Prototype - A Design Pattern for Object Cloning

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The Prototype is a creational design pattern that lets you copy existing objects without making your code dependent on their classes. It creates new objects by cloning existing prototypes rather than instantiating new ones.

What is the Prototype Pattern?

The Prototype pattern specifies the kinds of objects to create using a prototypical instance, and creates new objects by copying this prototype. This is particularly useful when object creation is costly or complex.

Why Use a Prototype?

Common Use Cases

Complex Object Duplication

Clone objects with complex initialization or configuration.

Performance Optimization

Avoid expensive initialization by cloning pre-configured objects.

Dynamic Object Creation

Create objects at runtime without knowing their concrete classes.

Snapshot/Memento

Save object states for undo/redo functionality.

Advantages and Disadvantages

Advantages ✅	Disadvantages ❌
Reduces object creation cost.	Cloning complex objects with circular references is tricky.
Avoids subclass proliferation.	Deep copying can be complex.
Adds/removes objects at runtime.	Requires implementing clone method for each class.
Specifies new objects by varying values.	Can be difficult with objects having private fields.

Shallow Copy vs Deep Copy

Understanding the difference between shallow and deep copying is crucial:

Shallow Copy

Copies primitive values
Copies references to nested objects (not the objects themselves)
Changes to nested objects affect both original and clone

Deep Copy

Copies primitive values
Recursively copies all nested objects
Clone is completely independent of the original

Examples in Different Languages

// Prototype Interface
interface Prototype {
clone(): Prototype;
}

// Concrete Prototype
class Shape implements Prototype {
x: number;
y: number;
color: string;

constructor(x: number, y: number, color: string) {
this.x = x;
this.y = y;
this.color = color;
}

clone(): Shape {
return new Shape(this.x, this.y, this.color);
}

describe(): string {
return `Shape at (${this.x}, ${this.y}) with color ${this.color}`;
}
}

class Circle extends Shape {
radius: number;

constructor(x: number, y: number, color: string, radius: number) {
super(x, y, color);
this.radius = radius;
}

clone(): Circle {
return new Circle(this.x, this.y, this.color, this.radius);
}

describe(): string {
return `Circle at (${this.x}, ${this.y}) with radius ${this.radius} and color ${this.color}`;
}
}

// Usage
const originalCircle = new Circle(10, 20, "red", 5);
const clonedCircle = originalCircle.clone();

clonedCircle.x = 30;
clonedCircle.color = "blue";

console.log(originalCircle.describe());
// Circle at (10, 20) with radius 5 and color red

console.log(clonedCircle.describe());
// Circle at (30, 20) with radius 5 and color blue

// Prototype Interface
interface Prototype extends Cloneable {
Prototype clone();
}

// Concrete Prototype
class Shape implements Prototype {
protected int x;
protected int y;
protected String color;

public Shape(int x, int y, String color) {
this.x = x;
this.y = y;
this.color = color;
}

@Override
public Shape clone() {
return new Shape(this.x, this.y, this.color);
}

public String describe() {
return String.format("Shape at (%d, %d) with color %s", x, y, color);
}
}

class Circle extends Shape {
private int radius;

public Circle(int x, int y, String color, int radius) {
super(x, y, color);
this.radius = radius;
}

@Override
public Circle clone() {
return new Circle(this.x, this.y, this.color, this.radius);
}

@Override
public String describe() {
return String.format("Circle at (%d, %d) with radius %d and color %s",
x, y, radius, color);
}
}

// Usage
Circle originalCircle = new Circle(10, 20, "red", 5);
Circle clonedCircle = originalCircle.clone();

clonedCircle.x = 30;
clonedCircle.color = "blue";

System.out.println(originalCircle.describe());
System.out.println(clonedCircle.describe());

// Prototype Interface
interface PrototypeInterface {
public function clone(): self;
}

