Mapper Implementation Patterns

Core Mapper Principles

1. API spec drives mapper logic - NOT the other way around
2. ZERO MISSING FIELDS - Interface field count MUST EQUAL mapped field count
3. PREFER map() utility over constructors
4. ALWAYS validate required fields from API spec
5. Use const output pattern in all mappers
6. Convert snake_case to camelCase consistently
7. Apply core types (UUID, Email, URL, DateTime) via map()
8. Use

   optional()

   for optional fields - Normalize null to undefined, preserve "", 0, false
9. NEVER weaken API spec to make mapper easier
10. Use InvalidState for validation errors - throw immediately on missing required fields
11. Declare helpers before exports - nested mappers declared BEFORE main mapper but NOT exported
12. map() handles null/undefined - No ternary checks needed with map(), it returns undefined automatically
13. No fallbacks between different fields - If API has 2 fields, output has 2 fields. No merging, no defaults
14. Use any for raw parameters - All raw parameters should be

    any

    for simplicity
15. Extract complex inline mappings - Nested objects with 3+ properties should be helper functions
16. Helper functions MUST validate - All helper functions need

    ensureProperties()

    for required fields
17. Enum arrays use map pattern -

    raw.field?.map((d: any) => toEnum(EnumType, d))

See type-mapping skill for complete type conversion reference.

Mapper Function Naming Convention

MANDATORY: All mapper functions MUST use the

to<Model>

naming pattern.

Pattern: to<Model>

// ✅ CORRECT - to<Model> pattern with any
export function toUser(raw: any): User { }
export function toGroup(raw: any): Group { }
export function toEntry(raw: any): Entry { }

// Helper functions (non-exported) also use to<Model>
function toUserIdentity(raw: any): UserIdentity { }
function toAddress(raw: any): Address { }

// ❌ WRONG - Using 'any' instead of any
export function mapUser(raw: any): User { }      // NO!
export function mapGroup(raw: any): Group { }    // NO!

Rationale

1. Concise and clear - "to" clearly indicates conversion/transformation
2. Consistent with helpers - Internal helpers already use

   to

   prefix
3. Industry standard - Common pattern in TypeScript/JavaScript
4. Matches other utilities - toEnum(), toString(), toDate() convention

Naming Rules

• Exported mappers:

  export function to<Model>(raw: any): <Model>

• Helper mappers:

  function to<NestedModel>(raw: any): <NestedModel>

  (not exported)
• Model name: Exact TypeScript interface name (User, Group, Entry, UserIdentity, etc.)
• NO map prefix:

  toUser

  not

  mapUser

• NO get prefix:

  toUser

  not

  getUser

Examples

// Main resource mappers
export function toUser(raw: any): User { }
export function toGroup(raw: any): Group { }
export function toRole(raw: any): Role { }
export function toSite(raw: any): Site { }
export function toEntry(raw: any): Entry { }
export function toZone(raw: any): Zone { }

// Nested object helpers (not exported)
function toUserIdentity(raw: any): UserIdentity { }
function toAddress(raw: any): Address { }
function toContactInfo(raw: any): ContactInfo { }
function toOrganizationRef(raw: any): OrganizationRef { }

// Extended models
export function toUserInfo(raw: any): UserInfo { }
export function toGroupInfo(raw: any): GroupInfo { }

Standard Mapper Function Structure

Canonical Format

All mapper functions MUST follow this exact structure:

export function to<Resource>(raw: any): <Resource> {
  // 1. Check for required fields
  ensureProperties(raw, ['id', 'required_field']);
  // Or manual validation:
  // if (!raw.id) {
  //   throw new InvalidStateError('Missing required field: id');
  // }

  // 2. Create output object
  const output: <Resource> = {
    // Property mappings here
  };

  return output;
}

Complete Example with All Patterns

// src/Mappers.ts
import { map } from '@zerobias-org/util-hub-module-utils';
import { UUID, Email, URL, DateTime, InvalidStateError } from '@zerobias-org/types-core-js';
import { User, Address, UserStatus, UserRole } from '../generated/model';
import { mapWith, ensureProperties, optional } from './util';  // Import helpers

// Helper function - NOT exported, declared BEFORE main mapper
function toAddress(raw: any): Address {
  // 1. Check for required fields
  ensureProperties(raw, ['street']);

