functional-patternslisted
Install: claude install-skill aiskillstore/marketplace
# Functional Patterns for Node.js/TypeScript
## Overview
Functional programming patterns that promote testability, composability, and maintainability.
## Pure Functions
### Definition
A pure function:
- Always returns the same output for the same input
- Has no side effects (no I/O, no mutation)
### Examples
```typescript
// Pure: Deterministic, no side effects
const add = (a: number, b: number): number => a + b;
const calculateTotal = (items: OrderItem[]): number =>
items.reduce((sum, item) => sum + item.price * item.quantity, 0);
const filterActiveUsers = (users: User[]): User[] =>
users.filter((user) => user.isActive);
// Impure: Has side effects
const saveUser = async (user: User): Promise<void> => {
await database.save(user); // I/O side effect
};
const logMessage = (msg: string): void => {
console.log(msg); // Side effect
};
const generateId = (): string => {
return crypto.randomUUID(); // Non-deterministic
};
```
### Pure Core, Impure Shell
```typescript
// Pure core - all business logic
const validateOrder = (order: Order): Result<Order, ValidationError> => {
if (!order.items.length) return Result.fail({ code: 'EMPTY_ORDER' });
if (order.total < 0) return Result.fail({ code: 'INVALID_TOTAL' });
return Result.ok(order);
};
const calculateDiscount = (order: Order, user: User): number => {
const baseDiscount = user.isPremium ? 0.15 : 0;
const volumeDiscount = order.items.length > 10 ? 0.05 : 0;
return baseDiscount + volumeDiscount;
};