// Concrete Prototype
class Shape implements PrototypeInterface {
protected $x;
protected $y;
protected $color;

public function __construct(int $x, int $y, string $color) {
$this->x = $x;
$this->y = $y;
$this->color = $color;
}

public function clone(): self {
return new self($this->x, $this->y, $this->color);
}

public function describe(): string {
return sprintf("Shape at (%d, %d) with color %s",
$this->x, $this->y, $this->color);
}
}

class Circle extends Shape {
private $radius;

public function __construct(int $x, int $y, string $color, int $radius) {
parent::__construct($x, $y, $color);
$this->radius = $radius;
}

public function clone(): self {
return new self($this->x, $this->y, $this->color, $this->radius);
}

public function describe(): string {
return sprintf("Circle at (%d, %d) with radius %d and color %s",
$this->x, $this->y, $this->radius, $this->color);
}
}

// Usage
$originalCircle = new Circle(10, 20, "red", 5);
$clonedCircle = $originalCircle->clone();

echo $originalCircle->describe() . "\n";
echo $clonedCircle->describe() . "\n";

// Prototype Trait
trait Prototype: Clone {
fn clone_prototype(&self) -> Self;
}

// Concrete Prototype
#[derive(Clone)]
struct Shape {
x: i32,
y: i32,
color: String,
}

impl Shape {
fn new(x: i32, y: i32, color: String) -> Self {
Shape { x, y, color }
}

fn describe(&self) -> String {
format!("Shape at ({}, {}) with color {}", self.x, self.y, self.color)
}
}

impl Prototype for Shape {
fn clone_prototype(&self) -> Self {
self.clone()
}
}

#[derive(Clone)]
struct Circle {
x: i32,
y: i32,
color: String,
radius: i32,
}

impl Circle {
fn new(x: i32, y: i32, color: String, radius: i32) -> Self {
Circle { x, y, color, radius }
}

fn describe(&self) -> String {
format!(
"Circle at ({}, {}) with radius {} and color {}",
self.x, self.y, self.radius, self.color
)
}
}

impl Prototype for Circle {
fn clone_prototype(&self) -> Self {
self.clone()
}
}

// Usage
fn main() {
let original_circle = Circle::new(10, 20, "red".to_string(), 5);
let mut cloned_circle = original_circle.clone_prototype();

cloned_circle.x = 30;
cloned_circle.color = "blue".to_string();

println!("{}", original_circle.describe());
println!("{}", cloned_circle.describe());
}

Example in Vue.js

Prototype for Component State

interface ComponentState {
  clone(): ComponentState;
}

class FormState implements ComponentState {
  fields: Map<string, any>;
  isDirty: boolean;
  errors: Map<string, string>;

  constructor() {
    this.fields = new Map();
    this.isDirty = false;
    this.errors = new Map();
  }

  clone(): FormState {
    const cloned = new FormState();
    cloned.fields = new Map(this.fields);
    cloned.isDirty = this.isDirty;
    cloned.errors = new Map(this.errors);
    return cloned;
  }

  setField(name: string, value: any): void {
    this.fields.set(name, value);
    this.isDirty = true;
  }

  getField(name: string): any {
    return this.fields.get(name);
  }

  setError(field: string, message: string): void {
    this.errors.set(field, message);
  }
}

export { FormState, ComponentState };

Usage in Vue Component with Undo/Redo

<script setup lang="ts">
import { ref } from 'vue';
import { FormState } from './ComponentStatePrototype';

const currentState = ref(new FormState());
const history = ref<FormState[]>([]);
const historyIndex = ref(-1);

const saveState = () => {
  // Remove any forward history
  history.value = history.value.slice(0, historyIndex.value + 1);

  // Add current state to history
  history.value.push(currentState.value.clone());
  historyIndex.value++;
};

const updateField = (name: string, value: any) => {
  saveState();
  currentState.value.setField(name, value);
};

const undo = () => {
  if (historyIndex.value > 0) {
    historyIndex.value--;
    currentState.value = history.value[historyIndex.value].clone();
  }
};

const redo = () => {
  if (historyIndex.value < history.value.length - 1) {
    historyIndex.value++;
    currentState.value = history.value[historyIndex.value].clone();
  }
};

const canUndo = computed(() => historyIndex.value > 0);
const canRedo = computed(() => historyIndex.value < history.value.length - 1);
</script>

<template>
  <div>
    <button @click="undo" :disabled="!canUndo">Undo</button>
    <button @click="redo" :disabled="!canRedo">Redo</button>

