The Y combinator

The Y combinator

const Y = (fn) => ((g) => g(g))((g) => fn((x) => g(g)(x)))

const factorialGenerator = (f) => (n) => n === 0 ? 1 : n * f(n - 1)
const factorial = Y(factorialGenerator)
factorial(5) // 120

const sumFromZeroToNGenerator = (f) => (n) => n <= 1 ? n : n + f(n - 1)
const sumFromZeroToN = Y(sumFromZeroToNGenerator)
sumFromZeroToN(5) // 15

const fibonaciGenerator = (f) => (n) => n < 2 ? 1 : f(n - 2) + f(n - 1)
const fibonaci = Y(fibonaciGenerator)
fibonaci(7) // 21