Common Refactoring… — Refactoring Toward Better… | OneNoughtOne

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System Design (LLD)Refactoring Toward Better Design

Refactoring Toward Better Design

LevelAdvanced

Duration75 mins

TopicRefactoring Toward Better Design

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Common Refactoring Patterns

The Refactoring Catalog: Tools for Transformation

Refactoring patterns are named, repeatable transformations that improve code structure while preserving behavior. Having a shared vocabulary of these patterns enables precise communication during code reviews, pair programming sessions, and LLD interviews. Martin Fowler's Refactoring catalog documents over 60 patterns, but most real-world refactoring draws from a smaller set of frequently used transformations. This page covers the essential patterns that address the most common code smells and structural problems. Each pattern follows a predictable structure: Motivation: Why and when to apply this pattern Mechanics: Step-by-step transformation process Example: Before and after code demonstrating the pattern Considerations: Trade-offs and related patterns

What You Will Learn

By the end of this page, you will have internalized the most important refactoring patterns, understand their mechanical application, and be able to identify which pattern addresses which code smell. This knowledge is directly applicable in LLD interviews where demonstrating clean code practices carries significant weight.

Extract Method

Extract Method is the single most important and frequently used refactoring pattern. It takes a code fragment and transforms it into a method whose name explains its purpose. Motivation: Long methods that are difficult to understand Duplicated code fragments across methods Code blocks with clear conceptual purpose Need to make code more testable The fundamental principle: A method should do one thing at one level of abstraction. When you can name a code fragment, you should extract it into a method with that name.

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// BEFORE: Long method with mixed abstraction levels
class OrderProcessor {
    processOrder(order: Order): void {
        // Validate order
        if (!order.items || order.items.length === 0) {
            throw new Error('Order must have at least one item');
        }
        if (!order.customer || !order.customer.id) {
            throw new Error('Order must have a valid customer');
        }
        for (const item of order.items) {
            if (!item.productId || item.quantity <= 0) {
                throw new Error('Invalid order item');
            }
        }
 
        // Calculate total
        let subtotal = 0;
        for (const item of order.items) {
            subtotal += item.price * item.quantity;
        }
        const tax = subtotal * 0.08;
        const shipping = subtotal > 100 ? 0 : 10;
        const total = subtotal + tax + shipping;
 
        // Create invoice
        const invoice = {
            orderId: order.id,
            customerId: order.customer.id,
            items: order.items,
            subtotal,
            tax,
            shipping,
            total,
            createdAt: new Date()
        };
        
        this.invoiceRepository.save(invoice);
        this.emailService.sendInvoice(order.customer.email, invoice);
    }
}
 
// AFTER: Extracted methods with clear responsibilities
class OrderProcessor {
    processOrder(order: Order): void {
        this.validateOrder(order);
        const totals = this.calculateTotals(order);
        const invoice = this.createInvoice(order, totals);
        this.deliverInvoice(order.customer, invoice);
    }
 
    private validateOrder(order: Order): void {
        this.validateOrderHasItems(order);
        this.validateOrderHasCustomer(order);
        this.validateOrderItems(order.items);
    }
 
    private validateOrderHasItems(order: Order): void {
        if (!order.items || order.items.length === 0) {
            throw new Error('Order must have at least one item');
        }
    }
 
    private validateOrderHasCustomer(order: Order): void {
        if (!order.customer || !order.customer.id) {
            throw new Error('Order must have a valid customer');
        }
    }
 
    private validateOrderItems(items: OrderItem[]): void {
        for (const item of items) {
            if (!item.productId || item.quantity <= 0) {
                throw new Error('Invalid order item');
            }
        }
    }
 
    private calculateTotals(order: Order): OrderTotals {
        const subtotal = this.calculateSubtotal(order.items);
        const tax = this.calculateTax(subtotal);
        const shipping = this.calculateShipping(subtotal);
        return { subtotal, tax, shipping, total: subtotal + tax + shipping };
    }
 
    private calculateSubtotal(items: OrderItem[]): number {
        return items.reduce((sum, item) => sum + item.price * item.quantity, 0);
    }
 
    private calculateTax(subtotal: number): number {
        return subtotal * this.TAX_RATE;
    }
 
    private calculateShipping(subtotal: number): number {
        return subtotal > this.FREE_SHIPPING_THRESHOLD ? 0 : this.SHIPPING_COST;
    }
 
    private createInvoice(order: Order, totals: OrderTotals): Invoice {
        return {
            orderId: order.id,
            customerId: order.customer.id,
            items: order.items,
            ...totals,
            createdAt: new Date()
        };
    }
 
    private deliverInvoice(customer: Customer, invoice: Invoice): void {
        this.invoiceRepository.save(invoice);
        this.emailService.sendInvoice(customer.email, invoice);
    }
}

Mechanics of Extract Method

Create a new method with an intention-revealing name. 2. Copy the extracted code into the new method. 3. Identify local variables used in the extracted code. 4. Determine if any variables are modified—these may become return values. 5. Compile to check for scope issues. 6. Replace the original code with a call to the new method. 7. Test to ensure behavior is preserved.

Extract Class

Extract Class creates a new class by moving a subset of fields and methods from an existing class. This is the primary remedy for the Large Class smell and violations of the Single Responsibility Principle. Motivation: A class has grown too large to understand easily A subset of fields and methods form a cohesive group The same data is always used together Multiple responsibilities have accumulated in one class Signs you need Extract Class: Methods that only use a subset of the class's fields Groups of fields that have similar prefixes (e.g., billingAddress, billingCity, billingZip) Data clumps that appear together across methods

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// BEFORE: Class with multiple responsibilities
class Employee {
    private id: string;
    private name: string;
    private email: string;
    private department: string;
    
    // Address-related fields - cohesive group
    private street: string;
    private city: string;
    private state: string;
    private zipCode: string;
    private country: string;
    
