How to use generators in a proper way with loops

Let's say that we have some data in an array and we need to save each array item in a separate document in mongodb

Here is a code how do I try to do that:

const co = require('co');

const Model = new require('./mongoose').Schema({...});

const data = [
    {...},
    {...},
    {...},
    {...}
];

function* saveData() {
    for (let i = 0; i < data.length; i++) {
        yield (new Model(data[i])).save(() => {
            console.log(i);
        });
    }

    yield function*() { console.log(`xxx`); };
}

co(saveData).then(() => {
    console.log(`The end. Do here some cool things`);
});

I expect that 'the end' will be outputed after all data saved and console would look like this:

0
1
2
3
xxx
The end. Do here some cool things

But what I get is:

0
1
2
xxx
The end. Do here some cool things
3

How to fix the code to :
1. make the code to output xxx after saving all the items
2. make the code to outpt The end... really at the end

Solution

Does this solve your issue? Change

yield (new Model(data[i])).save(() => {
            console.log(i);
        });

yield (new Model(data[i])).save().then(() => console.log(i));

Basically, since you're yielding a promise, my spidey sense is wondering how the ordering of that works. By using .then, you guarantee that the generator won't yield until the console.log completes.