  // 2. Create output object
  const output: Address = {
    street: raw.street,
    city: optional(raw.city),
    zipCode: optional(raw.zip_code)
  };

  return output;
}

// Main mapper - exported, uses helper
export function toUser(raw: any): User {
  // 1. Check for required fields
  ensureProperties(raw, ['id', 'email', 'first_name', 'status']);

  // 2. Create output object
  const output: User = {
    id: raw.id.toString(),                              // ID conversion
    email: raw.email,                                   // Direct mapping (required)
    firstName: raw.first_name,                          // snake_case → camelCase (required)
    lastName: optional(raw.last_name),                  // Optional - normalizes null to undefined, keeps ""
    createdAt: map(DateTime, raw.created_at),          // map() handles required/optional automatically
    updatedAt: map(DateTime, raw.updated_at),          // map() returns undefined if null/undefined
    dateOfBirth: map(Date, raw.date_of_birth),         // No ternary needed - map() handles it
    status: toEnum(UserStatus, raw.status),            // Enum conversion
    phoneNumber: optional(raw.phone_number),           // Optional - null→undefined, keeps "", 0, false
    address: mapWith(toAddress, raw.address),          // Nested object with mapWith
    roles: raw.roles?.map((r: any) => toEnum(UserRole, r))  // Array mapping
  };

  return output;
}

Property Mapping Patterns

1. Direct Mapping (Same Type)

For required fields - direct mapping:

{
  email: raw.email,        // Required - direct mapping
  firstName: raw.first_name // Required - snake_case → camelCase
}

For optional fields - use

optional()

:

{
  description: optional(raw.description), // Optional - null→undefined, keeps ""
  count: optional(raw.count),        // Optional - null→undefined, keeps 0
  active: optional(raw.active)       // Optional - null→undefined, keeps false
}

Rule: Direct mapping for required fields. Use

optional()

for optional fields to normalize null.

2. ID Conversion

Always convert numeric IDs to strings:

{
  id: raw.id.toString()  // or raw.id if already string
}

3. Constructor-Based Conversion with map()

Use

map()

helper for types with constructors (Date, UUID, Email, URL, etc.):

import { map } from '@zerobias-org/util-hub-module-utils';
import { UUID, Email, URL, DateTime } from '@zerobias-org/types-core-js';

{
  createdAt: map(DateTime, raw.created_at),    // Required or optional - map() handles both
  updatedAt: map(DateTime, raw.updated_at),    // map() returns undefined if raw.updated_at is null/undefined
  userId: map(UUID, raw.user_id),
  email: map(Email, raw.email)
}

Rule:

map()

handles undefined/null automatically and returns undefined. No need for ternary checks.

❌ WRONG - Don't use ternary with map():

{
  dateOfBirth: raw.dateOfBirth ? map(Date, raw.dateOfBirth) : undefined  // ❌ Redundant
}

✅ CORRECT - map() handles it:

{
  dateOfBirth: map(Date, raw.dateOfBirth)  // ✅ map() returns undefined if raw.dateOfBirth is null/undefined
}

Why map() is preferred:

• Handles optional/undefined values automatically - returns undefined if input is null/undefined
• Provides consistent error handling
• Validates input during conversion
• Cleaner, more concise code - no ternary needed

4. Enum Conversion with toEnum()

Use

toEnum()

helper for enum properties:

{
  status: toEnum(StatusEnum, raw.status)
}

Default behavior: Values are converted to

snake_case

before enum lookup.

Custom transformation: Pass a second parameter transformation function:

{
  type: toEnum(TypeEnum, raw.type, (v) => v.toUpperCase()),
  format: toEnum(FormatEnum, raw.format, (v) => v.toLowerCase())
}

5. Optional/Nullable Properties

Use

optional()

helper to normalize null while preserving all other values:

{
  description: optional(raw.description),  // null→undefined, preserves "", 0, false
  phoneNumber: optional(raw.phone_number), // Normalizes only null
  name: optional(raw.name),                // Keeps empty strings
  count: optional(raw.count),              // Keeps 0 as 0
  active: optional(raw.active)             // Keeps false as false
}

Why

optional()

?