    // Phone-related fields - another cohesive group
    private areaCode: string;
    private phoneNumber: string;
    private extension: string;
 
    getFullAddress(): string {
        return `${this.street}
${this.city}, ${this.state} ${this.zipCode}
${this.country}`;
    }
 
    getFormattedPhone(): string {
        let result = `(${this.areaCode}) ${this.phoneNumber}`;
        if (this.extension) {
            result += ` x${this.extension}`;
        }
        return result;
    }
 
    validateAddress(): boolean {
        return this.street?.length > 0 
            && this.city?.length > 0 
            && this.state?.length === 2 
            && this.zipCode?.match(/^\d{5}(-\d{4})?$/) !== null;
    }
 
    // ... many more methods mixing employee logic with address/phone logic
}
 
// AFTER: Cohesive classes with single responsibilities
class Address {
    constructor(
        private street: string,
        private city: string,
        private state: string,
        private zipCode: string,
        private country: string
    ) {}
 
    getFullAddress(): string {
        return `${this.street}
${this.city}, ${this.state} ${this.zipCode}
${this.country}`;
    }
 
    validate(): boolean {
        return this.street?.length > 0 
            && this.city?.length > 0 
            && this.state?.length === 2 
            && this.zipCode?.match(/^\d{5}(-\d{4})?$/) !== null;
    }
 
    // Address can now evolve independently
    getMailingLabel(): string { /* ... */ }
    distanceTo(other: Address): number { /* ... */ }
}
 
class PhoneNumber {
    constructor(
        private areaCode: string,
        private number: string,
        private extension?: string
    ) {}
 
    format(): string {
        let result = `(${this.areaCode}) ${this.number}`;
        if (this.extension) {
            result += ` x${this.extension}`;
        }
        return result;
    }
 
    validate(): boolean {
        return this.areaCode?.match(/^\d{3}$/) !== null
            && this.number?.match(/^\d{3}-\d{4}$/) !== null;
    }
 
    // PhoneNumber can now evolve independently
    isInternational(): boolean { /* ... */ }
}
 
class Employee {
    constructor(
        private id: string,
        private name: string,
        private email: string,
        private department: string,
        private address: Address,
        private phone: PhoneNumber
    ) {}
 
    // Employee is now focused on employee-specific behavior
    getContactInfo(): string {
        return `${this.name}
${this.address.getFullAddress()}
${this.phone.format()}`;
    }
 
    isContactValid(): boolean {
        return this.address.validate() && this.phone.validate();
    }
}

Deciding What to Extract

Look for fields that change together, methods that use the same subset of fields, and data that has meaning independent of the original class. Address and PhoneNumber are meaningful concepts on their own—they don't require Employee to make sense. This semantic independence signals extraction opportunity.

Move Method and Move Field

Move Method and Move Field relocate functionality to the class where it belongs. These patterns address Feature Envy—when a method uses more data from another class than from its own. Motivation: A method repeatedly accesses data from another object A field is used more by other classes than by its own Related functionality is scattered across classes Changes to one concept require changes in multiple classes The guiding principle: Things that change for the same reason should be together. Things that change for different reasons should be apart.

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// BEFORE: Method envies data from another class
class Account {
    private balance: number;
    private overdraftLimit: number;
    private accountType: AccountType;
 
    getBalance(): number { return this.balance; }
    getOverdraftLimit(): number { return this.overdraftLimit; }
    getAccountType(): AccountType { return this.accountType; }
}
 
class BankReportService {
    // This method uses Account data more than its own
    calculateOverdraftFee(account: Account, amount: number): number {
        if (account.getAccountType() === AccountType.PREMIUM) {
            return 0; // Premium accounts have no overdraft fees
        }
 
        const overdraftAmount = amount - account.getBalance();
        if (overdraftAmount <= 0) {
            return 0; // Not actually overdrawing
        }
 
        if (overdraftAmount > account.getOverdraftLimit()) {
            return 35.00; // Over limit: maximum fee
        }
 
        return overdraftAmount * 0.05; // 5% of overdraft amount
    }
}
 
// AFTER: Method moved to where the data lives
class Account {
    private balance: number;
    private overdraftLimit: number;
    private accountType: AccountType;
 
    // Method now lives with its data
    calculateOverdraftFee(withdrawalAmount: number): number {
        if (this.accountType === AccountType.PREMIUM) {
            return 0;
        }
 
        const overdraftAmount = withdrawalAmount - this.balance;
        if (overdraftAmount <= 0) {
            return 0;
        }
 
        if (overdraftAmount > this.overdraftLimit) {
            return Account.MAXIMUM_OVERDRAFT_FEE;
        }
 
        return overdraftAmount * Account.OVERDRAFT_FEE_RATE;
    }
 
    private static readonly MAXIMUM_OVERDRAFT_FEE = 35.00;
    private static readonly OVERDRAFT_FEE_RATE = 0.05;
}
 
class BankReportService {
    // Caller becomes simpler, delegating to Account
    generateOverdraftReport(account: Account, amount: number): Report {
        const fee = account.calculateOverdraftFee(amount);
        // ... reporting logic that is actually this service's concern
    }
}

Move Field mechanics: Move Field follows similar logic but for data rather than behavior: If the field is public, encapsulate it first (use getters/setters) Create the field in the target class with appropriate access Add a reference from the source class to the target class (if not already present) Change all references in the source class to use the target class's field Remove the field from the source class Compile and test Fields should live where their methods live. If you've moved methods to a class, the fields they use often should follow.

The Tell, Don't Ask Principle

Move Method often transforms code from 'Ask' style (querying data then computing) to 'Tell' style (telling an object to do something). The latter typically results in better encapsulation and clearer responsibilities.

Rename Method, Variable, and Class

Renaming is often underestimated as a refactoring technique, but it's one of the most powerful. Good names reveal intent, reduce the need for comments, and make code self-documenting. Motivation: Names no longer reflect what code does (after evolution) Names are too vague, cryptic, or misleading Names use inconsistent conventions Names expose implementation instead of intent The core principle: Code is read far more often than it's written. Investing in good names pays dividends on every future read.

