code.js

var flatten = (a, r, cb) => {
    if (typeof a.length === 'undefined') {
        r.push(a);
        return cb;
    } else if (a.length === 0) {
        return cb;
    } else {
        return flatten(a[0], r, () => flatten(a.slice(1), r, cb));
    }
};

var a = [1,2,[3,4,[5]],6];
var r = [];

var cb = () => flatten(a, r, () => console.log(r));

while (cb = cb());

This is still recursive, though, since flatten() calls itself.