What is a Value Object - Value Objects | OneNoughtOne

0/246

System Design (LLD)Value Objects

Value Objects in Domain-Driven Design

LevelIntermediate

Duration55 mins

TopicValue Objects

1 / 4

What is a Value Object

Beyond Identity: Objects That Describe

In Domain-Driven Design, we often focus on Entities—objects with distinct identities that we track throughout their lifecycle. A Customer with ID 42 remains the same customer even if they change their name or address. But there's another category of domain objects that's equally fundamental, yet frequently overlooked or misunderstood: Value Objects.

Value Objects represent the descriptive aspects of your domain. They don't have identity—they are defined entirely by their attributes. A Money object representing $100.00 USD doesn't have an identity; there's nothing distinguishing one "$100.00 USD" from another. What matters is what it describes, not which particular instance it is.

Understanding and properly implementing value objects is a hallmark of sophisticated domain modeling. They reduce bugs, improve expressiveness, and create self-documenting code that captures domain concepts with precision.

What You Will Learn

By the end of this page, you will understand what value objects are, how they differ from entities, and why they are essential building blocks in any well-designed domain model. You'll see concrete examples that illuminate when an object should be a value object and how to recognize them in your domain.

The Concept of Identity in Domain Modeling

To understand value objects, we must first crystallize the concept of identity in domain modeling.

Identity means:

An object remains "the same object" even when its attributes change
We track the object across time and through state transitions
Two objects with identical attributes are NOT the same if they have different identities

Consider a Customer entity:

customer-entity.ts
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
// Customer is an ENTITY - identity matters
class Customer {
    private readonly id: CustomerId;
    private name: string;
    private email: Email;
    private address: Address;
 
    constructor(id: CustomerId, name: string, email: Email, address: Address) {
        this.id = id;
        this.name = name;
        this.email = email;
        this.address = address;
    }
 
    // Customer 42 is STILL Customer 42 even after changing name
    changeName(newName: string): void {
        this.name = newName;
    }
 
    // Identity comparison - two customers are equal only if same ID
    equals(other: Customer): boolean {
        return this.id.equals(other.id);
    }
}
 
// Two different Customer objects with same attributes 
// are NOT the same customer
const customer1 = new Customer(new CustomerId(42), "Alice", email, address);
const customer2 = new Customer(new CustomerId(99), "Alice", email, address);
// customer1.equals(customer2) is FALSE - different identities

Now consider: Does the address within that customer have identity? If the customer moves from "123 Main St" to "456 Oak Ave", is it the same address that transformed, or did the customer simply switch to a different address?

The answer is crucial: The address itself doesn't have identity. We don't track addresses across their "lifecycle." We care about what the address is, not which specific address instance it is. An address at "123 Main St, New York, NY 10001" is indistinguishable from any other address with those same values.

This is the essence of a Value Object.

Defining Value Objects

A Value Object is an object that:

Has no conceptual identity — It is defined entirely by its attributes
Is immutable — Once created, its state never changes
Compares by value — Two value objects are equal if all their attributes are equal
Is replaceable — Interchangeable with any other instance having the same attributes
Is side-effect free — Operations return new instances rather than modifying existing ones

Eric Evans, in his seminal book Domain-Driven Design, describes value objects as objects that "describe some characteristic or attribute but carry no concept of identity."

The Litmus Test for Value Objects

Ask yourself: "If I have two instances with identical attributes, does it matter which one I use?" If the answer is NO—if they're completely interchangeable—you have a value object. If the answer is YES—if you need to track which specific instance—you have an entity.

Value Object vs Entity Comparison
Characteristic	Value Object	Entity
Identity	None — defined by attributes	Distinct identity, tracked over time
Equality	By value (all attributes)	By identity (ID only)
Mutability	Immutable	Typically mutable
Lifecycle	No lifecycle, replaceable	Has lifecycle, state transitions
Examples	Money, DateRange, Address, Email	Customer, Order, Account, Product
Persistence	Embedded in entity	Has own table/document

Canonical Value Object Examples

Let's examine several canonical examples of value objects that appear across countless domains. Understanding these patterns will help you recognize value objects in your own domain.

