Functional Programming in JavaScript: Map, Reduce, and Beyond | DevPrep

Functional Programming in JavaScript: Map, Reduce, and Beyond | DevPrep

// Map: transform each element
const prices = [10, 20, 30];
const withTax = prices.map(p => p * 1.18);

// Filter: select elements
const expensive = prices.filter(p => p > 15);

// Reduce: accumulate to single value
const total = prices.reduce((sum, p) => sum + p, 0);

// Chaining
const result = users
  .filter(u => u.active)
  .map(u => u.name)
  .sort();

const compose = (...fns) => (x) => fns.reduceRight((acc, fn) => fn(acc), x);
const pipe = (...fns) => (x) => fns.reduce((acc, fn) => fn(acc), x);

const processUser = pipe(
  validateEmail,
  normalizeUsername,
  hashPassword,
  createUser
);

const user = processUser(rawInput);

const multiply = (a) => (b) => a * b;
const double = multiply(2);
const triple = multiply(3);

double(5); // 10
triple(5); // 15

// Practical currying
const hasPermission = (role) => (action) => (resource) =>
  permissions[role]?.[resource]?.includes(action) ?? false;

const canAdminDelete = hasPermission("admin")("delete");
canAdminDelete("posts"); // true

// Array operations
const added = [...items, newItem];
const removed = items.filter(i => i.id !== targetId);
const updated = items.map(i => i.id === targetId ? { ...i, ...changes } : i);

// Nested object updates
const updatedState = {
  ...state,
  user: {
    ...state.user,
    address: { ...state.user.address, city: "Mumbai" }
  }
};