• Normalizes

  null

  →

  undefined

  (consistent "no value" representation) ✅
• Preserves empty strings

  ""

  (legitimate value) ✅
• Preserves zeros

  0

  (legitimate value) ✅
• Preserves

  false

  (legitimate boolean value) ✅
• One "no value" state (

  undefined

  ) instead of two (

  null

  and

  undefined

  )
• Cleaner, more semantic than

  ?? undefined

  ✅

❌ WRONG - Logical OR destroys legitimate values:

{
  name: raw.name || undefined,  // ❌ Converts "", 0, false to undefined (data loss!)
  count: raw.count || 0,        // ❌ Converts null/undefined to 0 (default injection!)
  active: raw.active || false,  // ❌ Converts null/undefined to false (default injection!)
  description: raw.description ? raw.description : undefined  // ❌ Converts "", 0, false to undefined
}

✅ CORRECT - optional() preserves legitimate falsy values:

{
  name: optional(raw.name),        // ✅ null→undefined, keeps ""
  count: optional(raw.count),      // ✅ null→undefined, keeps 0
  active: optional(raw.active),    // ✅ null→undefined, keeps false
  description: optional(raw.description)  // ✅ null→undefined, keeps ""
}

IMPORTANT - No Fallbacks or Defaults Between Different Fields:

❌ WRONG - Don't merge different API fields:

{
  phoneNumber: raw.mobilePhone || raw.phoneNumber || undefined  // ❌ NO!
}

✅ CORRECT - Map each API field to its own output field:

{
  mobilePhone: optional(raw.mobilePhone),
  phoneNumber: optional(raw.phoneNumber)
}

Rule: If the API has 2 different fields, your output should have 2 different fields. No fallbacks, no defaults, no merging. Use

optional()

for optional fields.

6. Nested Objects

For single nested objects: Use non-exported helper functions with

mapWith()

:

// Helper function - NOT exported, declared BEFORE main mapper
// Does NOT check for null - mapWith() handles that
function toSubResource(raw: any): SubResource {
  // 1. Check for required fields
  ensureProperties(raw, ['id']);

  // 2. Create output object
  const output: SubResource = {
    id: raw.id.toString(),
    name: optional(raw.name)
  };

  return output;
}

// Main mapper - exported, uses helper with mapWith()
export function toResource(raw: any): Resource {
  // 1. Check for required fields
  ensureProperties(raw, ['id']);

  // 2. Create output object
  const output: Resource = {
    id: raw.id.toString(),
    nestedObject: mapWith(toSubResource, raw.nested_object)  // ✅ mapWith handles null/undefined
  };

  return output;
}

For arrays: Call mapper directly (NO mapWith):

// Helper for array items - call directly, NO mapWith
function toSubResource(raw: any): SubResource {
  // 1. Check for required fields
  ensureProperties(raw, ['id']);

  // 2. Create output object
  const output: SubResource = {
    id: raw.id.toString(),
    name: optional(raw.name)
  };

  return output;
}

// Main mapper - arrays call helper directly
export function toResource(raw: any): Resource {
  const output: Resource = {
    id: raw.id.toString(),
    items: raw.items?.map(toSubResource)  // ✅ Direct call, no mapWith
  };

  return output;
}

Rules:

• Nested mappers are declared BEFORE the parent mapper but are NOT exported
• Helper functions assume valid input - they don't check for null/undefined
• Single objects: Use

  mapWith()

  - handles null/undefined at boundary
• Arrays: Call mapper directly (NO mapWith) -

  raw.items?.map(toMapper)

• Return type: Helpers return plain

  T

  , not

  T | undefined

  (mapWith adds the undefined)

Function Ordering Rules

Mapper functions in

Mappers.ts

MUST be ordered as follows:

1. Helper Functions First

All non-exported helper functions declared BEFORE any exported functions:

// Helper 1 - NOT exported
function toAddress(raw: any): Address {
  // Implementation
}

// Helper 2 - NOT exported
function toContactInfo(raw: any): ContactInfo {
  // Implementation
}

// Exported mapper that uses helpers
export function toUser(raw: any): User {
  const output: User = {
    address: toAddress(raw.address),
    contact: toContactInfo(raw.contact)
  };
  return output;
}

2. Dependency Order

If mapper A uses mapper B, B MUST be declared first:

// B declared first
function toAddress(raw: any): Address { }

// A declared second (depends on B)
function toUser(raw: any): User {
  return {
    address: toAddress(raw.address)  // Uses B
  };
}

3. Alphabetical Within Groups

Within each group (helpers, exports), order functions alphabetically:

// Helpers in alphabetical order
function toAddress(raw: any): Address { }
function toContactInfo(raw: any): ContactInfo { }
function toMetadata(raw: any): Metadata { }

// Exports in alphabetical order
export function toOrganization(raw: any): Organization { }
export function toUser(raw: any): User { }
export function toWebhook(raw: any): Webhook { }

Exception: Dependency order overrides alphabetical order.

