Javascript for-loop async File execution

Question

In the next code, I want to process several files at the same time without wait to the end of each other. For this reason, I first read the files (array) and then the callback is called to process an element of this array instance.

I have found a problem into this javascript code, exactly in a async for-loop, where this process is executed as a sync code instead of async.

var array = ['string1','string2','string3','string4'];

function processArray (arrayString,callback){
    //Read file Example.csv thought sync way
    try{
        var ifs = new InputFileStream('Example.csv','utf8');
        table = ifs.read(0);
        ifs.close();
    }catch(err){
        console.log(err.stack);
    }

    callback(arrayString, table);

}

//Async for
for (var i=0; i<array.length; i++) {                
    processArray(array[i], function(arrayString, table){
        //Here process the file values thought async way
        console.log('processed_'+i);
    });
}

What makes you think this construction is supposed to behave asynchronously? — Cerbrus
– Cerbrus, Commented Aug 18, 2015 at 12:16
I guess that this example is the right way to implement a for-loop async. Do you know any other method to do it? — JosepB
– JosepB, Commented Aug 18, 2015 at 12:19
What I was hinting at is that this isn't async. What makes you think it is? — Cerbrus
– Cerbrus, Commented Aug 18, 2015 at 12:21
what is InputFileStream - not standard javascript that I know — Jaromanda X
– Jaromanda X, Commented Aug 18, 2015 at 12:24

bhspencer · Accepted Answer · 2015-08-18 13:32:36Z

1

You could put the call back in a setTimeout with a delay of 1ms. That will run it in the next block of execution and your loop will continue on.

e.g. use this:

setTimeout(function() { callback(arrayString, table); }, 1);

instead of this:

callback(arrayString, table);

An alternative to this is to run the callback on a separate thread using Web Workers. I don't think it would appropiate to provide a long answer describing how to do multi threaded JavaScript here so I'll just leave the link to the docs. https://developer.mozilla.org/en-US/docs/Web/API/Web_Workers_API/Using_web_workers

edited Aug 18, 2015 at 13:32

answered Aug 18, 2015 at 12:41

bhspencer

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Fernando Carvajal · Accepted Answer · 2018-01-25 00:18:47Z

where this process is executed as a sync code instead of async

I've seen that you just have find out the answers of your question, remember that JavaScript is single thread.

So, for that when you execute operations that require full use of CPU like for..loops, while, etc; you just will get your code running synchronous and not only that,

You will get your web page freeze if they are huge loops

Let me give you an example, this is a while loop that will run for 6 seconds, look how you cannot do anything in stackoverflow.

function blocker (ms) {
    console.log('You cannot do anything')
    var now = new Date().getTime();
    while(true) {
        if (new Date().getTime() > now +ms)
            return;
    }   
}
blocker(6000) //This stop your entire web page for 6 seconds

If you really want to achieve running blocking code in the background read about Web Workers or you just can use a small library I wrote, that allow you to execute a blocking CPU function in the background, I called it GenericWebWorker

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