zip-vs-array-from.js

console.log("1. zip");
for (const [i, x] of zip(naturalNumbers(), naturalNumbers())) {
    console.log(i, x);
}

console.log("2. Array.from");
Array.from(naturalNumbers(), function (x, i) {
    console.log(i, x);
});

function* naturalNumbers() {
    for (let n=0; n < 10; n++) {
	console.log("Yield", n);
        yield n;
    }
};

function zip(...iterables) {
    const iterators = iterables.map(i => i[Symbol.iterator]());
    let done = false;
    return {
        [Symbol.iterator]() {
            return this;
        },
        next() {
            if (!done) {
                const items = iterators.map(i => i.next());
                done = items.some(item => item.done);
                if (!done) {
                    return { value: items.map(i => i.value) };
                }
                // Done for the first time: close all iterators
                for (const iterator of iterators) {
                    if (typeof iterator.return === 'function') {
                        iterator.return();
                    }
                }
            }
            // We are done
            return { done: true };
        }
    }
}