7. Array Mapping

For arrays of nested objects - call mapper directly (NO mapWith):

{
  // Array of nested objects - call mapper directly
  items: Array.isArray(raw.items) ? raw.items.map(toSubResource) : undefined
}

For required arrays (no optional chaining):

{
  // Required array - validate and map
  items: Array.isArray(raw.items) ? raw.items.map(toSubResource) : []
}

For array of enums:

{
  // Single enum array
  roles: Array.isArray(raw.roles) ? raw.roles.map(r => toEnum(UserRole, r)) : undefined,

  // ✅ CORRECT - daysOfWeek enum array pattern
  daysOfWeek: Array.isArray(raw.daysOfWeek) ? raw.daysOfWeek.map(d => toEnum(ScheduleEvent.DaysOfWeekEnum, d)) : undefined
}

Key points:

• Arrays: use

  Array.isArray()

  check first for type safety
• Helper functions don't check null/undefined - they assume valid input
• Enum arrays use same pattern with

  toEnum()

  in map callback

Required Field Validation

Pattern: Validate Before Mapping

// API spec says 'id' is required
export function toWebhook(raw: any): Webhook {
  // 1. Check for required fields
  if (!raw.id) {
    throw new InvalidStateError('Missing required field: id');
  }

  // 2. Create output object
  const output: Webhook = {
    id: raw.id.toString(),
    // ... other fields
  };

  return output;
}

Key points:

• Check for required fields BEFORE any mapping
• Use

  InvalidStateError

  from

  @zerobias-org/types-core-js

• Use

  ensureProperties()

  helper for multiple fields or manual

  if

  checks for single fields
• Throw immediately - don't return undefined for missing required fields
• Use section comment:

  // 1. Check for required fields

Handling Falsy Values

ensureProperties()

correctly handles all falsy values:

// ✅ CORRECT - ensureProperties handles 0, "", false correctly
ensureProperties(raw, ['id', 'count', 'active', 'name']);
// Passes validation for: id=0, count=0, active=false, name=""
// Fails validation for: id=null, id=undefined

The helper only checks for

null

and

undefined

, so all other falsy values (

0

,

''

,

false

) pass validation correctly.

Optional Field Handling

Pattern: Use

optional()

to Normalize Null

// Handle optional fields with optional()
const output: Webhook = {
  name: optional(raw.name),             // null→undefined, keeps ""
  email: map(Email, raw.email),         // map() handles undefined/null
  count: optional(raw.count),           // null→undefined, keeps 0
  active: optional(raw.active),         // null→undefined, keeps false
  tags: optional(raw.tags),             // null→undefined, keeps []
  metadata: mapWith(toMetadata, raw.metadata)  // mapWith handles optional nested objects
};

Key patterns:

• raw.field

  - Direct mapping for required fields

• optional(raw.field)

  - Normalize null to undefined for optional fields

• map(Type, raw.field)

  - map() automatically handles undefined/null

• mapWith(toNested, raw.nested)

  - mapWith handles optional nested objects

❌ NEVER use logical OR or inject defaults:

// ❌ WRONG - Logical OR destroys legitimate values
name: raw.name || undefined   // Converts "", 0, false to undefined
count: raw.count || 0         // Converts null/undefined to 0 (default injection!)
tags: raw.tags || []          // Converts null/undefined to [] (default injection!)
active: raw.active || false   // Converts null/undefined to false (default injection!)

// ✅ CORRECT - optional() preserves legitimate values
name: optional(raw.name)    // null→undefined, keeps ""
count: optional(raw.count)  // null→undefined, keeps 0
tags: optional(raw.tags)    // null→undefined, keeps []
active: optional(raw.active) // null→undefined, keeps false

snake_case to camelCase Conversion

Always Convert Field Names

// API returns snake_case, internal types use camelCase
const output: Webhook = {
  createdAt: map(DateTime, data.created_at),              // ✅
  updatedAt: map(DateTime, data.updated_at),              // ✅
  lastTriggeredAt: map(DateTime, data.last_triggered_at)  // ✅
};

Conversion rules:

• created_at

  →

  createdAt

• updated_at

  →

  updatedAt

• last_triggered_at

  →

  lastTriggeredAt

• content_type

  →

  contentType

• insecure_ssl

  →

  insecureSsl

API Spec is Source of Truth

❌ WRONG APPROACH: Weakening spec for mapper

# api.yml - DON'T DO THIS
Organization:
  type: object
  properties:    # ❌ NO! Don't remove required to make mapper easier
    id:
      type: string

Why wrong: Removes critical API contract information just to avoid validation in mapper.

