Strategy Implementation

Introduction

The Strategy pattern defines a family of interchangeable algorithms and lets you choose which one to use at runtime.

Basic Strategy Pattern

const strategies = {
  add: (a, b) => a + b,
  subtract: (a, b) => a - b,
  multiply: (a, b) => a * b,
  divide: (a, b) => a / b
};

function calculate(strategy, a, b) {
  if (!strategies[strategy]) throw new Error('Unknown strategy');
  return strategies[strategy](a, b);
}

calculate('add', 5, 3);      // 8
calculate('multiply', 5, 3); // 15

Strategy with Classes

class SortingStrategy {
  sort(data) { throw new Error('Must implement sort()'); }
}

class BubbleSort extends SortingStrategy {
  sort(data) {
    const arr = [...data];
    for (let i = 0; i < arr.length; i++) {
      for (let j = 0; j < arr.length - i - 1; j++) {
        if (arr[j] > arr[j + 1]) [arr[j], arr[j + 1]] = [arr[j + 1], arr[j]];
      }
    }
    return arr;
  }
}

class QuickSort extends SortingStrategy {
  sort(data) {
    if (data.length <= 1) return data;
    const pivot = data[0];
    const left = data.slice(1).filter(x => x < pivot);
    const right = data.slice(1).filter(x => x >= pivot);
    return [...this.sort(left), pivot, ...this.sort(right)];
  }
}

const sorter = new QuickSort();
sorter.sort([3, 1, 4, 1, 5]); // [1, 1, 3, 4, 5]

Summary

Define a family of algorithms as interchangeable strategies
Context delegates to the selected strategy
New strategies can be added without changing existing code
Each strategy is independently testable

Strategy Implementation

Strategy Implementation

Introduction

Basic Strategy Pattern

Strategy with Classes

Summary

Further Reading