Higher-Order Functions

// map, filter, reduce are all HOFs
const numbers = [1, 2, 3, 4, 5];

numbers.map(n => n * 2);           // [2, 4, 6, 8, 10]
numbers.filter(n => n > 3);        // [4, 5]
numbers.reduce((sum, n) => sum + n, 0); // 15

// Custom HOF
function repeat(fn, times) {
  for (let i = 0; i < times; i++) {
    fn(i);
  }
}

repeat(console.log, 3); // 0, 1, 2

function createGreeter(greeting) {
  return function (name) {
    return `${greeting}, ${name}!`;
  };
}

const hello = createGreeter("Hello");
const bye = createGreeter("Goodbye");

hello("Shiva"); // "Hello, Shiva!"
bye("Shiva");   // "Goodbye, Shiva!"

function createValidator(rules) {
  return function (value) {
    return rules.every(rule => rule(value));
  };
}

const isValidPassword = createValidator([
  (v) => v.length >= 8,
  (v) => /[A-Z]/.test(v),
  (v) => /[0-9]/.test(v),
]);

isValidPassword("MyPass123"); // true
isValidPassword("weak");      // false

function withAuth(handler) {
  return function (req, res) {
    if (!req.headers.authorization) {
      return res.status(401).json({ error: "Unauthorized" });
    }
    return handler(req, res);
  };
}

const getProfile = withAuth((req, res) => {
  res.json({ name: "Shiva" });
});

// Pipe: left to right
const pipe = (...fns) => (input) =>
  fns.reduce((result, fn) => fn(result), input);

// Compose: right to left
const compose = (...fns) => (input) =>
  fns.reduceRight((result, fn) => fn(result), input);

const processName = pipe(
  (s) => s.trim(),
  (s) => s.toLowerCase(),
  (s) => s.replace(/\s+/g, "-")
);

processName("  Hello World  "); // "hello-world"