✅ CORRECT APPROACH: Mapper validates spec

# api.yml - Keep spec accurate
Organization:
  type: object
  required:
    - id       # ✅ YES! Spec reflects API reality
  properties:
    id:
      type: string

// Mapper validates required fields
export function toOrganization(raw: any): Organization {
  // 1. Check for required fields
  if (!raw.id) {
    throw new InvalidStateError('Missing required field: id');
  }

  // 2. Create output object
  const output: Organization = {
    id: raw.id.toString(),
    name: raw.name
  };

  return output;
}

Why correct: API spec stays accurate, mapper enforces the contract.

Nested Object Mapping

Pattern: Create Separate Mapper Functions

// Parent mapper
export function toWebhook(data: any): Webhook | undefined {
  if (!data) return undefined;

  const output: Webhook = {
    id: map(UUID, data.id),
    config: toWebhookConfig(data.config),  // Call nested mapper
    metadata: toMetadata(data.metadata)     // Another nested mapper
  };

  return output;
}

// Nested mapper
export function toWebhookConfig(data: any): WebhookConfig | undefined {
  if (!data) return undefined;

  const output: WebhookConfig = {
    url: map(URL, data.url),
    contentType: data.content_type || 'json'
  };

  return output;
}

Array Mapping

Pattern: Map and Filter

// Array mapper
export function toWebhookArray(data: any): Webhook[] {
  if (!Array.isArray(data)) return [];
  return data.map(toWebhook).filter((w): w is Webhook => w !== undefined);
}

Key points:

• Check

  Array.isArray()

  first
• Use

  .map()

  to transform each item
• Use

  .filter()

  to remove undefined results
• Type predicate:

  (w): w is Webhook => w !== undefined

Const Output Pattern

Always Use const output

export function toWebhook(data: any): Webhook | undefined {
  if (!data) return undefined;

  // ✅ YES - Use const output pattern
  const output: Webhook = {
    id: map(UUID, data.id),
    name: data.name || undefined,
    // ... all fields
  };

  return output;
}

Why this pattern:

• Clear type declaration
• All fields visible in one place
• Easy to review completeness
• TypeScript catches missing fields

Anti-pattern:

// ❌ NO - Building object incrementally
export function toWebhook(data: any): Webhook | undefined {
  if (!data) return undefined;

  const webhook: Partial<Webhook> = {};
  webhook.id = map(UUID, data.id);
  webhook.name = data.name;
  // Easy to miss fields
  return webhook as Webhook;
}

Validation Checklist

Verify Mapper Implementation

# Mappers.ts exists
ls src/Mappers.ts

# Uses map() utility
grep "import.*map.*from.*@zerobias-org/util-hub-module-utils" src/Mappers.ts
# Should show import

# Prefers map() over constructors
CONSTRUCTOR_COUNT=$(grep -E "new (UUID|Email|URL|DateTime)\(" src/Mappers.ts | wc -l)
MAP_COUNT=$(grep -E "map\((UUID|Email|URL|DateTime)," src/Mappers.ts | wc -l)
# MAP_COUNT should be >= CONSTRUCTOR_COUNT

# Validates required fields
grep "Missing required field" src/Mappers.ts
# Should show validation for required fields

# Const output pattern
grep "const output:" src/Mappers.ts
# Should show const pattern

# No environment variables
grep "process.env" src/Mappers.ts
# Should return nothing

Standard Output Format

When documenting mapper implementation:

# Mapper Implementation: Mappers.ts

## Mapper Functions Created

### toWebhook(data: any): Webhook | undefined
- **Validates**: id (required field)
- **Converts**: created_at → createdAt (DateTime)
- **Handles**: Optional fields (name, description)
- **Nested**: config → WebhookConfig via toWebhookConfig()
- **Uses**: map() for UUID, DateTime conversions

### toWebhookArray(data: any): Webhook[]
- **Handles**: Array conversion
- **Filters**: undefined results

### toWebhookConfig(data: any): WebhookConfig | undefined
- **Converts**: content_type → contentType
- **Uses**: map() for URL conversion
- **Handles**: Optional fields

## Type Conversions Applied
✅ UUID via map()
✅ DateTime via map() (snake_case → camelCase)
✅ URL via map()
✅ Optional fields handled
✅ Required fields validated

