How to Run Multiple Asynchronous Functions Then Execute Callbacks?
In My Node. Js Code I Need to Make 2 or 3 Api Calls, and Each Will Return Some Data. After All Api Calls Are Complete, I Want to Collect All the Data into a...
In my Node.js code I need to make 2 or 3 API calls, and each will return some data. After all API calls are complete, I want to collect all the data into a single JSON object to send to the frontend.
I know how to do this using the API callbacks (the next call will happen in the previous call's callback) but this would be slow:
//1st request
request(' function (err1, res1, body) {
//2nd request
request(' function (err2, res2, body2) {
//combine data and do something with it
});
});
I know you could also do something similar and neater with promises, but I think the same concept applies where the next call won't execute until the current one has finished.
Is there a way to call all functions at the same time, but for my final block of code to wait for all API calls to complete and supply data before executing?
4 Answers
Promises give you Promise.all() (this is true for native promises as well as library ones like bluebird's).
Update: Since Node 8, you can use util.promisify() like you would with Bluebird's .promisify()
var requestAsync = util.promisify(request); // const util = require('util')
var urls = ['url1', 'url2'];
Promise.all(urls.map(requestAsync)).then(allData => {
// All data available here in the order of the elements in the array
});
So what you can do (native):
function requestAsync(url) {
return new Promise(function(resolve, reject) {
request(url, function(err, res, body) {
if (err) { return reject(err); }
return resolve([res, body]);
});
});
}
Promise.all([requestAsync('url1'), requestAsync('url2')])
.then(function(allData) {
// All data available here in the order it was called.
});
If you have bluebird, this is even simpler:
var requestAsync = Promise.promisify(request);
var urls = ['url1', 'url2'];
Promise.all(urls.map(requestAsync)).then(allData => {
// All data available here in the order of the elements in the array
});
Sounds like async.parallel() would also do the job if you'd like to use async:
var async = require('async');
async.parallel({
one: function(parallelCb) {
request(' function (err, res, body) {
parallelCb(null, {err: err, res: res, body: body});
});
},
two: function(parallelCb) {
request(' function (err, res, body) {
parallelCb(null, {err: err, res: res, body: body});
});
},
three: function(parallelCb) {
request(' function (err, res, body) {
parallelCb(null, {err: err, res: res, body: body});
});
}
}, function(err, results) {
// results will have the results of all 3
console.log(results.one);
console.log(results.two);
console.log(results.three);
});
Promise.all is now included with ES6 so you don't need any 3rd party libraries at all.
"Promise.all waits for all fulfillments (or the first rejection)"
I've setup a gist to demonstrate Promise.all() with refactoring itterations at:
I'm using an IIFE (Immediately Invoked Function Expression). If you're not familiar, you'll want to be for the example below though the gist shows how with using an IIFE.
TL;DR
( function( promises ){
return new Promise( ( resolve, reject ) => {
Promise.all( promises )
.then( values => {
console.log("resolved all promises")
console.dir( values );
resolve( values.reduce( (sum,value) => { return sum+value }) ); //Use Array.prototype.reduce() to sum the values in the array
})
.catch( err => {
console.dir( err );
throw err;
});
});
})([
new Promise( ( resolve, reject ) => {
console.log("resolving 1");
resolve( 1 );
}),
new Promise( ( resolve, reject ) => {
console.log("resolving 2");
resolve( 2 );
})
]).then( sum => { console.dir( { sum: sum } ) } )
I had a similar use case where I had to do 10 concurrent calls. I did it with the combination of async/await and Promise.all.
async function getData() {
try {
let result = null
const ids = [1,2,3,4,5,6,7,8,9,10,11,12,13,14,15]
let promises = ids.map(async (id) => {
return fetch(
`
).then((data) => data.json());
});
result = await Promise.all(promises)
return result
} catch(err) {
console.log("error: ", err)
}
}
getData().then(data => console.log(data))David Miller brings 15 years of experience in global economics, personal finance strategy, and market dynamics. He specializes in turning complex economic trends into actionable insights for everyday readers.
