make asynchronous functions within for loop run in series

Question

So in Node.js let's say I have the following code:

for (var i = 0; i < 1000; i++) {
    someAsynchronousFunction(function(err,res) {
        // Do a bunch of stuff
        callback(null,res);
    });
}

But I want this to run synchronously. I know this is not advised in Node JS, but I am just trying to understand the language. I tried implementing the following solution, but it just ends up hanging during runtime:

for (var i = 0; i < 1000; i++) {
    var going = true;
    someAsynchronousFunction(function(err,res) {
        // Do a bunch of stuff
        callback(null,res);
        going = false;
    });
    while (going) {

    }
}

What is going wrong and what is the correct way to do this?

David Mulder · Accepted Answer · 2017-09-14 09:09:03Z

2

One of the best way to do that is to use the async library.

async.timesSeries(1000, function(n, next){
    someAsynchronousFunction(next);
});

Or you can do that with async.series() function.

.times() documentation : http://caolan.github.io/async/docs.html#.timesSeries

edited Sep 14, 2017 at 9:09

David Mulder

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answered Aug 5, 2016 at 12:40

F. Kauder

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3 Comments

Hurricane Development Over a year ago

Awesome, I was actually looking for an Async function to do this, but I could not find it.

Hurricane Development Over a year ago

Did you mean timesSeries()? Also, form a performance standpoint on strictly asynchronous loop in parallel. Is it faster to run Async.times() or simply do the for loop?

F. Kauder Over a year ago

Yes ! Because .times() run in parallel. For the faster way, I don't know. I think it must be similar, but we need some benchmarks to concluded that.

Jose Hermosilla Rodrigo · Accepted Answer · 2016-08-05 12:43:02Z

1

Another way to do this is using Promises to generate a sequential execution of them thanks to Array#reduce :

// Function that returns Promise that is fllfiled after a second.
function asyncFunc (x){
  return new Promise((rs, rj)=>{
    setTimeout( ()=>{
      console.log('Hello ', x);
      rs();
    }, 1000)
  });
}
// Generate an array filed with values : [0, 1, 2, 3, ...]
Array.from({length : 1000}, (el, i)=> i)
// loop througth the array chaining the promises.
.reduce( (promise, value) => 
   promise.then(asyncFunc.bind(null, value))
, Promise.resolve(null));

answered Aug 5, 2016 at 12:43

Jose Hermosilla Rodrigo

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