## Validation
✅ map() utility used (preferred)
✅ Required fields validated
✅ const output pattern
✅ snake_case → camelCase
✅ Core types applied
✅ No environment variables

## Code Location
- src/Mappers.ts

Success Criteria

Mapper implementation MUST meet all criteria:

• ✅ All mappers use const output pattern
• ✅ map() utility preferred over constructors
• ✅ Required fields validated per API spec
• ✅ Optional fields handled correctly (still map them!)
• ✅ snake_case converted to camelCase consistently
• ✅ Core types applied (UUID, Email, URL, DateTime)
• ✅ No API spec weakening for mapper convenience
• ✅ ZERO MISSING FIELDS - all interface fields mapped
• ✅ One mapper function per type
• ✅ Nested objects use separate mapper functions
• ✅ Arrays use array mapper pattern with filter

Utility Functions

toEnum() Helper

The

toEnum()

function converts string values to enum values with optional transformation:

/**
 * Converts a string value to an enum value
 * @param enumType - The enum object
 * @param value - The string value to convert
 * @param transform - Optional transformation function (default: converts to snake_case)
 */
function toEnum<T>(
  enumType: object,
  value: string,
  transform?: (v: string) => string
): T {
  // Implementation expected to be available in the module
}

Usage examples:

// Default: converts to snake_case
status: toEnum(UserStatus, raw.status)
// raw.status = "activeUser" → converted to "active_user" → matched to enum

// Custom transformation: uppercase
type: toEnum(TypeEnum, raw.type, (v) => v.toUpperCase())
// raw.type = "admin" → "ADMIN" → matched to enum

// Custom transformation: lowercase
format: toEnum(FormatEnum, raw.format, (v) => v.toLowerCase())

map() Helper

The

map()

utility function handles type conversion with automatic optional/undefined handling:

import { map } from '@zerobias-org/util-hub-module-utils';
import { UUID, Email, URL, DateTime } from '@zerobias-org/types-core-js';

// Automatically handles optional/undefined
id: map(UUID, raw.id)           // Required
email: map(Email, raw.email)     // Optional - returns undefined if raw.email is undefined
createdAt: map(DateTime, raw.created_at)

ensureProperties() Helper for Required Field Validation

For validating multiple required fields at once, use

ensureProperties()

from

src/util.ts

:

import { ensureProperties } from './util';

Type Signature:

function ensureProperties<K extends string>(
  raw: unknown,
  properties: readonly K[]
): asserts raw is Record<K, NonNullable<unknown>>

Usage:

// Before - Manual validation
export function toUser(raw: any): User {
  // 1. Check for required fields
  if (!raw.id) {
    throw new InvalidStateError('Missing required field: id');
  }
  if (!raw.email) {
    throw new InvalidStateError('Missing required field: email');
  }
  if (!raw.status) {
    throw new InvalidStateError('Missing required field: status');
  }

  // 2. Create output object
  const output: User = {
    id: raw.id.toString(),
    email: raw.email,
    status: toEnum(UserStatus, raw.status)
  };

  return output;
}

// After - Using ensureProperties with TypeScript type inference
export function toUser(raw: any): User {
  // 1. Check for required fields
  ensureProperties(raw, ['id', 'email', 'status']);
  // TypeScript now knows: raw.id, raw.email, raw.status exist and are not null/undefined

  // 2. Create output object
  const output: User = {
    id: raw.id.toString(),    // ✅ TypeScript knows raw.id exists
    email: raw.email,          // ✅ TypeScript knows raw.email exists
    status: toEnum(UserStatus, raw.status)  // ✅ TypeScript knows raw.status exists
  };

  return output;
}

Benefits:

• Less boilerplate - One line instead of multiple if statements
• TypeScript type inference - Assertion signature tells TypeScript properties exist
• Consistent error messages - All use same format
• Easy to see requirements - Array shows all required fields at a glance
• Correctly handles falsy values - Only checks

  null

  /

  undefined

  , allows

  0

  ,

  false

  ,

  ""

  ✅
• Better IDE support - Autocomplete and type checking after validation

Falsy Value Handling:

ensureProperties()

implementation explicitly checks

value === null || value === undefined

, which means:

• ✅

  0

  passes validation (legitimate numeric value)
• ✅

  ""

  passes validation (legitimate empty string)
• ✅

  false

  passes validation (legitimate boolean value)
• ❌

  null

  fails validation (missing value)
• ❌

  undefined

  fails validation (missing value)

