How to make 2 asynchronous requests and track the end of each?

For example, there are 2 requests:

User.find({}, (err, users) => { users = users; }); Category.find({}, (err, categories) => { categories = categories; }); res.json({ users, categories });

And when the answer comes from 2 requests, in order not to lose productivity, return the answer from the server?

    1 answer 1

    I recommend reading about Promise and using Promise.all .

    In short: this method accepts an array of promises and will be fulfilled when all promises are fulfilled (promises)

     const p1 = Promise.resolve(3); const p2 = new Promise((resolve, reject) => { setTimeout(resolve, 100, "foo"); }); Promise.all([p1, p3]).then(values => { console.log(values); // [3, "foo"] через 0.1сек });

    For your case, p1 and p2 should return promises that the data will load (just wrap your methods in promises, and resolve() in callbacks).

    Alternative option:

     const users = null; const categories = null const sendData = () => { if (users && categories) { res.json({ users, categories }); // next(); } }; User.find({}, (err, u) => { users = u; sendData(); }); Category.find({}, (err, c) => { categories = c; sendData(); });

    Another option is to use a chain of callbacks, but then performance is lost, although this is the simplest option that was almost always used before the appearance of ES6.