law-of-demeter

name	law-of-demeter
description	Use when accessing nested object properties. Use when chaining method calls. Use when reaching through objects to get data.

Law of Demeter (Don't Talk to Strangers)

Overview

Only talk to your immediate friends, not strangers.

A method should only call methods on: itself, its parameters, objects it creates, or its direct components. Never reach through an object to access another object's internals.

When to Use

• Accessing nested properties: obj.a.b.c
• Chaining method calls: obj.getA().getB().getC()
• Reaching through objects for data
• Long dot chains in your code

The Iron Rule

NEVER chain through objects. Ask, don't reach.

No exceptions:

• Not for "it's simpler"
• Not for "it's just one chain"
• Not for "the data is there"
• Not for "fewer lines of code"

Detection: The Chain Smell

If you see multiple dots, you're violating LoD:

// ❌ VIOLATION: Reaching through objects
function getEmployeeCity(company: Company, employeeId: string): string {
  return company.employees
    .find(e => e.id === employeeId)
    ?.address.city;  // Reaching into employee, then into address
}

// More violations:
user.getProfile().getAddress().getZipCode();
order.getCustomer().getPaymentMethod().getLast4();

The Correct Pattern: Ask, Don't Reach

Let objects expose what's needed:

// ✅ CORRECT: Ask the object directly
class Employee {
  constructor(
    private name: string,
    private address: Address
  ) {}
  
  getCity(): string {
    return this.address.city;  // Employee asks its own address
  }
}

class Company {
  getEmployeeCities(): Map<string, string> {
    return new Map(
      this.employees.map(e => [e.id, e.getCity()])
    );
  }
  
  getEmployeeCity(employeeId: string): string | undefined {
    return this.employees.find(e => e.id === employeeId)?.getCity();
  }
}

// Usage: Ask company, don't reach through it
const city = company.getEmployeeCity(employeeId);

Why Chains Are Bad

Problem	Impact
Tight coupling	Caller knows internal structure
Fragile code	Structure changes break all callers
Hidden dependencies	Not obvious what's needed
Hard to test	Must mock entire chain
Null danger	Each . is a potential null

Allowed Method Calls

A method m of class C should only call methods on:

1. this - C's own methods
2. Parameters - Objects passed to m
3. Created objects - Objects m creates
4. Components - C's direct instance variables
5. Globals - Accessible global objects (sparingly)

class OrderProcessor {
  constructor(private logger: Logger) {}  // Component

  process(order: Order): Receipt {         // Parameter
    this.validate(order);                  // this
    const receipt = new Receipt(order);    // Created
    this.logger.log('Processed');          // Component
    return receipt;
  }

  // ❌ NOT ALLOWED: order.customer.address.city
  // ✅ ALLOWED: order.getShippingCity()
}

Pressure Resistance Protocol

1. "It's Simpler"

Pressure: "One line with dots is simpler than adding methods"

Response: Simple to write ≠ simple to maintain. Chains create fragile code.

Action: Add methods that expose needed data.

2. "It's Just One Chain"

Pressure: "It's only two dots, not a big deal"

Response: Two dots = two objects you're coupled to. Both can change and break you.

Action: Even short chains should be eliminated.

3. "The Data Is Right There"

Pressure: "The structure has the data, why wrap it?"

Response: Structure changes. Wrapping isolates you from changes.

Action: Ask the owner for the data.

4. "It's Read-Only"

Pressure: "I'm just reading, not modifying"

Response: Reading through chains still couples you to structure.

Action: Ask for what you need.

Red Flags - STOP and Reconsider

If you notice ANY of these, refactor:

• Multiple dots: a.b.c.d
• Chained getters: getA().getB().getC()
• Optional chains: a?.b?.c?.d
• Null checks for nested access
• Structure knowledge in calling code
• Mocking chains in tests

All of these mean: Add a method to ask directly.

Refactoring Chains

// ❌ BEFORE: Chain
const zip = user.getProfile().getAddress().getZipCode();

// ✅ AFTER: Ask
// In User class:
getZipCode(): string {
  return this.profile.getZipCode();
}

// In Profile class:
getZipCode(): string {
  return this.address.zipCode;
}

// Usage:
const zip = user.getZipCode();

Quick Reference

Chain (Bad)	Ask (Good)
company.employees[0].address.city	company.getEmployeeCity(id)
order.customer.paymentMethod.last4	order.getPaymentLast4()
user.profile.settings.theme	user.getTheme()
car.engine.fuel.level	car.getFuelLevel()

Common Rationalizations (All Invalid)

Excuse	Reality
"It's simpler"	Chains are simpler to write, harder to maintain.
"Just one chain"	One chain = multiple couplings.
"Data is right there"	Expose it properly through methods.
"It's read-only"	Reading chains still couples you.
"Fewer lines"	Lines don't matter. Maintainability does.
"It's obvious what it does"	Obvious coupling is still coupling.

The Bottom Line

Ask objects for what you need. Don't reach through them.

When you need data from nested objects: add a method on the owner that returns it. Never chain through multiple objects. Each dot is a dependency you're taking on.