Using map() on an iterator

Question

Say we have a Map: let m = new Map();, using m.values() returns a map iterator.

But I can\'t use forEach() or map() o

Answer 1

You could define another iterator function to loop over this:

function* generator() {
    for (let i = 0; i < 10; i++) {
        console.log(i);
        yield i;
    }
}

function* mapIterator(iterator, mapping) {
    for (let i of iterator) {
        yield mapping(i);
    }
}

let values = generator();
let mapped = mapIterator(values, (i) => {
    let result = i*2;
    console.log(`x2 = ${result}`);
    return result;
});

console.log('The values will be generated right now.');
console.log(Array.from(mapped).join(','));

Now you might ask: why not just use Array.from instead? Because this will run through the entire iterator, save it to a (temporary) array, iterate it again and then do the mapping. If the list is huge (or even potentially infinite) this will lead to unnecessary memory usage.

Of course, if the list of items is fairly small, using Array.from should be more than sufficient.