This is the correct behavior - we preserve all legitimate values and only reject truly missing ones.

optional() Helper for Normalizing Null to Undefined

For optional fields that may be

null

, use

optional()

from

src/util.ts

to normalize

null

→

undefined

:

import { optional } from './util';

Type Signature:

function optional<T>(value: T | null | undefined): T | undefined {
  return value ?? undefined;
}

Usage:

// Before - Manual null normalization
export function toUser(raw: any): User {
  ensureProperties(raw, ['id', 'email']);

  const output: User = {
    id: raw.id.toString(),
    email: raw.email,
    phoneNumber: raw.phoneNumber ?? undefined,
    avatarUrl: raw.avatarUrl ?? undefined,
    middleName: raw.middleName ?? undefined
  };

  return output;
}

// After - Using optional() for cleaner code
export function toUser(raw: any): User {
  ensureProperties(raw, ['id', 'email']);

  const output: User = {
    id: raw.id.toString(),
    email: raw.email,
    phoneNumber: optional(raw.phoneNumber),
    avatarUrl: optional(raw.avatarUrl),
    middleName: optional(raw.middleName)
  };

  return output;
}

Benefits:

• Cleaner code -

  optional(raw.field)

  vs

  raw.field ?? undefined

• More semantic - Clearly indicates "this field is optional"
• Preserves falsy values - Only converts

  null

  to

  undefined

  , keeps

  0

  ,

  ''

  ,

  false

• Consistent pattern - Works alongside

  map()

  ,

  mapWith()

  ,

  ensureProperties()

What optional() does:

• ✅

  optional(null)

  →

  undefined

  (normalizes null)
• ✅

  optional(undefined)

  →

  undefined

  (passes through)
• ✅

  optional(0)

  →

  0

  (preserves zero)
• ✅

  optional("")

  →

  ""

  (preserves empty string)
• ✅

  optional(false)

  →

  false

  (preserves false)
• ✅

  optional("value")

  →

  "value"

  (preserves value)

mapWith() Helper for Single Nested Objects

For single nested objects, use

mapWith()

from

src/util.ts

which works like

map()

but for custom mapper functions:

import { mapWith } from './util';

/**
 * Applies a mapper function to a value, handling null/undefined at the boundary
 * Works like map() but for custom mapper functions instead of constructors
 * @param mapper - The mapper function to apply (assumes valid input)
 * @param value - The value to map
 * @returns Mapped value or undefined if input is null/undefined
 */
function mapWith<T>(mapper: (raw: any) => T, value: any): T | undefined {
  if (value === null || value === undefined) {
    return undefined;
  }
  return mapper(value);
}

Usage examples:

// Helper function - does NOT check for null (mapWith handles it)
function toSubResource(raw: any): SubResource {
  ensureProperties(raw, ['id']);

  const output: SubResource = {
    id: raw.id.toString(),
    name: optional(raw.name)
  };

  return output;
}

// Single nested object - mapWith handles null/undefined
export function toResource(raw: any): Resource {
  ensureProperties(raw, ['id']);

  const output: Resource = {
    id: raw.id.toString(),
    subResource: mapWith(toSubResource, raw.sub_resource),  // ✅ Clean! mapWith handles null
    contact: mapWith(toContactInfo, raw.contact)
  };

  return output;
}

// For arrays - DON'T use mapWith, call mapper directly
export function toUser(raw: any): User {
  const output: User = {
    id: raw.id.toString(),
    // ✅ Call mapper directly (NO mapWith for arrays)
    addresses: raw.addresses?.map(toAddress)
  };

  return output;
}

// Helper for arrays - same structure, no null check
function toAddress(raw: any): Address {
  ensureProperties(raw, ['street']);

  return {
    street: raw.street,
    city: optional(raw.city)
  };
}

Benefits:

• Consistent with

  map()

  pattern - handles null/undefined automatically
• Separation of concerns - null handling in

  mapWith()

  , validation in helpers
• Only for single nested objects - arrays use direct mapper call
• No ternary clutter:

  mapWith(toSubResource, raw.sub)

  vs

  raw.sub ? toSubResource(raw.sub) : undefined

• Helper functions are simpler - no null checks, just validation + transformation

Implementation location:

• Currently:

  src/util.ts

  - import with

  import { mapWith } from './util'

• Future: Will be moved to

  @zerobias-org/util-hub-module-utils

  alongside

  map()

  and

  toEnum()

Inline Object Mapping - ANTI-PATTERN

❌ AVOID: Complex Inline Object Mappings

Never create complex inline object mappings with 3+ properties. Extract to helper functions instead.