Money: The Quintessential Value Object

Money is perhaps the most universally applicable value object. A $50.00 USD is completely interchangeable with any other $50.00 USD. We don't track individual instances of "fifty dollars"—we care only about the amount and currency.

money-value-object.ts
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
// Money - a canonical value object
class Money {
    private readonly amount: number;
    private readonly currency: Currency;
 
    private constructor(amount: number, currency: Currency) {
        // Validation at construction
        if (amount < 0) {
            throw new Error("Amount cannot be negative");
        }
        this.amount = Money.roundToMinorUnits(amount, currency);
        this.currency = currency;
    }
 
    // Factory methods for creation
    static of(amount: number, currency: Currency): Money {
        return new Money(amount, currency);
    }
 
    static usd(amount: number): Money {
        return new Money(amount, Currency.USD);
    }
 
    static zero(currency: Currency): Money {
        return new Money(0, currency);
    }
 
    // Operations return NEW instances (immutability)
    add(other: Money): Money {
        this.ensureSameCurrency(other);
        return new Money(this.amount + other.amount, this.currency);
    }
 
    subtract(other: Money): Money {
        this.ensureSameCurrency(other);
        const result = this.amount - other.amount;
        if (result < 0) {
            throw new Error("Insufficient amount");
        }
        return new Money(result, this.currency);
    }
 
    multiply(factor: number): Money {
        return new Money(this.amount * factor, this.currency);
    }
 
    // Value equality - two Money objects are equal if same amount AND currency
    equals(other: Money): boolean {
        return this.amount === other.amount && 
               this.currency === other.currency;
    }
 
    // Immutable - no setters, no mutation methods
    getAmount(): number {
        return this.amount;
    }
 
    getCurrency(): Currency {
        return this.currency;
    }
 
    private ensureSameCurrency(other: Money): void {
        if (this.currency !== other.currency) {
            throw new Error(`Currency mismatch: ${this.currency} vs ${other.currency}`);
        }
    }
 
    private static roundToMinorUnits(amount: number, currency: Currency): number {
        const minorUnits = currency.getMinorUnits();
        const factor = Math.pow(10, minorUnits);
        return Math.round(amount * factor) / factor;
    }
}
 
// Usage
const price = Money.usd(99.99);
const tax = Money.usd(8.00);
const total = price.add(tax); // Returns NEW Money instance
 
// These are equal - same value
const fifty1 = Money.usd(50.00);
const fifty2 = Money.usd(50.00);
console.log(fifty1.equals(fifty2)); // true

DateRange: Capturing Temporal Concepts

A date range defines a period between two dates. The specific instance doesn't matter—what matters is the start and end dates.

date-range.ts
TypeScript
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
// DateRange - a value object for temporal spans
class DateRange {
    private readonly start: Date;
    private readonly end: Date;
 
    private constructor(start: Date, end: Date) {
        if (start > end) {
            throw new Error("Start date must be before or equal to end date");
        }
        this.start = new Date(start); // Defensive copy
        this.end = new Date(end);
    }
 
    static between(start: Date, end: Date): DateRange {
        return new DateRange(start, end);
    }
 
    static singleDay(date: Date): DateRange {
        return new DateRange(date, date);
    }
 
    // Query methods
    contains(date: Date): boolean {
        return date >= this.start && date <= this.end;
    }
 
    overlaps(other: DateRange): boolean {
        return this.start <= other.end && this.end >= other.start;
    }
 
    getDurationInDays(): number {
        const diffTime = this.end.getTime() - this.start.getTime();
        return Math.ceil(diffTime / (1000 * 60 * 60 * 24)) + 1;
    }
 