Before and After Naming
Before	After	Why Better
`data`	`userPreferences`	Reveals what data represents
`process()`	`validateAndEnrichOrder()`	Describes what processing does
`flag`	`isEligibleForDiscount`	Boolean reads naturally in conditionals
`tmp`	`remainingBalance`	Domain term instead of implementation detail
`Manager`	`OrderRepository`	Specific responsibility instead of vague role
`doIt()`	`submitPayment()`	Action is clear from name
`list`	`pendingOrders`	Contents and status are visible
`x, y`	`latitude, longitude`	Domain semantics, not arbitrary labels

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// BEFORE: Cryptic names require mental translation
function calc(a: number[], b: number): number {
    let r = 0;
    for (let i = 0; i < a.length; i++) {
        if (a[i] > b) {
            r += a[i];
        }
    }
    return r;
}
 
// AFTER: Names reveal intent
function sumValuesAboveThreshold(values: number[], threshold: number): number {
    let total = 0;
    for (const value of values) {
        if (value > threshold) {
            total += value;
        }
    }
    return total;
}
 
// EVEN BETTER: Using modern constructs that name the pattern
function sumValuesAboveThreshold(values: number[], threshold: number): number {
    return values
        .filter(value => value > threshold)
        .reduce((sum, value) => sum + value, 0);
}
 
// CLASS RENAMING EXAMPLE
// BEFORE: What does this class do?
class Manager {
    handle(input: unknown): unknown { /* ... */ }
}
 
// AFTER: Clear responsibility
class OrderValidationService {
    validateOrder(order: Order): ValidationResult { /* ... */ }
}

Naming Guidelines

Methods: Use verb phrases that describe the action (validate, calculate, fetch, send). Classes: Use nouns that describe the abstraction (Order, Customer, PaymentGateway). Variables: Use nouns describing content (pendingOrders, currentUser). Booleans: Use 'is/has/can/should' prefixes (isValid, hasPermission, canRetry).

Replace Conditional with Polymorphism

Replace Conditional with Polymorphism transforms complex switch statements or type-checking conditionals into polymorphic method calls. This is one of the most powerful object-oriented refactorings. Motivation: Switch statements that check the same object type in multiple places If-else chains testing instance types (instanceof/typeof) Adding new types requires modifying existing code (violating Open-Closed Principle) Type-specific behavior is scattered across the codebase The principle: When behavior varies by type, let the type itself define the behavior through polymorphism.

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// BEFORE: Switch statement on type code
enum EmployeeType {
    ENGINEER = 'ENGINEER',
    MANAGER = 'MANAGER',
    SALESPERSON = 'SALESPERSON'
}
 
class Employee {
    type: EmployeeType;
    baseSalary: number;
    commissionRate?: number;
    teamSize?: number;
 
    calculatePay(): number {
        switch (this.type) {
            case EmployeeType.ENGINEER:
                return this.baseSalary;
            
            case EmployeeType.MANAGER:
                return this.baseSalary + (this.teamSize! * 100);
            
            case EmployeeType.SALESPERSON:
                return this.baseSalary * (1 + this.commissionRate!);
            
            default:
                throw new Error(`Unknown employee type: ${this.type}`);
        }
    }
 
    getTitle(): string {
        switch (this.type) {
            case EmployeeType.ENGINEER:
                return 'Software Engineer';
            case EmployeeType.MANAGER:
                return 'Engineering Manager';
            case EmployeeType.SALESPERSON:
                return 'Sales Representative';
            default:
                throw new Error(`Unknown employee type: ${this.type}`);
        }
    }
 
    // Every new employee type requires changes to EVERY switch statement
}
 
// AFTER: Polymorphic classes
abstract class Employee {
    constructor(protected baseSalary: number) {}
 
    abstract calculatePay(): number;
    abstract getTitle(): string;
}
 
class Engineer extends Employee {
    calculatePay(): number {
        return this.baseSalary;
    }
 
    getTitle(): string {
        return 'Software Engineer';
    }
}
 
class Manager extends Employee {
    constructor(baseSalary: number, private teamSize: number) {
        super(baseSalary);
    }
 
    calculatePay(): number {
        return this.baseSalary + this.teamSize * Manager.TEAM_BONUS_PER_MEMBER;
    }
 
    getTitle(): string {
        return 'Engineering Manager';
    }
 
    private static readonly TEAM_BONUS_PER_MEMBER = 100;
}
 
class Salesperson extends Employee {
    constructor(baseSalary: number, private commissionRate: number) {
        super(baseSalary);
    }
 
    calculatePay(): number {
        return this.baseSalary * (1 + this.commissionRate);
    }
 
    getTitle(): string {
        return 'Sales Representative';
    }
}
 
// Adding a new employee type: just add a new class
class Intern extends Employee {
    calculatePay(): number {
        return this.baseSalary * 0.5; // Interns get half
    }
 
    getTitle(): string {
        return 'Engineering Intern';
    }
}
 
// Factory creates the right type
class EmployeeFactory {
    static create(type: EmployeeType, ...args: any[]): Employee {
        switch (type) {
            case EmployeeType.ENGINEER:
                return new Engineer(args[0]);
            case EmployeeType.MANAGER:
                return new Manager(args[0], args[1]);
            case EmployeeType.SALESPERSON:
                return new Salesperson(args[0], args[1]);
        }
    }
}

When to Keep the Switch

Not all switch statements should become polymorphism. Keep conditionals when: (1) the behavior doesn't justify a class hierarchy, (2) the types are external and can't be extended, (3) there's only one conditional location, or (4) the domain is truly procedural. Apply polymorphism when the same conditional appears in multiple places.

Introduce Parameter Object

Introduce Parameter Object replaces a long parameter list with a single object that encapsulates those parameters. This pattern reduces parameter coupling and often reveals new abstractions. Motivation: Methods with 4+ parameters are hard to understand and call correctly The same group of parameters appears in multiple method signatures Related parameters change together Parameter order is error-prone (positional arguments) The insight: If the same parameters always travel together, they probably represent a concept that deserves its own name.