    <input
      :value="currentState.getField('name')"
      @input="updateField('name', ($event.target as HTMLInputElement).value)"
      placeholder="Name"
    />
  </div>
</template>

Real-World Example: Deep Cloning Complex Objects

Deep Clone Implementation

class Address {
street: string;
city: string;
country: string;

constructor(street: string, city: string, country: string) {
this.street = street;
this.city = city;
this.country = country;
}

clone(): Address {
return new Address(this.street, this.city, this.country);
}
}

class Person {
name: string;
age: number;
address: Address;
hobbies: string[];

constructor(name: string, age: number, address: Address, hobbies: string[]) {
this.name = name;
this.age = age;
this.address = address;
this.hobbies = hobbies;
}

// Shallow clone - shares references
shallowClone(): Person {
return new Person(this.name, this.age, this.address, this.hobbies);
}

// Deep clone - completely independent
deepClone(): Person {
return new Person(
this.name,
this.age,
this.address.clone(),
[...this.hobbies]
);
}

describe(): string {
return `${this.name}, ${this.age} years old, lives in ${this.address.city}`;
}
}

// Usage - Demonstrating the difference
const originalAddress = new Address("123 Main St", "New York", "USA");
const originalPerson = new Person(
"John Doe",
30,
originalAddress,
["reading", "gaming"]
);

// Shallow clone - address is shared
const shallowClone = originalPerson.shallowClone();
shallowClone.address.city = "Los Angeles";
console.log(originalPerson.describe());
// John Doe, 30 years old, lives in Los Angeles (MODIFIED!)

// Deep clone - address is independent
const deepClone = originalPerson.deepClone();
deepClone.address.city = "Chicago";
console.log(originalPerson.describe());
// John Doe, 30 years old, lives in Los Angeles (NOT MODIFIED)
console.log(deepClone.describe());
// John Doe, 30 years old, lives in Chicago

Real-World Example: Prototype Registry

Registry Pattern

// Abstract Prototype
abstract class GameCharacter {
name: string;
health: number;
attack: number;
defense: number;

constructor(name: string, health: number, attack: number, defense: number) {
this.name = name;
this.health = health;
this.attack = attack;
this.defense = defense;
}

abstract clone(): GameCharacter;

describe(): string {
return `${this.name}: HP=${this.health}, ATK=${this.attack}, DEF=${this.defense}`;
}
}

// Concrete Prototypes
class Warrior extends GameCharacter {
constructor() {
super("Warrior", 150, 30, 25);
}

clone(): Warrior {
const cloned = new Warrior();
cloned.name = this.name;
cloned.health = this.health;
cloned.attack = this.attack;
cloned.defense = this.defense;
return cloned;
}
}

class Mage extends GameCharacter {
mana: number;

constructor() {
super("Mage", 80, 50, 10);
this.mana = 100;
}

clone(): Mage {
const cloned = new Mage();
cloned.name = this.name;
cloned.health = this.health;
cloned.attack = this.attack;
cloned.defense = this.defense;
cloned.mana = this.mana;
return cloned;
}

describe(): string {
return `${super.describe()}, MP=${this.mana}`;
}
}

class Archer extends GameCharacter {
constructor() {
super("Archer", 100, 40, 15);
}

clone(): Archer {
const cloned = new Archer();
cloned.name = this.name;
cloned.health = this.health;
cloned.attack = this.attack;
cloned.defense = this.defense;
return cloned;
}
}

// Prototype Registry
class CharacterRegistry {
private prototypes: Map<string, GameCharacter> = new Map();

registerPrototype(key: string, prototype: GameCharacter): void {
this.prototypes.set(key, prototype);
}

createCharacter(key: string): GameCharacter | undefined {
const prototype = this.prototypes.get(key);
return prototype ? prototype.clone() : undefined;
}

listAvailable(): string[] {
return Array.from(this.prototypes.keys());
}
}

// Usage
const registry = new CharacterRegistry();

// Register prototypes
registry.registerPrototype("warrior", new Warrior());
registry.registerPrototype("mage", new Mage());
registry.registerPrototype("archer", new Archer());

