zip-vs-array-from.js

// Lazy (=on-demand) zip()
for (const [i, x] of zip(naturalNumbers(), naturalNumbers())) {
    console.log(i, x);
    if (i >= 2) break;
}
// Output:
// 0 0
// 1 1
// 2 2

// Eager Array.from() (.entries() is lazy)
for (const [i, x] of Array.from(naturalNumbers()).entries()) {
    console.log(i, x);
    if (i >= 2) break;
}
// Infinite loop

function* naturalNumbers() {
    for (let n=0;; 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 };
        }
    }
}