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// BEFORE: Long parameter lists repeated across methods
class ReportGenerator {
    generateSalesReport(
        startDate: Date,
        endDate: Date,
        region: string,
        includeReturns: boolean,
        groupByCategory: boolean,
        format: ReportFormat
    ): Report {
        // ...
    }
 
    generateInventoryReport(
        startDate: Date,
        endDate: Date,
        region: string,
        includeInTransit: boolean,
        groupByWarehouse: boolean,
        format: ReportFormat
    ): Report {
        // ...
    }
 
    // What does a call look like?
    // generator.generateSalesReport(start, end, 'NA', true, false, 'PDF')
    // Which boolean is which? Easy to get wrong.
}
 
// AFTER: Parameter object encapsulates related data
interface DateRange {
    startDate: Date;
    endDate: Date;
}
 
interface SalesReportOptions {
    dateRange: DateRange;
    region: string;
    includeReturns: boolean;
    groupByCategory: boolean;
    format: ReportFormat;
}
 
interface InventoryReportOptions {
    dateRange: DateRange;
    region: string;
    includeInTransit: boolean;
    groupByWarehouse: boolean;
    format: ReportFormat;
}
 
class ReportGenerator {
    generateSalesReport(options: SalesReportOptions): Report {
        // Clearer access: options.dateRange.startDate
        // Easy to add new options without changing signature
    }
 
    generateInventoryReport(options: InventoryReportOptions): Report {
        // ...
    }
}
 
// Call sites are now self-documenting
const report = generator.generateSalesReport({
    dateRange: { startDate: lastMonth, endDate: today },
    region: 'NA',
    includeReturns: true,
    groupByCategory: false,
    format: 'PDF'
});
 
// DateRange can grow behavior of its own
class DateRange {
    constructor(
        public readonly startDate: Date,
        public readonly endDate: Date
    ) {
        this.validate();
    }
 
    private validate(): void {
        if (this.startDate > this.endDate) {
            throw new Error('Start date must be before end date');
        }
    }
 
    getDurationInDays(): number {
        const diffTime = this.endDate.getTime() - this.startDate.getTime();
        return Math.ceil(diffTime / (1000 * 60 * 60 * 24));
    }
 
    contains(date: Date): boolean {
        return date >= this.startDate && date <= this.endDate;
    }
 
    overlaps(other: DateRange): boolean {
        return this.startDate <= other.endDate && other.startDate <= this.endDate;
    }
}

Parameter Objects as Domain Discovery

Often, introducing a parameter object reveals a missing domain concept. 'DateRange' is not just a convenience—it's a genuine business concept that deserves its own abstraction. The refactoring surfaces implicit concepts and makes them explicit.

Extract Interface

Extract Interface creates a new interface from the existing public methods of a class. This enables polymorphism without inheritance and supports dependency inversion. Motivation: Need to support multiple implementations of the same concept Want to mock a class for testing Need to break a dependency cycle Class exposes too many methods; want to narrow the contract Preparing coding for the Dependency Inversion Principle

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// BEFORE: Concrete class makes testing difficult
class PaymentGateway {
    async chargeCard(
        cardNumber: string,
        amount: number,
        currency: string
    ): Promise<PaymentResult> {
        // Real HTTP calls to payment provider
        const response = await fetch('https://payment.provider.com/charge', {
            method: 'POST',
            body: JSON.stringify({ cardNumber, amount, currency }),
            headers: { 'Authorization': `Bearer ${this.apiKey}` }
        });
        return response.json();
    }
 
    async refund(transactionId: string, amount: number): Promise<RefundResult> {
        // Real HTTP calls to payment provider
    }
}
 
class OrderService {
    constructor(private gateway: PaymentGateway) {} // Hard dependency
 
    async completeOrder(order: Order): Promise<void> {
        // This is hard to test - always makes real API calls
        const result = await this.gateway.chargeCard(
            order.paymentCardNumber,
            order.total,
            'USD'
        );
        // ...
    }
}
 
// AFTER: Interface allows substitution
interface PaymentProcessor {
    chargeCard(
        cardNumber: string,
        amount: number,
        currency: string
    ): Promise<PaymentResult>;
    
    refund(transactionId: string, amount: number): Promise<RefundResult>;
}
 
// Real implementation
class StripePaymentGateway implements PaymentProcessor {
    constructor(private apiKey: string) {}
 
    async chargeCard(
        cardNumber: string,
        amount: number,
        currency: string
    ): Promise<PaymentResult> {
        // Real Stripe API calls
    }
 
    async refund(transactionId: string, amount: number): Promise<RefundResult> {
        // Real Stripe API calls
    }
}
 
// Test implementation
class FakePaymentGateway implements PaymentProcessor {
    private charges: { cardNumber: string; amount: number }[] = [];
 
    async chargeCard(
        cardNumber: string,
        amount: number,
        currency: string
    ): Promise<PaymentResult> {
        this.charges.push({ cardNumber, amount });
        return { success: true, transactionId: 'fake-txn-123' };
    }
 
    async refund(transactionId: string, amount: number): Promise<RefundResult> {
        return { success: true };
    }
 
    // Test helper methods
    getCharges() { return this.charges; }
    simulateFailure() { /* ... */ }
}
 
class OrderService {
    constructor(private paymentProcessor: PaymentProcessor) {} // Depends on interface
 
    async completeOrder(order: Order): Promise<void> {
        const result = await this.paymentProcessor.chargeCard(
            order.paymentCardNumber,
            order.total,
            'USD'
        );
        // Now testable without real API calls
    }
}
 
// In tests:
const fakeGateway = new FakePaymentGateway();
const orderService = new OrderService(fakeGateway);
 
await orderService.completeOrder(testOrder);
 
expect(fakeGateway.getCharges()).toHaveLength(1);
expect(fakeGateway.getCharges()[0].amount).toBe(testOrder.total);

Interface Segregation Alignment

When extracting interfaces, include only the methods that clients actually need. If different clients use different subsets of a class's methods, create separate, focused interfaces. This aligns with the Interface Segregation Principle.

