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I am trying to query my database several times and construct an object which stores every response from my database in a field. Here is my code:
router.post('/search', (req, res) => {
var collection = db.get().collection('styles')
var data = [];
collection.distinct('make.name', (err, docs) => {
data.push({'make': docs });
});
collection.distinct('model', (function (err, docs) {
data.push({'model': docs });
}))
res.send(data);
});
Since NodeJS/Express is asynchronous, this isn't working as I would like. How can I reconstruct this endpoint to make several database calls (from the same collection) and return an object containing it?
I am trying to query my database several times and construct an object which stores every response from my database in a field. Here is my code:
router.post('/search', (req, res) => {
var collection = db.get().collection('styles')
var data = [];
collection.distinct('make.name', (err, docs) => {
data.push({'make': docs });
});
collection.distinct('model', (function (err, docs) {
data.push({'model': docs });
}))
res.send(data);
});
Since NodeJS/Express is asynchronous, this isn't working as I would like. How can I reconstruct this endpoint to make several database calls (from the same collection) and return an object containing it?
Share Improve this question asked Apr 10, 2017 at 14:55 MosheMoshe 2,6744 gold badges30 silver badges72 bronze badges 3- You should use promises. – SLaks Commented Apr 10, 2017 at 14:55
- @SLaks, I understand the concept of Promises, yet I have never applied them. If you can post a code sample, I would mark your answer as correct. – Moshe Commented Apr 10, 2017 at 14:59
- blog.slaks/2015-01-05/introducing-promises – SLaks Commented Apr 10, 2017 at 15:35
2 Answers
Reset to default 5There's more than one way to do it:
Nested callbacks
Without promises you could nest the callbacks:
router.post('/search', (req, res) => {
var collection = db.get().collection('styles')
var data = [];
collection.distinct('make.name', (err, docs) => {
if (err) {
// ALWAYS HANDLE ERRORS!
}
data.push({'make': docs });
collection.distinct('model', (function (err, docs) {
if (err) {
// ALWAYS HANDLE ERRORS!
}
data.push({'model': docs });
res.send(data);
}))
});
});
This would be the easiest way, but note that it is not efficient if those two requests could be done in parallel.
The async
module
You can use the async
module:
router.post('/search', (req, res) => {
var collection = db.get().collection('styles')
var data = [];
async.parallel({
make: cb => collection.distinct('make.name', cb),
model: cb => collection.distinct('model', cb),
}, (err, responses) => {
if (err) {
// ALWAYS HANDLE ERRORS!
}
data.push({'make': responses.make });
data.push({'model': responses.model });
res.send(data);
});
});
See: https://caolan.github.io/async/docs.html#parallel
But this may still not be the most convenient method.
ES2017 async
/await
The most flexible way of doing that if you have 30 calls to make would be to:
- Use functions that return promises instead of functions that take callbacks
- Use async/await if you can or at least generator based coroutines
- Await on promises (or yield promises) when the logic needs to run in sequence
- Use
Promise.all()
for anything that can be done in parallel
With async/await your code could look like this:
// in sequence:
var make = await collection.distinct('make.name');
var model = await collection.distinct('model');
// use 'make' and 'model'
Or:
// in parallel:
var array = await Promise.all([
collection.distinct('make.name'),
collection.distinct('model'),
]);
// use array[0] and array[1]
A big advantage of async
/await
is the error handling:
try {
var x = await asyncFunc1();
var array = await Promise.all([asyncFunc2(x), asyncFunc3(x)]);
var y = asyncFunc4(array);
console.log(await asyncFunc5(y));
} catch (err) {
// handle any error here
}
You can only use it inside of a function created with the async
keyword. For more info, see:
- https://developer.mozilla/en-US/docs/Web/JavaScript/Reference/Statements/async_function
- https://developer.mozilla/en-US/docs/Web/JavaScript/Reference/Operators/await
For support in browsers, see:
- http://caniuse./async-functions
For support in Node, see:
- http://node.green/#ES2017-features-async-functions
In places where you don't have native support for async
and await
you can use Babel:
- https://babeljs.io/docs/plugins/transform-async-to-generator/
or with a slightly different syntax a generator based approach like in co
or Bluebird coroutines:
- https://www.npmjs./package/co
- http://bluebirdjs./docs/api/promise.coroutine.html
See those answers for more info:
- try/catch blocks with async/await
- node.js ~ constructing chained sequence of Promise resolves
- How to run Generator Functions in Parallel?
- node.js ~ constructing chained sequence of Promise resolves
- Using async/await + Bluebird to promisifyAll
- jQuery: Return data after ajax call success
You can do it with Promises
router.post('/search', (req, res) => {
var collection = db.get().collection('styles');
// Create promise for "make.name" query
let firstQuery = new Promise((resolve, reject) => {
collection.distinct('make.name', (err, docs) => {
if (!err) {
resolve(docs);
} else {
reject(err);
}
});
});
// Create promise for "model" query
let secondQuery = new Promise((resolve, reject) => {
collection.distinct('model', (function (err, docs) {
if (!err) {
resolve(docs);
} else {
reject(err);
}
}))
})
// Run both queries at the same time and handle both resolve results or first reject
Promise.all([firstQuery, secondQuery])
.then((results) => {
res.send({ "make.name": results[0], "model": results[1] });
})
.catch((err) => {
// Catch error
res.send({});
});
});
Also you can use destructuring in callback functions like that:
Promise.all([firstQuery, secondQuery])
.then(([makeName, model]) => res.send({ "make.name": makeName, model }))
UPD: If you have a bunch of collection to request you can create an array of collections name, map it to promise requests and handle with Promise.all, for example
let collections = ["firstCollection", "secondCollection", "nCollection"];
let promises = collections.map((collectionName) => {
return new Promise((resolve, reject) => {
collection.distinct(collectionName, (err, docs) => {
if (!err) {
resolve(docs)
} else {
reject(err);
}
});
})
});
Promise.all(promises)
.then(results => {
// Do what you want to do
})
.catch(error => {
// or catch
});
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