Explain the concept of the Strategy pattern

TL;DR

The Strategy pattern is a behavioral design pattern that allows you to define a family of algorithms, encapsulate each one as a separate class, and make them interchangeable. This pattern lets the algorithm vary independently from the clients that use it. For example, if you have different sorting algorithms, you can define each one as a strategy and switch between them without changing the client code.

class Context {
  constructor(strategy) {
    this.strategy = strategy;
  }

  executeStrategy(data) {
    return this.strategy.doAlgorithm(data);
  }
}

class ConcreteStrategyA {
  doAlgorithm(data) {
    // Implementation of algorithm A
  }
}

class ConcreteStrategyB {
  doAlgorithm(data) {
    // Implementation of algorithm B
  }
}

// Usage
const context = new Context(new ConcreteStrategyA());
context.executeStrategy(data);

The Strategy pattern

Definition

The Strategy pattern is a behavioral design pattern that enables selecting an algorithm's behavior at runtime. It defines a family of algorithms, encapsulates each one, and makes them interchangeable. This pattern allows the algorithm to vary independently from the clients that use it.

Components

1. Context: Maintains a reference to a Strategy object and is configured with a ConcreteStrategy object.
2. Strategy: An interface common to all supported algorithms. The Context uses this interface to call the algorithm defined by a ConcreteStrategy.
3. ConcreteStrategy: Implements the Strategy interface to provide a specific algorithm.

Example

Consider a scenario where you have different sorting algorithms and you want to switch between them without changing the client code.

// Strategy interface
class Strategy {
  doAlgorithm(data) {
    throw new Error('This method should be overridden!');
  }
}

// ConcreteStrategyA
class ConcreteStrategyA extends Strategy {
  doAlgorithm(data) {
    return data.sort((a, b) => a - b); // Example: ascending sort
  }
}

// ConcreteStrategyB
class ConcreteStrategyB extends Strategy {
  doAlgorithm(data) {
    return data.sort((a, b) => b - a); // Example: descending sort
  }
}

// Context
class Context {
  constructor(strategy) {
    this.strategy = strategy;
  }

  setStrategy(strategy) {
    this.strategy = strategy;
  }

  executeStrategy(data) {
    return this.strategy.doAlgorithm(data);
  }
}

// Usage
const data = [3, 1, 4, 1, 5, 9];
const context = new Context(new ConcreteStrategyA());
console.log(context.executeStrategy(data)); // Output: [1, 1, 3, 4, 5, 9]

context.setStrategy(new ConcreteStrategyB());
console.log(context.executeStrategy(data)); // Output: [9, 5, 4, 3, 1, 1]

Benefits

• Flexibility: You can change the algorithm at runtime.
• Maintainability: Adding new strategies does not affect existing code.
• Encapsulation: Each algorithm is encapsulated in its own class.

Drawbacks

• Overhead: Increased number of classes and objects.
• Complexity: Can make the system more complex if not used judiciously.

Further reading

• Strategy pattern on Wikipedia
• Refactoring Guru: Strategy pattern
• MDN Web Docs: JavaScript classes