System Design (LLD)Refactoring Toward Better Design

Refactoring Toward Better Design

LevelAdvanced

Duration75 mins

TopicRefactoring Toward Better Design

2 / 4

Common Refactoring Patterns

The Refactoring Catalog: Tools for Transformation

Refactoring patterns are named, repeatable transformations that improve code structure while preserving behavior. Having a shared vocabulary of these patterns enables precise communication during code reviews, pair programming sessions, and LLD interviews. Martin Fowler's Refactoring catalog documents over 60 patterns, but most real-world refactoring draws from a smaller set of frequently used transformations. This page covers the essential patterns that address the most common code smells and structural problems. Each pattern follows a predictable structure: Motivation: Why and when to apply this pattern Mechanics: Step-by-step transformation process Example: Before and after code demonstrating the pattern Considerations: Trade-offs and related patterns

What You Will Learn

By the end of this page, you will have internalized the most important refactoring patterns, understand their mechanical application, and be able to identify which pattern addresses which code smell. This knowledge is directly applicable in LLD interviews where demonstrating clean code practices carries significant weight.

Extract Method

Extract Method is the single most important and frequently used refactoring pattern. It takes a code fragment and transforms it into a method whose name explains its purpose. Motivation: Long methods that are difficult to understand Duplicated code fragments across methods Code blocks with clear conceptual purpose Need to make code more testable The fundamental principle: A method should do one thing at one level of abstraction. When you can name a code fragment, you should extract it into a method with that name.

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// BEFORE: Long method with mixed abstraction levels
class OrderProcessor {
    processOrder(order: Order): void {
        // Validate order
        if (!order.items || order.items.length === 0) {
            throw new Error('Order must have at least one item');
        }
        if (!order.customer || !order.customer.id) {
            throw new Error('Order must have a valid customer');
        }
        for (const item of order.items) {
            if (!item.productId || item.quantity <= 0) {
                throw new Error('Invalid order item');
            }
        }
 
        // Calculate total
        let subtotal = 0;
        for (const item of order.items) {
            subtotal += item.price * item.quantity;
        }
        const tax = subtotal * 0.08;
        const shipping = subtotal > 100 ? 0 : 10;
        const total = subtotal + tax + shipping;
 
        // Create invoice
        const invoice = {
            orderId: order.id,
            customerId: order.customer.id,
            items: order.items,
            subtotal,
            tax,
            shipping,
            total,
            createdAt: new Date()
        };
        
        this.invoiceRepository.save(invoice);
        this.emailService.sendInvoice(order.customer.email, invoice);
    }
}
 
// AFTER: Extracted methods with clear responsibilities
class OrderProcessor {
    processOrder(order: Order): void {
        this.validateOrder(order);
        const totals = this.calculateTotals(order);
        const invoice = this.createInvoice(order, totals);
        this.deliverInvoice(order.customer, invoice);
    }
 
    private validateOrder(order: Order): void {
        this.validateOrderHasItems(order);
        this.validateOrderHasCustomer(order);
        this.validateOrderItems(order.items);
    }
 
    private validateOrderHasItems(order: Order): void {
        if (!order.items || order.items.length === 0) {
            throw new Error('Order must have at least one item');
        }
    }
 
    private validateOrderHasCustomer(order: Order): void {
        if (!order.customer || !order.customer.id) {
            throw new Error('Order must have a valid customer');
        }
    }
 
    private validateOrderItems(items: OrderItem[]): void {
        for (const item of items) {
            if (!item.productId || item.quantity <= 0) {
                throw new Error('Invalid order item');
            }
        }
    }
 
    private calculateTotals(order: Order): OrderTotals {
        const subtotal = this.calculateSubtotal(order.items);
        const tax = this.calculateTax(subtotal);
        const shipping = this.calculateShipping(subtotal);
        return { subtotal, tax, shipping, total: subtotal + tax + shipping };
    }
 
    private calculateSubtotal(items: OrderItem[]): number {
        return items.reduce((sum, item) => sum + item.price * item.quantity, 0);
    }
 
    private calculateTax(subtotal: number): number {
        return subtotal * this.TAX_RATE;
    }
 
    private calculateShipping(subtotal: number): number {
        return subtotal > this.FREE_SHIPPING_THRESHOLD ? 0 : this.SHIPPING_COST;
    }
 
    private createInvoice(order: Order, totals: OrderTotals): Invoice {
        return {
            orderId: order.id,
            customerId: order.customer.id,
            items: order.items,
            ...totals,
            createdAt: new Date()
        };
    }
 
    private deliverInvoice(customer: Customer, invoice: Invoice): void {
        this.invoiceRepository.save(invoice);
        this.emailService.sendInvoice(customer.email, invoice);
    }
}

Mechanics of Extract Method

Create a new method with an intention-revealing name. 2. Copy the extracted code into the new method. 3. Identify local variables used in the extracted code. 4. Determine if any variables are modified—these may become return values. 5. Compile to check for scope issues. 6. Replace the original code with a call to the new method. 7. Test to ensure behavior is preserved.

Extract Class

Extract Class creates a new class by moving a subset of fields and methods from an existing class. This is the primary remedy for the Large Class smell and violations of the Single Responsibility Principle. Motivation: A class has grown too large to understand easily A subset of fields and methods form a cohesive group The same data is always used together Multiple responsibilities have accumulated in one class Signs you need Extract Class: Methods that only use a subset of the class's fields Groups of fields that have similar prefixes (e.g., billingAddress, billingCity, billingZip) Data clumps that appear together across methods

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// BEFORE: Class with multiple responsibilities
class Employee {
    private id: string;
    private name: string;
    private email: string;
    private department: string;
    
    // Address-related fields - cohesive group
    private street: string;
    private city: string;
    private state: string;
    private zipCode: string;
    private country: string;
    
    // Phone-related fields - another cohesive group
    private areaCode: string;
    private phoneNumber: string;
    private extension: string;
 
    getFullAddress(): string {
        return `${this.street}
${this.city}, ${this.state} ${this.zipCode}
${this.country}`;
    }
 
    getFormattedPhone(): string {
        let result = `(${this.areaCode}) ${this.phoneNumber}`;
        if (this.extension) {
            result += ` x${this.extension}`;
        }
        return result;
    }
 
    validateAddress(): boolean {
        return this.street?.length > 0 
            && this.city?.length > 0 
            && this.state?.length === 2 
            && this.zipCode?.match(/^\d{5}(-\d{4})?$/) !== null;
    }
 
