nth fib

fib.js

const fib = num => num < 3
  ? 1
  : fib(num - 1) + fib(num - 2)

console.log(fib(7))

y_combinator_fib.js

const Y = f => (x => f(x(x)))(x => f((...y) => x(x)(...y)))

const fib = Y(
  f => num => num < 3
    ? 1
    : f(num - 1) + f(num - 2)
)

console.log(fib(7))