Functional Programming

// JavaScript — sum of squares of even numbers
const result = [1, 2, 3, 4, 5, 6]
    .filter(n => n % 2 === 0)   // [2, 4, 6]
    .map(n => n * n)            // [4, 16, 36]
    .reduce((a, b) => a + b, 0); // 56

Same logic in procedural style would need a loop, a temp variable, and three if/accumulator steps.

const add = a => b => a + b;     // curried
const add5 = add(5);             // partial application
add5(3); // 8

Building specialized functions from general ones, by fixing some arguments up front.

-- Haskell: factorial
factorial :: Int -> Int
factorial 0 = 1
factorial n = n * factorial (n - 1)

FP languages often optimize tail recursion into loops, so you don't blow the stack.

-- Haskell: define an algebraic type, match on its shape
data Shape = Circle Double | Square Double | Rectangle Double Double

area :: Shape -> Double
area (Circle r)         = pi * r * r
area (Square s)         = s * s
area (Rectangle w h)    = w * h

Algebraic data types + pattern matching = a uniquely powerful way to model data.

A monad is a wrapper around a value with a standard way to chain operations. Examples:

• Maybe / Option — value or nothing (no nulls).
• Either / Result — success or error (no exceptions).
• Promise / Future — async value.
• IO — controlled side effects.

If you've used .then() on JavaScript Promises, you've used a monad — without the scary name.

Don't compute a value until it's actually needed. Enables infinite data structures (like an infinite list of primes) and big performance wins.