Prototype Design Pattern in JavaScript

1. Definition

The Prototype Design Pattern involves creating objects based on a template of an existing object through cloning. It's a way to create duplicate objects without depending on their class.

2. Problem Statement

Consider a scenario where instantiation of a new object is more costly than copying an existing one. Maybe the object creation involves fetching data from a database, performing some heavy computation, or has other overhead. In such cases, direct instantiation for each new object could be inefficient or might not even be feasible.

3. Solution

The Prototype pattern provides a mechanism to copy the original or prototype object and then modify it according to our needs, rather than creating a new instance from scratch. This way, you can avoid costly instantiation and use a pre-existing, partially constructed version as a starting point.

4. Real-World Use Cases

1. Spawning new game entities that are slightly different variations of an existing one.

2. A document editor which can create a copy of a document to serve as a backup.

3. Cloning structures in DNA replication in biology.

5. Implementation Steps

1. Declare a prototype interface with a clone method.

2. The concrete class, which needs cloning capability, implements this interface to return a copy of itself.

3. Client code creates a new instance by copying an existing instance.

6. Implementation in JavaScript

// Prototype interface
function Prototype() {
  this.clone = function() {};
}
// Concrete prototype
function ConcretePrototype(name) {
  this.name = name;
  this.clone = function() {
    return new ConcretePrototype(this.name);
  };
}
// Client code
const prototype1 = new ConcretePrototype('Prototype1');
const prototype2 = prototype1.clone();
prototype2.name = 'Prototype2';
console.log(prototype1.name);  // Prototype1
console.log(prototype2.name);  // Prototype2

Output:

Prototype1
Prototype2

Explanation:

1. Prototype is a simple interface with a clone method, laying out the contract that any object wishing to have prototype-based creation should follow.

2. ConcretePrototype is the actual object that we want to duplicate. It has a property 'name' and implements the clone method to return a new instance of itself.

3. The client creates an object (prototype1) and then creates a clone of it (prototype2). We then modify the clone to differentiate it from the original.

4. When displayed, both the original and the clone have different names, proving that they are distinct objects.

7. When to use?

Use the Prototype pattern when:

1. The classes to instantiate are specified at runtime.

2. Objects can be cloned into different configurations without involving subclassing.

3. Creating a new instance of an object is more expensive than copying an existing one.

4. You want to keep the number of classes in a system to a minimum when the state configurations of objects may be very high.