    // ... many more methods mixing employee logic with address/phone logic
}
 
// AFTER: Cohesive classes with single responsibilities
class Address {
    constructor(
        private street: string,
        private city: string,
        private state: string,
        private zipCode: string,
        private country: string
    ) {}
 
    getFullAddress(): string {
        return `${this.street}
${this.city}, ${this.state} ${this.zipCode}
${this.country}`;
    }
 
    validate(): boolean {
        return this.street?.length > 0 
            && this.city?.length > 0 
            && this.state?.length === 2 
            && this.zipCode?.match(/^\d{5}(-\d{4})?$/) !== null;
    }
 
    // Address can now evolve independently
    getMailingLabel(): string { /* ... */ }
    distanceTo(other: Address): number { /* ... */ }
}
 
class PhoneNumber {
    constructor(
        private areaCode: string,
        private number: string,
        private extension?: string
    ) {}
 
    format(): string {
        let result = `(${this.areaCode}) ${this.number}`;
        if (this.extension) {
            result += ` x${this.extension}`;
        }
        return result;
    }
 
    validate(): boolean {
        return this.areaCode?.match(/^\d{3}$/) !== null
            && this.number?.match(/^\d{3}-\d{4}$/) !== null;
    }
 
    // PhoneNumber can now evolve independently
    isInternational(): boolean { /* ... */ }
}
 
class Employee {
    constructor(
        private id: string,
        private name: string,
        private email: string,
        private department: string,
        private address: Address,
        private phone: PhoneNumber
    ) {}
 
    // Employee is now focused on employee-specific behavior
    getContactInfo(): string {
        return `${this.name}
${this.address.getFullAddress()}
${this.phone.format()}`;
    }
 
    isContactValid(): boolean {
        return this.address.validate() && this.phone.validate();
    }
}

Deciding What to Extract

Look for fields that change together, methods that use the same subset of fields, and data that has meaning independent of the original class. Address and PhoneNumber are meaningful concepts on their own—they don't require Employee to make sense. This semantic independence signals extraction opportunity.

Move Method and Move Field

Move Method and Move Field relocate functionality to the class where it belongs. These patterns address Feature Envy—when a method uses more data from another class than from its own. Motivation: A method repeatedly accesses data from another object A field is used more by other classes than by its own Related functionality is scattered across classes Changes to one concept require changes in multiple classes The guiding principle: Things that change for the same reason should be together. Things that change for different reasons should be apart.

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// BEFORE: Method envies data from another class
class Account {
    private balance: number;
    private overdraftLimit: number;
    private accountType: AccountType;
 
    getBalance(): number { return this.balance; }
    getOverdraftLimit(): number { return this.overdraftLimit; }
    getAccountType(): AccountType { return this.accountType; }
}
 
class BankReportService {
    // This method uses Account data more than its own
    calculateOverdraftFee(account: Account, amount: number): number {
        if (account.getAccountType() === AccountType.PREMIUM) {
            return 0; // Premium accounts have no overdraft fees
        }
 
        const overdraftAmount = amount - account.getBalance();
        if (overdraftAmount <= 0) {
            return 0; // Not actually overdrawing
        }
 
        if (overdraftAmount > account.getOverdraftLimit()) {
            return 35.00; // Over limit: maximum fee
        }
 
        return overdraftAmount * 0.05; // 5% of overdraft amount
    }
}
 
// AFTER: Method moved to where the data lives
class Account {
    private balance: number;
    private overdraftLimit: number;
    private accountType: AccountType;
 
    // Method now lives with its data
    calculateOverdraftFee(withdrawalAmount: number): number {
        if (this.accountType === AccountType.PREMIUM) {
            return 0;
        }
 
        const overdraftAmount = withdrawalAmount - this.balance;
        if (overdraftAmount <= 0) {
            return 0;
        }
 
        if (overdraftAmount > this.overdraftLimit) {
            return Account.MAXIMUM_OVERDRAFT_FEE;
        }
 
        return overdraftAmount * Account.OVERDRAFT_FEE_RATE;
    }
 
    private static readonly MAXIMUM_OVERDRAFT_FEE = 35.00;
    private static readonly OVERDRAFT_FEE_RATE = 0.05;
}
 
class BankReportService {
    // Caller becomes simpler, delegating to Account
    generateOverdraftReport(account: Account, amount: number): Report {
        const fee = account.calculateOverdraftFee(amount);
        // ... reporting logic that is actually this service's concern
    }
}

Move Field mechanics: Move Field follows similar logic but for data rather than behavior: If the field is public, encapsulate it first (use getters/setters) Create the field in the target class with appropriate access Add a reference from the source class to the target class (if not already present) Change all references in the source class to use the target class's field Remove the field from the source class Compile and test Fields should live where their methods live. If you've moved methods to a class, the fields they use often should follow.

The Tell, Don't Ask Principle

Move Method often transforms code from 'Ask' style (querying data then computing) to 'Tell' style (telling an object to do something). The latter typically results in better encapsulation and clearer responsibilities.

Rename Method, Variable, and Class

Renaming is often underestimated as a refactoring technique, but it's one of the most powerful. Good names reveal intent, reduce the need for comments, and make code self-documenting. Motivation: Names no longer reflect what code does (after evolution) Names are too vague, cryptic, or misleading Names use inconsistent conventions Names expose implementation instead of intent The core principle: Code is read far more often than it's written. Investing in good names pays dividends on every future read.

