Strategy Pattern
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Basic Strategy
A strategy represents one algorithm behind a common interface, while a context object holds a reference to whichever strategy it was given and delegates the actual work to it. This means the context never needs an if/else chain to pick behavior — it just calls the strategy it was handed.
Example: Basic Strategy
interface PaymentStrategy {
pay(amount: number): string;
}
class CreditCard implements PaymentStrategy {
pay(amount: number) { return `Paid ${amount} by credit card`; }
}
class Context {
constructor(private strategy: PaymentStrategy) {}
checkout(amount: number) { return this.strategy.pay(amount); }
}
console.log(new Context(new CreditCard()).checkout(100));Switching Strategies
A context can expose a setter so its strategy can be swapped while the program is running, letting the same object change behavior in response to user input, configuration, or runtime conditions without being recreated.
Example: Switching Strategies
interface PaymentStrategy {
pay(amount: number): string;
}
class PayPal implements PaymentStrategy {
pay(amount: number) { return `Paid ${amount} via PayPal`; }
}
class Context {
private strategy: PaymentStrategy;
constructor(strategy: PaymentStrategy) { this.strategy = strategy; }
setStrategy(strategy: PaymentStrategy) { this.strategy = strategy; }
checkout(amount: number) { return this.strategy.pay(amount); }
}
const ctx = new Context(new PayPal());
console.log(ctx.checkout(50));Function Strategies
For simple algorithms, a typed function can serve as a strategy without creating a separate class for it — TypeScript's structural typing means any function matching the expected signature is automatically compatible.
Example: Function Strategies
type SortStrategy = (a: number, b: number) => number;
const ascending: SortStrategy = (a, b) => a - b;
const descending: SortStrategy = (a, b) => b - a;
function sortWith(nums: number[], strategy: SortStrategy) {
return [...nums].sort(strategy);
}
console.log(sortWith([3, 1, 2], ascending));Benefits of Strategy
Strategy reduces large conditional blocks and isolates each algorithm so it can be tested, replaced, or extended independently, which matters most once a codebase has several interchangeable variants of the same operation.
Example: Benefits of Strategy
interface Discount {
apply(price: number): number;
}
class NoDiscount implements Discount {
apply(price: number) { return price; }
}
class TenPercentOff implements Discount {
apply(price: number) { return price * 0.9; }
}
function checkout(price: number, discount: Discount) {
return discount.apply(price);
}
console.log(checkout(100, new TenPercentOff()));When to Use Strategy
Use Strategy when an operation genuinely has multiple interchangeable approaches that might change at runtime or grow over time. Avoid it when one simple algorithm is sufficient — the extra interface adds indirection you don't need yet.
Example: When to Use Strategy
interface Formatter {
format(value: number): string;
}
class PlainFormatter implements Formatter {
format(value: number) { return String(value); }
}
function display(value: number, formatter: Formatter) {
return formatter.format(value);
}
console.log(display(42, new PlainFormatter()));Chapter Quiz — Complete all 7 topics to unlock
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