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I have such a loop :

var someArray = [];

for(var i = 0; i < myArray.length; i++) {
    var tempArray = [];

    arangodb.query('somequery')
        .then(
            cursor => cursor.all()
        ).then(
            keys => tempArray  = keys,
            err => console.error('Failed to execute query:', err)
        ).then(function () {
            someArray.push.apply(someArray, tempArray);
        });
}

I want to do other operations when all tempArrays are collected in someArray. But since Node.js is async, I don't know how to do it. Can you help me with that? Thanks in advance.

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7
  • node.js isn't async, arangodb.query is Commented Aug 6, 2017 at 8:19
  • can arangodb.query('somequery') be run in parallel or do they have to be in series? also, if one fails, continue processing or stop? Commented Aug 6, 2017 at 8:22
  • They can run in parallel, if you refer to my answer Promise.all() would reject if even 1 of the promises fail Commented Aug 6, 2017 at 8:26
  • @marvel308 I was asking what the OP wanted, not what your answer does - perhaps the OP needs in series, perhaps one failure shouldn't reject the whole process - it's up to the OP, not up to your answer Commented Aug 6, 2017 at 8:28
  • @JaromandaX It does not matter if they run in parallel or in series. I just want to process the complete array. And it shouldn't stop when one fails. Commented Aug 6, 2017 at 8:34

3 Answers 3

Reset to default
3

This will result in a flat array of keys from cursor.all()

any arangodb.query that fails will be ignored (still with console output though)

Promise.all(myArray.map(item => 
    arangodb.query('somequery')
    .then(cursor => cursor.all()))
    .catch(err => console.error('Failed to execute query:', err))
)
// remove rejections, which will be undefined
.then(results => results.filter(result => !!result))
// flatten the results
.then(results => [].concat(...results))
.then(results => {
    // do things with the array of results
})
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9 Comments

Will this line exactly be like this : .then(results => [].concat(...results)) ? I mean should there be three dots?
Yes. In es2016 or is it es2017. Anyway. Latest node will like it. Use babel otherwise
Thank you, one last question : in the last part should it be like this : .then(results => { someArray = results; }) Or should there be some other code as well?
Why. You can't access the data synchronously
@jason, yes the result is what you want, but you can't use it outside this function (or a function invoked by this function) because of time. This task is async; you don't know when this function is called and therefore when the result is available. For you this might be just a few milliseconds, but JS can run a lot of code in that time.
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1

you need to use Promise.all()

var someArray = [];

function queryDB(){
    return arangodb.query('somequery')
        .then(
            cursor => cursor.all()).then(
            keys => tempArray  = keys,
            err => console.error('Failed to execute query:', err)
         ).catch(function(err){
            console.log('Failed');
         })
}
var promiseArray = [];
for(var i = 0; i < myArray.length; i++)
{
   promiseArray.push(queryDB());
} 

Promise.all(promiseArray).then(function(results){

    someArray = results.filter(result => !!result);
})

basically queryDB() would return a promise, you can do Promise.all() to wait for all promises to resolve and then you can access the result in result

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8 Comments

This won't work for failures and might signal early completion falsely. Quoting from MDN Promise.all fail-fast behaviour Promise.all is rejected if any of the elements are rejected. For example, if you pass in four promises that resolve after a timeout and one promise that rejects immediately, then Promise.all will reject immediately.
tempArray is undefined, your result would be a array of arrays - you've rewritten too much of the OP's intent
@ManasJayanth so should I pass to someArray after each promise?
@ManasJayanth it wasn't clear in the question, Promise.all() would only resolve if each of the promises resolved and would do so in parallel if it need to be done serially try bluebirdjs.com/docs/api/promise.each.html
@marvel308 You are overlooking cases where some of the tasks fail. Promise.all fails immediately when even one of the tasks fail. Bluebird's docs speak only for their library - which hopefully is spec compliant
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1

The only way to track if all your async operations are complete or not is to simply keep count of success callback triggers and failure callback triggers. Following should help you out.

let count = 0;

const checkCompletion = (curr, total) => {
    if (curr < total) {
        // Not all tasks finised
    } else {
        // All done
    }
};

for(var i = 0; i < myArray.length; i++) {
    var   tempArray = [];
    arangodb.query('somequery')
        .then(cursor => cursor.all())
        .then(keys => {
            // success
            count += 1;
            checkCompletion(count, myArray.length);
        }).catch(e => {
            // failure
            count += 1;
            checkCompletion(count, myArray.length);
        });

}
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