hilbert.js

const pass = (fn, value) => {
    fn(value);
    return value;
};

const UP = (point, fn) => pass(fn, [point[0], point[1] + 1]),
    DOWN = (point, fn) => pass(fn, [point[0], point[1] - 1]),
    LEFT = (point, fn) => pass(fn, [point[0] - 1, point[1]]),
    RIGHT = (point, fn) => pass(fn, [point[0] + 1, point[1]]);

const A = (p, fn, n) => n == 0 ? p : 
    B(DOWN(A(RIGHT(A(UP(D(p, fn, n - 1), fn), fn, n - 1), fn), fn, n - 1), fn), fn, n - 1);

const B = (p, fn, n) => n == 0 ? p : 
    A(RIGHT(B(DOWN(B(LEFT(C(p, fn, n - 1), fn), fn, n - 1), fn), fn, n - 1), fn), fn, n - 1);

const C = (p, fn, n) => n == 0 ? p : 
    D(UP(C(LEFT(C(DOWN(B(p, fn, n - 1), fn), fn, n - 1), fn), fn, n - 1), fn), fn, n - 1);

const D = (p, fn, n) => n == 0 ? p : 
    C(LEFT(D(UP(D(RIGHT(A(p, fn, n - 1), fn), fn, n - 1), fn), fn, n - 1), fn), fn, n - 1);