Prevents expensive queries from overwhelming the server
 * @remarks
 * - Uses field extensions estimator for custom complexity values
 * - Falls back to simple estimator with default complexity of 1
 * - Rejects queries that exceed the configured maximum complexity
 */

Use inline for single remarks:

/**
 * Creates all DataLoader instances for a single request
 * @returns Object containing all typed DataLoaders
 * @remarks Called in GraphQL context factory - creates fresh instances per request
 */

Parameter Descriptions

Bad: Restating the Name

/**
 * @param id - The id
 * @param name - The name
 * @param options - The options
 */

Good: Adding Value

/**
 * @param id - The user's UUID (not the legacy numeric ID from v1 API)
 * @param name - Display name, max 50 characters, sanitized for XSS
 * @param options - Configuration for the query, see QueryOptions type
 */

Parameter Description Guidelines

Include	Avoid
Valid value ranges	Restating the parameter name
Format requirements	Restating the type
Default behavior	Obvious information
Edge cases	Implementation details
Units (ms, bytes, etc.)	Internal variable names

Return Value Descriptions

Bad: Restating the Type

/**
 * @returns The user
 * @returns A promise
 * @returns The result
 */

Good: Explaining Behavior

/**
 * @returns The user if found, null if not found or soft-deleted
 * @returns Promise resolving to array of entities in same order as input
 * @returns Authentication tokens on success, error message on failure
 */

Anti-Patterns to Avoid

Don't Document the Obvious

// Wrong: Adds no value
/**
 * Constructor
 */
constructor() {}

/**
 * Gets the name
 * @returns The name
 */
getName(): string { return this.name; }

Don't Duplicate TypeScript Types

// Wrong: Type is already in signature
/**
 * @param id - {string} The user ID
 * @returns {Promise<User>} The user
 */
async getUser(id: string): Promise<User> { ... }

// Correct: Description only, type from TypeScript
/**
 * @param id - The user's UUID identifier
 * @returns The user entity with populated relations
 */
async getUser(id: string): Promise<User> { ... }

Don't Write Implementation Comments

// Wrong: Documents how, not why
/**
 * Loops through users and filters by active status
 */
const activeUsers = users.filter(u => u.active);

// Correct: Self-documenting code needs no comment
// If explanation is needed, explain WHY:
// Active users are filtered first to avoid unnecessary permission checks
const activeUsers = users.filter(u => u.active);

Escaping @ Symbols in JSDoc

When documenting code that contains TypeScript/NestJS decorators (like @Injectable(), @Processor('queue-name')), JSDoc will interpret the @ as a tag marker. This causes lint errors because JSDoc sees @Processor('qpr-v2') as a single unknown tag name (including the parentheses and arguments).

The problem: Adding decorator names to definedTags doesn't help because JSDoc parses the entire string @Processor('qpr-v2') as the tag name, not just @Processor.

Solution 1: Backticks in Prose

When mentioning decorators in description text, wrap them in backticks:

/**
 * Queue processor for QPR calculations
 * @description Handles jobs from the `@Processor('qpr-v2')` queue
 * @remarks Uses `@Injectable()` scope for request isolation
 */

Solution 2: Escape in @example Blocks

In @example blocks, use fenced code blocks and escape @ as \@:

/**
 * Creates a queue processor
 * @example
 * ```typescript
 * \@Processor('my-queue')
 * export class MyProcessor {
 *   \@Process()
 *   async handle(job: Job) { ... }
 * }
 * ```
 */

Quick Reference for Escaping

Context	Approach	Example
Prose/description	Wrap in backticks	`@Injectable()`
@example block	Escape with backslash	\@Processor('name')
Code comments	No escaping needed	// Uses @Injectable

Quick Reference

Required Structure for Services

/**
 * @file feature.service.ts
 * @description Service providing feature functionality
 * @module feature
 */

/**
 * Service for feature operations
 * @description Brief description of what this service handles
 * @remarks
 * - Important architectural decisions
 * - Usage patterns or constraints
 */
@Injectable()
export class FeatureService {
  /**
   * Brief description of what this method does
   * @param paramName - What this parameter represents and any constraints
   * @returns What is returned and under what conditions
   * @remarks Any non-obvious behavior or usage notes
   */
  methodName(paramName: Type): ReturnType { ... }
}

Required Structure for Interfaces

/**
 * Interface for feature operations
 * @description Explains the contract this interface defines
 */
export interface IFeature {
  /**
   * Method description
   * @param param - Parameter description with constraints
   * @returns Return description with conditions
   */
  method(param: Type): ReturnType;
}

Required Structure for Types

/**
 * Represents a feature configuration
 * @description Used to configure feature behavior at initialization
 */
export type FeatureConfig = {
  /** Maximum retry attempts before failing */
  maxRetries: number;
  /** Timeout in milliseconds */
  timeoutMs: number;
};

Verification Checklist

Before committing code, verify:

1. File preamble exists: @file, @description, @module
2. Function declarations have JSDoc: Required by ESLint
3. Interfaces have JSDoc: Required by ESLint
4. Type aliases have JSDoc: Required by ESLint
5. Parameters have meaningful descriptions: Not just restating the name
6. Returns have meaningful descriptions: Explain conditions and edge cases
7. @remarks used for "why": Design decisions, constraints, non-obvious behavior
8. No TypeScript types in JSDoc: Types come from the signature