Generator.js

        const Generator = Object.getPrototypeOf(function* () { });
        Generator.prototype.map = function* (mapper) {
            for (const it of this) {
                yield mapper(it);
            }
        };
        Generator.prototype.filter = function* (predicate) {
            for (const it of this) {
                if (predicate(it)) yield it;
            }
        };
        Generator.prototype.take = function* (n) {
            let i = 0;
            for (const it of this) {
                if (i >= n) break;
                yield it;
                i++;
            }
        };
        Generator.prototype.toList = () => Array.from(this);
        /// aka toList()[0]
        Generator.prototype.first = function (n) {
            const it = toList();
            return it ? it[0] : null;
        };