Before and After Naming
Before	After	Why Better
`data`	`userPreferences`	Reveals what data represents
`process()`	`validateAndEnrichOrder()`	Describes what processing does
`flag`	`isEligibleForDiscount`	Boolean reads naturally in conditionals
`tmp`	`remainingBalance`	Domain term instead of implementation detail
`Manager`	`OrderRepository`	Specific responsibility instead of vague role
`doIt()`	`submitPayment()`	Action is clear from name
`list`	`pendingOrders`	Contents and status are visible
`x, y`	`latitude, longitude`	Domain semantics, not arbitrary labels

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// BEFORE: Cryptic names require mental translation
function calc(a: number[], b: number): number {
    let r = 0;
    for (let i = 0; i < a.length; i++) {
        if (a[i] > b) {
            r += a[i];
        }
    }
    return r;
}
 
// AFTER: Names reveal intent
function sumValuesAboveThreshold(values: number[], threshold: number): number {
    let total = 0;
    for (const value of values) {
        if (value > threshold) {
            total += value;
        }
    }
    return total;
}
 
// EVEN BETTER: Using modern constructs that name the pattern
function sumValuesAboveThreshold(values: number[], threshold: number): number {
    return values
        .filter(value => value > threshold)
        .reduce((sum, value) => sum + value, 0);
}
 
// CLASS RENAMING EXAMPLE
// BEFORE: What does this class do?
class Manager {
    handle(input: unknown): unknown { /* ... */ }
}
 
// AFTER: Clear responsibility
class OrderValidationService {
    validateOrder(order: Order): ValidationResult { /* ... */ }
}

Naming Guidelines

Methods: Use verb phrases that describe the action (validate, calculate, fetch, send). Classes: Use nouns that describe the abstraction (Order, Customer, PaymentGateway). Variables: Use nouns describing content (pendingOrders, currentUser). Booleans: Use 'is/has/can/should' prefixes (isValid, hasPermission, canRetry).

Replace Conditional with Polymorphism

Replace Conditional with Polymorphism transforms complex switch statements or type-checking conditionals into polymorphic method calls. This is one of the most powerful object-oriented refactorings. Motivation: Switch statements that check the same object type in multiple places If-else chains testing instance types (instanceof/typeof) Adding new types requires modifying existing code (violating Open-Closed Principle) Type-specific behavior is scattered across the codebase The principle: When behavior varies by type, let the type itself define the behavior through polymorphism.

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// BEFORE: Switch statement on type code
enum EmployeeType {
    ENGINEER = 'ENGINEER',
    MANAGER = 'MANAGER',
    SALESPERSON = 'SALESPERSON'
}
 
class Employee {
    type: EmployeeType;
    baseSalary: number;
    commissionRate?: number;
    teamSize?: number;
 
    calculatePay(): number {
        switch (this.type) {
            case EmployeeType.ENGINEER:
                return this.baseSalary;
            
            case EmployeeType.MANAGER:
                return this.baseSalary + (this.teamSize! * 100);
            
            case EmployeeType.SALESPERSON:
                return this.baseSalary * (1 + this.commissionRate!);
            
            default:
                throw new Error(`Unknown employee type: ${this.type}`);
        }
    }
 
    getTitle(): string {
        switch (this.type) {
            case EmployeeType.ENGINEER:
                return 'Software Engineer';
            case EmployeeType.MANAGER:
                return 'Engineering Manager';
            case EmployeeType.SALESPERSON:
                return 'Sales Representative';
            default:
                throw new Error(`Unknown employee type: ${this.type}`);
        }
    }
 
    // Every new employee type requires changes to EVERY switch statement
}
 
// AFTER: Polymorphic classes
abstract class Employee {
    constructor(protected baseSalary: number) {}
 
    abstract calculatePay(): number;
    abstract getTitle(): string;
}
 
class Engineer extends Employee {
    calculatePay(): number {
        return this.baseSalary;
    }
 
    getTitle(): string {
        return 'Software Engineer';
    }
}
 
class Manager extends Employee {
    constructor(baseSalary: number, private teamSize: number) {
        super(baseSalary);
    }
 
    calculatePay(): number {
        return this.baseSalary + this.teamSize * Manager.TEAM_BONUS_PER_MEMBER;
    }
 
    getTitle(): string {
        return 'Engineering Manager';
    }
 
    private static readonly TEAM_BONUS_PER_MEMBER = 100;
}
 
class Salesperson extends Employee {
    constructor(baseSalary: number, private commissionRate: number) {
        super(baseSalary);
    }
 
    calculatePay(): number {
        return this.baseSalary * (1 + this.commissionRate);
    }
 
    getTitle(): string {
        return 'Sales Representative';
    }
}
 
// Adding a new employee type: just add a new class
class Intern extends Employee {
    calculatePay(): number {
        return this.baseSalary * 0.5; // Interns get half
    }
 
    getTitle(): string {
        return 'Engineering Intern';
    }
}
 
// Factory creates the right type
class EmployeeFactory {
    static create(type: EmployeeType, ...args: any[]): Employee {
        switch (type) {
            case EmployeeType.ENGINEER:
                return new Engineer(args[0]);
            case EmployeeType.MANAGER:
                return new Manager(args[0], args[1]);
            case EmployeeType.SALESPERSON:
                return new Salesperson(args[0], args[1]);
        }
    }
}

When to Keep the Switch

Not all switch statements should become polymorphism. Keep conditionals when: (1) the behavior doesn't justify a class hierarchy, (2) the types are external and can't be extended, (3) there's only one conditional location, or (4) the domain is truly procedural. Apply polymorphism when the same conditional appears in multiple places.

Introduce Parameter Object

Introduce Parameter Object replaces a long parameter list with a single object that encapsulates those parameters. This pattern reduces parameter coupling and often reveals new abstractions. Motivation: Methods with 4+ parameters are hard to understand and call correctly The same group of parameters appears in multiple method signatures Related parameters change together Parameter order is error-prone (positional arguments) The insight: If the same parameters always travel together, they probably represent a concept that deserves its own name.