// Create characters from prototypes
const player1 = registry.createCharacter("warrior");
const player2 = registry.createCharacter("mage");
const player3 = registry.createCharacter("archer");

if (player1) console.log(player1.describe());
if (player2) console.log(player2.describe());
if (player3) console.log(player3.describe());

// Modify clones without affecting prototypes
if (player1) {
player1.name = "Player 1 Warrior";
player1.health = 200;
}

// Create another warrior - still has original stats
const player4 = registry.createCharacter("warrior");
if (player4) console.log(player4.describe());
// Warrior: HP=150, ATK=30, DEF=25

Getting Started with Prototype

Identify Clonable Objects Find objects that are expensive to create or need to be duplicated.
Create Prototype Interface Define a clone method in a common interface.
Implement Clone Method Add cloning logic to each concrete class.
Handle Deep vs Shallow Decide whether you need shallow or deep copying.
Optional: Create Registry Build a registry to manage and retrieve prototypes.
Test Cloning Verify that clones are independent of originals.

Best Practices

Use Deep Copy Carefully

Deep copying can be expensive - use it only when necessary.

Handle Circular References

Implement safeguards against circular reference issues.

Consider Built-in Cloning

Use language built-in cloning mechanisms when available.

Document Clone Behavior

Clearly document whether cloning is shallow or deep.

Common Mistakes to Avoid

Cloning Strategies in JavaScript/TypeScript

Using Spread Operator (Shallow)

const original = { name: "John", age: 30 };
const clone = { ...original };

Using Object.assign (Shallow)

const clone = Object.assign({}, original);

Using JSON (Deep, with limitations)

// Limited: doesn't handle functions, dates, undefined, etc.
const clone = JSON.parse(JSON.stringify(original));

Using structuredClone (Deep, modern browsers)

// Modern approach - handles most types
const clone = structuredClone(original);

Prototype vs Factory Method

Prototype	Factory Method
Clones existing objects	Creates new objects from scratch
Runtime object creation	Compile-time class selection
Best for complex initialization	Best for simple instantiation
Requires existing instance	Doesn’t need existing instance

When to Use Prototype?

When object creation is more expensive than copying
When you need to create objects at runtime
When you want to avoid subclass proliferation
When you need to save object states (snapshots)
When objects have many configuration options

When to Avoid Prototype?

When objects are simple and cheap to create
When deep cloning is complex and error-prone
When objects have many circular references
When built-in language features suffice

Advanced: Prototype with Copy Constructor

class Document {
  title: string;
  content: string;
  metadata: Map<string, any>;
  createdAt: Date;

  constructor(title: string, content: string) {
    this.title = title;
    this.content = content;
    this.metadata = new Map();
    this.createdAt = new Date();
  }

  // Copy constructor
  static fromPrototype(prototype: Document): Document {
    const doc = new Document(prototype.title, prototype.content);
    doc.metadata = new Map(prototype.metadata);
    doc.createdAt = new Date(prototype.createdAt);
    return doc;
  }

  clone(): Document {
    return Document.fromPrototype(this);
  }

  addMetadata(key: string, value: any): void {
    this.metadata.set(key, value);
  }
}

// Usage
const template = new Document("Template", "Template content");
template.addMetadata("author", "System");

const doc1 = template.clone();
doc1.title = "Document 1";
doc1.addMetadata("editor", "User 1");

const doc2 = template.clone();
doc2.title = "Document 2";
doc2.addMetadata("editor", "User 2");

Handling Complex Cloning Scenarios

class Node {
  value: number;
  next: Node | null = null;
  visited: boolean = false;

  constructor(value: number) {
    this.value = value;
  }

  // Deep clone with circular reference handling
  clone(visited: Map<Node, Node> = new Map()): Node {
    // Check if already cloned
    if (visited.has(this)) {
      return visited.get(this)!;
    }

    // Create new node
    const cloned = new Node(this.value);
    visited.set(this, cloned);

    // Clone next node if exists
    if (this.next) {
      cloned.next = this.next.clone(visited);
    }

    return cloned;
  }
}

// Usage with circular reference
const node1 = new Node(1);
const node2 = new Node(2);
const node3 = new Node(3);

node1.next = node2;
node2.next = node3;
node3.next = node1; // Circular reference

const clonedNode1 = node1.clone();
// Successfully clones the entire circular structure