Writing nested non-blocking loops in node.js

Question

How to properly write the following set of loops in Node.Js without blocking the event loop or without causing out of memory error.

What I have tried so far includes

Combinations of setImmediate()/setInterval()
The Async module see my code here
Thread_a_gogo (this module is no more maintained)

The code.

for(var i = 0; i < 2000; i++)
    for(var j = 0; i < 2000; j++)
        for(var k = 0; k < 2000; k++)
            console.log(i + ":" + j + ":" + k);

Also created a JSFiddle to play around here

I don't think the event loop is what you want to use for computations. Its purpose is for handling async IO. Spinning up a new process that does for loops would be better. — tcooc
– tcooc, Commented Nov 29, 2016 at 16:09
Your async.js code should work, you just have to actually use the asynchronous setImmediate(cb) (etc) instead of immediately calling cb(). — Bergi
– Bergi, Commented Nov 29, 2016 at 19:31

nanobar · Accepted Answer · 2016-11-29 18:42:26Z

1

Not really sure what your use-case is. Javascript is blocking as it's single threaded, it would have to run the loop before moving to something else.

You could for example use a generator to run each iteration on an event though.

function* ticker() {
  for(let i = 0; i < 10; i++)
    for(let j = 0; i < 10; j++)
        for(let k = 0; k < 10; k++)
            yield[i, j, k];
}

const gen = ticker();

setInterval(() => console.log(gen.next().value), 500);

console.log('I am at the end (called straight away)');

edited Nov 29, 2016 at 18:42

answered Nov 29, 2016 at 16:00

nanobar

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1 Comment

Bergi Over a year ago

This never stops. You need to test for .next().done and when it's true, clear the interval.

Bergi · Accepted Answer · 2016-11-29 19:37:29Z

1

You need to combine setImmediate/setTimeout/etc. with the async.js library. async.each is only for orchestration, it doesn't provide any asynchrony itself.

function getPerm(reducedWordList, callback) {
    var sevenLtrWords = _.filter(reducedWordList, word => word.length == 7); 
    var fourLtrWords = _.filter(reducedWordList, word => word.length == 4); 

    async.each(sevenLtrWords, (i, cb) => {
        async.each(sevenLtrWords, (j, cb) => {
            async.each(fourLtrWords, (k, cb) => {
                console.log(i + " " + j + " " + k); 
                setTimeout(cb, 0);
            }, cb);
        }, cb);
    }, callback);
}

answered Nov 29, 2016 at 19:37

Bergi

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2 Comments

C-- Over a year ago

If I understand this correctly, the setTimeOut() in the above code, will only let the innermost async to tick the event loop right?

Bergi Over a year ago

It's setTimeout itself that ticks the event loop. All of these cbs just advance their respective iteration when called, and when the array end is met then it calls the next outer callback instead.

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