Strategy pattern

The strategy pattern defines a family of interchangeable algorithms and lets you choose which one to use at runtime. In JavaScript, since functions are values, it is implemented very naturally by storing functions in an object.

const strategies = {
  normal: (price) => price,
  member: (price) => price * 0.9,    // 10% discount
  vip: (price) => price * 0.8,       // 20% discount
};

function calculatePrice(price, customerType) {
  const strategy = strategies[customerType] || strategies.normal;
  return strategy(price);
}

console.log(calculatePrice(100, "vip"));    // 80
console.log(calculatePrice(100, "member")); // 90

Why is it useful?

It eliminates long if/else or switch chains.
Adding a new behavior is as easy as adding a function to the object.
Each strategy stays isolated and is easy to test separately.

Examples

Strategy to sort in different ways

const sorters = {
  asc: (a, b) => a - b,
  desc: (a, b) => b - a,
};
const numbers = [3, 1, 2];
console.log([...numbers].sort(sorters.asc));
console.log([...numbers].sort(sorters.desc));