// ❌ WRONG - Complex inline mapping (hard to read, test, maintain)
export function toEntryUser(raw: any): EntryUser {
  ensureProperties(raw, ['id']);

  const output: EntryUser = {
    id: String(raw.id),
    identity: raw.identity ? {  // ❌ COMPLEX INLINE MAPPING
      id: (raw.identity as any).id ? String((raw.identity as any).id) : undefined,
      firstName: optional((raw.identity as any).firstName),
      middleName: optional((raw.identity as any).middleName),
      lastName: optional((raw.identity as any).lastName),
      fullName: optional((raw.identity as any).fullName),
      initials: optional((raw.identity as any).initials),
      email: map(Email, (raw.identity as any).email as string),
    } : undefined,
  };
  return output;
}

✅ CORRECT: Extract to Helper Function

// Helper function - declared before main mapper
function toEntryUserIdentity(raw: any): EntryUserIdentity {
  ensureProperties(raw, ['id', 'email']);  // ✅ Validates required fields

  const output: EntryUserIdentity = {
    id: String(raw.id),
    firstName: optional(raw.firstName),
    middleName: optional(raw.middleName),
    lastName: optional(raw.lastName),
    fullName: optional(raw.fullName),
    initials: optional(raw.initials),
    email: map(Email, raw.email),
  };
  return output;
}

// Main mapper - uses helper with mapWith
export function toEntryUser(raw: any): EntryUser {
  ensureProperties(raw, ['id']);

  const output: EntryUser = {
    id: String(raw.id),
    identity: mapWith(toEntryUserIdentity, raw.identity),  // ✅ Clean, testable
  };
  return output;
}

Benefits of Helper Functions:

1. Readability - Each function has single responsibility
2. Testability - Can unit test helpers independently
3. Reusability - Helper can be used by multiple mappers
4. Maintainability - Changes isolated to one function
5. Validation - Helper can validate its own required fields

Rule: If an inline object mapping has 3+ properties OR requires type casting, extract it to a helper function.

Common Patterns Quick Reference

// Required field validation - use ensureProperties helper
ensureProperties(raw, ['id', 'email', 'status']);
// ✅ Correctly handles 0, "", false - only rejects null/undefined

// Core type conversion - map() handles undefined automatically
id: map(UUID, raw.id)
email: map(Email, raw.email)
url: map(URL, raw.url)
createdAt: map(DateTime, raw.created_at)
dateOfBirth: map(Date, raw.dateOfBirth)  // No ternary needed!

// Enum conversion
status: toEnum(StatusEnum, raw.status)
type: toEnum(TypeEnum, raw.type, (v) => v.toUpperCase())

// Optional field handling - use optional() to normalize null
name: optional(raw.name)           // null→undefined, keeps ""
description: optional(raw.description)  // null→undefined, preserves ""
phoneNumber: optional(raw.phone_number) // null→undefined, keeps "", 0, false
count: optional(raw.count)         // null→undefined, keeps 0
active: optional(raw.active)       // null→undefined, keeps false

// Single nested object - use mapWith()
config: mapWith(toConfig, raw.config)      // mapWith handles null/undefined at boundary
address: mapWith(toAddress, raw.address)

// Array of nested objects - call mapper directly (NO mapWith)
items: raw.items?.map(toSubResource)       // toSubResource assumes valid input
contacts: raw.contacts?.map(toContact)     // toContact returns Contact

// Array of enums
roles: raw.roles?.map((r: any) => toEnum(UserRole, r))

// ❌ NEVER use logical OR - destroys legitimate values
name: raw.name || undefined        // ❌ WRONG - converts "" to undefined
count: raw.count || 0              // ❌ WRONG - default injection

// ❌ NEVER merge different API fields
phoneNumber: raw.mobilePhone || raw.phoneNumber  // ❌ WRONG - fallback between fields

// ✅ Use optional() for optional fields
name: optional(raw.name)        // ✅ CORRECT - null→undefined, keeps ""
count: optional(raw.count)      // ✅ CORRECT - null→undefined, keeps 0

// ✅ Map each field separately
mobilePhone: optional(raw.mobilePhone)  // ✅ CORRECT
phoneNumber: optional(raw.phoneNumber)  // ✅ CORRECT