    // Operations return new instances
    extend(days: number): DateRange {
        const newEnd = new Date(this.end);
        newEnd.setDate(newEnd.getDate() + days);
        return new DateRange(this.start, newEnd);
    }
 
    intersect(other: DateRange): DateRange | null {
        if (!this.overlaps(other)) {
            return null;
        }
        const newStart = this.start > other.start ? this.start : other.start;
        const newEnd = this.end < other.end ? this.end : other.end;
        return new DateRange(newStart, newEnd);
    }
 
    // Value equality
    equals(other: DateRange): boolean {
        return this.start.getTime() === other.start.getTime() &&
               this.end.getTime() === other.end.getTime();
    }
 
    // Immutable accessors (return copies to prevent mutation)
    getStart(): Date {
        return new Date(this.start);
    }
 
    getEnd(): Date {
        return new Date(this.end);
    }
}

Address: Compositional Value Objects

Addresses are composed of multiple parts (street, city, postal code), but the address as a whole has no identity. It describes a location.

address-value-object.ts
TypeScript
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
// Address - a composite value object
class Address {
    private readonly street: string;
    private readonly city: string;
    private readonly state: string;
    private readonly postalCode: PostalCode;
    private readonly country: Country;
 
    private constructor(
        street: string, 
        city: string, 
        state: string,
        postalCode: PostalCode,
        country: Country
    ) {
        // Validation
        if (!street.trim()) {
            throw new Error("Street is required");
        }
        if (!city.trim()) {
            throw new Error("City is required");
        }
        
        this.street = street.trim();
        this.city = city.trim();
        this.state = state.trim();
        this.postalCode = postalCode;
        this.country = country;
    }
 
    static create(
        street: string,
        city: string,
        state: string,
        postalCode: string,
        country: string
    ): Address {
        return new Address(
            street,
            city,
            state,
            PostalCode.of(postalCode),
            Country.of(country)
        );
    }
 
    // Returns new address with changed street
    // (used when customer provides correction, for example)
    withStreet(newStreet: string): Address {
        return new Address(
            newStreet,
            this.city,
            this.state,
            this.postalCode,
            this.country
        );
    }
 
    isSameCity(other: Address): boolean {
        return this.city === other.city && 
               this.country.equals(other.country);
    }
 
    formatForMailing(): string {
        return `${this.street}
${this.city}, ${this.state} ${this.postalCode}
${this.country}`;
    }
 
    // Value equality - all fields must match
    equals(other: Address): boolean {
        return this.street === other.street &&
               this.city === other.city &&
               this.state === other.state &&
               this.postalCode.equals(other.postalCode) &&
               this.country.equals(other.country);
    }
}
 
// PostalCode is ALSO a value object
class PostalCode {
    private readonly value: string;
 
    private constructor(value: string) {
        if (!PostalCode.isValid(value)) {
            throw new Error(`Invalid postal code: ${value}`);
        }
        this.value = value.toUpperCase().replace(/\s/g, '');
    }
 
    static of(value: string): PostalCode {
        return new PostalCode(value);
    }
 
    private static isValid(value: string): boolean {
        // Simple validation - adjust for your country
        const normalized = value.replace(/\s/g, '');
        return /^[A-Z0-9]{3,10}$/.test(normalized.toUpperCase());
    }
 
    equals(other: PostalCode): boolean {
        return this.value === other.value;
    }
 
    toString(): string {
        return this.value;
    }
}

Value Objects vs Primitive Obsession

One of the most common code smells addressed by value objects is Primitive Obsession—the overuse of primitive types (strings, numbers, booleans) to represent domain concepts.

Consider an email address. In many codebases, you'll see:

primitive-obsession.ts
TypeScript
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
// ❌ PRIMITIVE OBSESSION: Email as string
class Customer {
    private email: string; // Just a string!
    
    setEmail(email: string): void {
        // Validation scattered everywhere email is used
        if (!email.includes('@')) {
            throw new Error('Invalid email');
        }
        this.email = email;
    }
}
 
// Same validation repeated in different classes
class Newsletter {
    subscribe(email: string): void {
        if (!email.includes('@')) {
            throw new Error('Invalid email');
        }
        // ... subscribe logic
    }
}
 
// Bug: someone forgot to validate
class NotificationService {
    send(email: string, message: string): void {
        // Oops, no validation here!
        this.emailClient.send(email, message);
    }
}

Problems with Primitive Obsession

When domain concepts are represented as primitives: (1) Validation is scattered and duplicated, (2) Invalid values can slip through cracks, (3) Domain logic has no natural home, (4) Code loses domain expressiveness, (5) Mixing incompatible values (email vs phone number) causes silent bugs.

