fix test

[mesteche]

In your test of the fix combinator, I think you don't actually test the combinator since f is redefined inside it's anonymous parent function. Maybe try replacing this:

const result = fix((f) => {
                return function f(n) {
                    return n === 0 ? 1 : n * f(n - 1);
                };
})(5);

with this:

const result = fix(f => n => n === 0 ? 1 : n * f(n - 1))(5);