Y Combinator and Trampolines in Javascript

why.js

const assert = require('assert');

// const is_even = n =>
//     (n === 0) || !is_even(n - 1);

const mock_is_even = (myself, n) =>
    (n === 0) || !myself(myself, n - 1);

const mockingbird =
    fn =>
        (...args) =>
            fn(fn, ...args);
      
// const is_even = mockingbird(mock_is_even);
// assert(is_even(10));
// assert(!is_even(11));

const memoized = fn => {
    const map = new Map();
    
    return (...args) => {
        const key = JSON.stringify(args);
        return map[key] || (map[key] = fn(...args));
    }
}

// const is_even_memo = mockingbird(memoized(mock_is_even));
// assert(is_even_memo(10));
// assert(!is_even_memo(11));

// console.time('slow');
// is_even_memo(100);
// console.timeEnd('slow');

// console.time('fast');
// is_even_memo(100);
// console.timeEnd('fast');

const Y_is_even = (myself, n) => (n === 0) || !myself(n - 1);

const Y = fn => {
    const wrapper = (...args) => fn(wrapper, ...args);
    return wrapper;
}

const is_even = Y(Y_is_even);
assert(is_even(10));
assert(!is_even(11));

const is_even_memo = Y(memoized(Y_is_even));

console.time('slow');
is_even_memo(100);
console.timeEnd('slow');

console.time('fast');
is_even_memo(100);
console.timeEnd('fast');

const longtailed = fn => (...args0) => {

    class Thunk {
        constructor (delayed) {
            this.delayed = delayed;
        }

        evaluate () {
            return this.delayed();
        }
    }

    const wrapper = (...args) =>
        new Thunk(() => fn(wrapper, ...args));

    let value = fn(wrapper, ...args0);

    while (value instanceof Thunk)
        value = value.evaluate();

    return value;
}

const Y_is_even_tailrec = (myself, n) =>
    n === 0 ? true : 
    n === 1 ? false :
    myself(n - 2);

const iter_is_even = longtailed(Y_is_even_tailrec);
assert(iter_is_even(10));
assert(!iter_is_even(11));

// No problem!
iter_is_even(1000000);

// Maximum call stack size exceeded
// is_even(1000000);

const iter_is_even_memo = longtailed(memoized(Y_is_even_tailrec));
assert(iter_is_even_memo(10));
assert(!iter_is_even_memo(11));

console.time('slow');
iter_is_even_memo(100);
console.timeEnd('slow');

console.time('fast');
iter_is_even_memo(100);
console.timeEnd('fast');