The solution is to create a proper value object:

email-value-object.ts
TypeScript
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
// ✅ PROPER VALUE OBJECT: Email with behavior
class Email {
    private readonly value: string;
 
    private constructor(value: string) {
        const normalized = value.toLowerCase().trim();
        if (!Email.isValid(normalized)) {
            throw new InvalidEmailError(value);
        }
        this.value = normalized;
    }
 
    static of(value: string): Email {
        return new Email(value);
    }
 
    private static isValid(email: string): boolean {
        // Comprehensive validation in ONE place
        const emailRegex = /^[^\s@]+@[^\s@]+\.[^\s@]+$/;
        return emailRegex.test(email) && email.length <= 254;
    }
 
    getDomain(): string {
        return this.value.split('@')[1];
    }
 
    getLocalPart(): string {
        return this.value.split('@')[0];
    }
 
    isBusinessEmail(): boolean {
        const personalDomains = ['gmail.com', 'yahoo.com', 'hotmail.com'];
        return !personalDomains.includes(this.getDomain());
    }
 
    equals(other: Email): boolean {
        return this.value === other.value;
    }
 
    toString(): string {
        return this.value;
    }
}
 
// Now the Customer class is cleaner and safer
class Customer {
    private email: Email; // Type system enforces validity
    
    setEmail(email: Email): void {
        this.email = email; // Already validated at creation
    }
}
 
// Newsletter can't receive invalid emails
class Newsletter {
    subscribe(email: Email): void {
        // email is GUARANTEED to be valid
        if (email.isBusinessEmail()) {
            this.addToCorporateList(email);
        } else {
            this.addToPersonalList(email);
        }
    }
}
 
// Type system prevents mixing incompatible values
function processCustomer(
    email: Email,        // Must pass Email object
    phone: PhoneNumber   // Must pass PhoneNumber object
): void {
    // Can't accidentally pass phone where email is expected
}

Benefits of Value Objects over Primitives

•Validation in one place — Create once, trust everywhere. If you have an Email object, it's valid.
•Type safety — Can't accidentally pass a phone number where email is expected.
•Domain behavior has a home — email.getDomain(), email.isBusinessEmail() belong here.
•Self-documenting code — Method signatures reveal intent: send(Email email) vs send(String email).
•Encapsulated normalization — Lowercase, trim, format conversions happen automatically.

Characteristics of Well-Designed Value Objects

A well-designed value object is:

Essential Characteristics

•Immutable — No setters, no mutation methods. All operations create new instances. This guarantees thread-safety and eliminates aliasing bugs.
•Self-validating — Invalid instances cannot exist. Validation happens at construction, throwing immediately if constraints are violated.
•Equality by value — Two objects are equal if and only if all their attributes are equal. Implements equals() and hashCode() properly.
•Side-effect free — Methods query state or return new values; they never modify the object or produce observable side effects.
•Expressiveness via behavior — Contains domain-relevant methods that reveal what can be done with this concept.

Immutability is Non-Negotiable

Immutability is the cornerstone of value objects. A mutable value object is a contradiction—if it can change, it has state that persists across changes, which implies tracking identity. Immutability also provides thread-safety for free: you can share value objects across threads without synchronization.