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// BEFORE: Long parameter lists repeated across methods
class ReportGenerator {
    generateSalesReport(
        startDate: Date,
        endDate: Date,
        region: string,
        includeReturns: boolean,
        groupByCategory: boolean,
        format: ReportFormat
    ): Report {
        // ...
    }
 
    generateInventoryReport(
        startDate: Date,
        endDate: Date,
        region: string,
        includeInTransit: boolean,
        groupByWarehouse: boolean,
        format: ReportFormat
    ): Report {
        // ...
    }
 
    // What does a call look like?
    // generator.generateSalesReport(start, end, 'NA', true, false, 'PDF')
    // Which boolean is which? Easy to get wrong.
}
 
// AFTER: Parameter object encapsulates related data
interface DateRange {
    startDate: Date;
    endDate: Date;
}
 
interface SalesReportOptions {
    dateRange: DateRange;
    region: string;
    includeReturns: boolean;
    groupByCategory: boolean;
    format: ReportFormat;
}
 
interface InventoryReportOptions {
    dateRange: DateRange;
    region: string;
    includeInTransit: boolean;
    groupByWarehouse: boolean;
    format: ReportFormat;
}
 
class ReportGenerator {
    generateSalesReport(options: SalesReportOptions): Report {
        // Clearer access: options.dateRange.startDate
        // Easy to add new options without changing signature
    }
 
    generateInventoryReport(options: InventoryReportOptions): Report {
        // ...
    }
}
 
// Call sites are now self-documenting
const report = generator.generateSalesReport({
    dateRange: { startDate: lastMonth, endDate: today },
    region: 'NA',
    includeReturns: true,
    groupByCategory: false,
    format: 'PDF'
});
 
// DateRange can grow behavior of its own
class DateRange {
    constructor(
        public readonly startDate: Date,
        public readonly endDate: Date
    ) {
        this.validate();
    }
 
    private validate(): void {
        if (this.startDate > this.endDate) {
            throw new Error('Start date must be before end date');
        }
    }
 
    getDurationInDays(): number {
        const diffTime = this.endDate.getTime() - this.startDate.getTime();
        return Math.ceil(diffTime / (1000 * 60 * 60 * 24));
    }
 
    contains(date: Date): boolean {
        return date >= this.startDate && date <= this.endDate;
    }
 
    overlaps(other: DateRange): boolean {
        return this.startDate <= other.endDate && other.startDate <= this.endDate;
    }
}

Parameter Objects as Domain Discovery

Often, introducing a parameter object reveals a missing domain concept. 'DateRange' is not just a convenience—it's a genuine business concept that deserves its own abstraction. The refactoring surfaces implicit concepts and makes them explicit.

Extract Interface

Extract Interface creates a new interface from the existing public methods of a class. This enables polymorphism without inheritance and supports dependency inversion. Motivation: Need to support multiple implementations of the same concept Want to mock a class for testing Need to break a dependency cycle Class exposes too many methods; want to narrow the contract Preparing coding for the Dependency Inversion Principle

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// BEFORE: Concrete class makes testing difficult
class PaymentGateway {
    async chargeCard(
        cardNumber: string,
        amount: number,
        currency: string
    ): Promise<PaymentResult> {
        // Real HTTP calls to payment provider
        const response = await fetch('https://payment.provider.com/charge', {
            method: 'POST',
            body: JSON.stringify({ cardNumber, amount, currency }),
            headers: { 'Authorization': `Bearer ${this.apiKey}` }
        });
        return response.json();
    }
 
    async refund(transactionId: string, amount: number): Promise<RefundResult> {
        // Real HTTP calls to payment provider
    }
}
 
class OrderService {
    constructor(private gateway: PaymentGateway) {} // Hard dependency
 
    async completeOrder(order: Order): Promise<void> {
        // This is hard to test - always makes real API calls
        const result = await this.gateway.chargeCard(
            order.paymentCardNumber,
            order.total,
            'USD'
        );
        // ...
    }
}
 
// AFTER: Interface allows substitution
interface PaymentProcessor {
    chargeCard(
        cardNumber: string,
        amount: number,
        currency: string
    ): Promise<PaymentResult>;
    
    refund(transactionId: string, amount: number): Promise<RefundResult>;
}
 
// Real implementation
class StripePaymentGateway implements PaymentProcessor {
    constructor(private apiKey: string) {}
 
    async chargeCard(
        cardNumber: string,
        amount: number,
        currency: string
    ): Promise<PaymentResult> {
        // Real Stripe API calls
    }
 
    async refund(transactionId: string, amount: number): Promise<RefundResult> {
        // Real Stripe API calls
    }
}
 
// Test implementation
class FakePaymentGateway implements PaymentProcessor {
    private charges: { cardNumber: string; amount: number }[] = [];
 
    async chargeCard(
        cardNumber: string,
        amount: number,
        currency: string
    ): Promise<PaymentResult> {
        this.charges.push({ cardNumber, amount });
        return { success: true, transactionId: 'fake-txn-123' };
    }
 
    async refund(transactionId: string, amount: number): Promise<RefundResult> {
        return { success: true };
    }
 
    // Test helper methods
    getCharges() { return this.charges; }
    simulateFailure() { /* ... */ }
}
 
class OrderService {
    constructor(private paymentProcessor: PaymentProcessor) {} // Depends on interface
 
    async completeOrder(order: Order): Promise<void> {
        const result = await this.paymentProcessor.chargeCard(
            order.paymentCardNumber,
            order.total,
            'USD'
        );
        // Now testable without real API calls
    }
}
 
// In tests:
const fakeGateway = new FakePaymentGateway();
const orderService = new OrderService(fakeGateway);
 
await orderService.completeOrder(testOrder);
 
expect(fakeGateway.getCharges()).toHaveLength(1);
expect(fakeGateway.getCharges()[0].amount).toBe(testOrder.total);

Interface Segregation Alignment

When extracting interfaces, include only the methods that clients actually need. If different clients use different subsets of a class's methods, create separate, focused interfaces. This aligns with the Interface Segregation Principle.

Common Refactoring Patterns

The Refactoring Catalog: Tools for Transformation

Extract Method

Extract Class

Move Method and Move Field

Rename Method, Variable, and Class

Replace Conditional with Polymorphism

Introduce Parameter Object

Extract Interface

There’s more to understand.

Common Refactoring Patterns

The Refactoring Catalog: Tools for Transformation

Extract Method

Extract Class

Move Method and Move Field

Rename Method, Variable, and Class

Replace Conditional with Polymorphism

Introduce Parameter Object

Extract Interface

There’s more to understand.