Implementing Proper Equality

Value equality must be implemented correctly. Two Money objects of $50.00 USD must be equal regardless of which instance they are:

value-equality.ts
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
class Money {
    private readonly amount: number;
    private readonly currency: Currency;
 
    // ... constructor and other methods ...
 
    equals(other: Money | null | undefined): boolean {
        if (other === null || other === undefined) {
            return false;
        }
        if (!(other instanceof Money)) {
            return false;
        }
        return this.amount === other.amount && 
               this.currency === other.currency;
    }
 
    // In TypeScript/JavaScript, also override for collections
    // that might use string keys
    hashCode(): number {
        let hash = 17;
        hash = hash * 31 + Math.floor(this.amount * 100);
        hash = hash * 31 + this.currency.hashCode();
        return hash;
    }
}
 
// Usage demonstrating value equality
const m1 = Money.usd(100);
const m2 = Money.usd(100);
const m3 = Money.usd(50);
 
console.log(m1.equals(m2)); // true - same value
console.log(m1.equals(m3)); // false - different amounts
console.log(m1 === m2);     // false - different references (expected!)

Common Domain Value Objects

While value objects are domain-specific, certain patterns appear across most business domains. Recognizing these helps you identify candidates in your own domain:

Common Value Objects Across Domains
Category	Value Objects	Key Behaviors
Monetary	Money, Currency, Tax, Discount, Price	Arithmetic, formatting, currency conversion
Temporal	DateRange, TimeSlot, Duration, Deadline	Overlap detection, containment, comparison
Geographic	Address, Coordinates, Distance, Region	Formatting, proximity, containment
Identity Tokens	Email, PhoneNumber, URL, SSN, VIN	Validation, normalization, parsing
Quantities	Weight, Volume, Length, Temperature	Unit conversion, arithmetic, comparison
Business	SKU, OrderNumber, InvoiceNumber	Validation, formatting, parsing
Ranges	PriceRange, AgeRange, QuantityRange	Containment, overlap, intersection
Composite	FullName, PersonName, CompanyName	Formatting, parsing, comparison

Value Object Discovery Heuristic

Look for attributes in your entities that are primitives but carry domain meaning. Ask: 'What validation should this have? What operations make sense on this value?' If there are behaviors beyond simple get/set, you likely have a value object hiding inside a primitive.

Value Objects Across Languages

Different programming languages offer varying levels of support for value object implementation. Some languages have built-in constructs; others require more ceremony.

Java 16+ introduced records, which are ideal for simple value objects:

Java
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
// Java Record - immutable value object with minimal boilerplate
public record Money(BigDecimal amount, Currency currency) {
    
    // Compact constructor for validation
    public Money {
        if (amount.compareTo(BigDecimal.ZERO) < 0) {
            throw new IllegalArgumentException("Amount cannot be negative");
        }
        amount = amount.setScale(2, RoundingMode.HALF_UP);
    }
 
    // Additional factory methods
    public static Money usd(double amount) {
        return new Money(BigDecimal.valueOf(amount), Currency.USD);
    }
 
    // Operations return new instances
    public Money add(Money other) {
        requireSameCurrency(other);
        return new Money(amount.add(other.amount), currency);
    }
}
 
// Records provide:
// - equals() based on all components
// - hashCode() based on all components  
// - toString() showing all components
// - Immutability (fields are final)

Summary: What is a Value Object

We've established the foundational understanding of value objects. Let's consolidate the key insights:

Key Takeaways

•Value objects have no identity — They are defined entirely by their attributes. Two objects with the same values are interchangeable.
•Value objects are immutable — Once created, they never change. Operations return new instances.
•Value objects compare by value — Equality is based on all attributes, not object reference or ID.
•Value objects combat primitive obsession — They encapsulate validation and behavior for domain concepts.
•Value objects are self-validating — Invalid instances cannot exist; validation happens at construction.
•Value objects reveal domain language — They make code more expressive and self-documenting.

What's next:

Now that we understand what value objects are, we'll dive deep into their two most critical characteristics: immutability and equality. The next page explores why immutability is non-negotiable, how to implement proper equality, and the subtle bugs that arise when these are done incorrectly.

Page Complete

You now understand the fundamental nature of value objects—identity-less, immutable objects defined by their attributes. You can recognize when a domain concept should be a value object and understand how they differ from entities. Next, we'll explore the critical topics of immutability and equality in depth.