async parallel and using bind for iteration

Question

Been struggling with this, so thought I would see if anyone have a quick answer. Been trying to get async to work doing parallel calls, and have managed to do that (thanks to others :) ).

However, I now have the problem that I need to send a variable into the callback function. Basically, I am trying to use mongoose to make five requests to the database to find level 0-5 in a table.

Set up a for loop that iterates from 0 to 4, each loop makes a queries.push. Once that loop is done, it calls async.parallel to wait for replies. All that works, except reading i, which contains the number between 0-4. At first I thought I could just send it in by adding (i) in the closure. But that brakes the call to async.parallel (think it finds it and identifies it as not being a function). Think bind is the answer, but not certain to what I should bind to.

console.log("Lets see if we can't figure out this one once and for all");
var queries= [];
var maxLevels = 1;
for ( var i = 0; i < maxLevels; i++ ){
    console.log("Looking for "+ i)
    queries.push(function (cb) {
        console.log("Seaching for "+ i)
        Skill.find({level: i}).exec(function (err, docs) {
            if (err) {
                throw cb(err);
            }

            // do some stuff with docs & pass or directly pass it
            cb(null, docs);
        });
    });
}
console.log("In theory we now have 5 requests");
async.parallel(queries, function(err, docs) {
    // if any query fails
    if (err) {
        throw err;
    }
    console.log("This is what we got back")
    for (var doc in docs){
        console.log("Lets find 0")
        cuDoc = docs
        if (cuDoc === undefined ){
            console.log("Got nothing on "+ cuDoc);
        } else {
            console.log("Looking at " + cuDoc);
        }

    }

})

replace `var i` with `let i` and add the --harmony flag to your app start command. `node --harmony app` — Kevin B, Aug 11 '15 at 14:31
possible duplicate of [JavaScript closure inside loops – simple practical example](http://stackoverflow.com/questions/750486/javascript-closure-inside-loops-simple-practical-example) — Kevin B, Aug 11 '15 at 14:32

score 1 · Answer 1 · answered Aug 11 '15 at 21:15

Why don't you use something like find({level: { $gte: 0, $lte: 5 }}) ? If you want the query to match only integers, you could use {$in: [0,1,2,3,4,5]}.

Anyway, I would have used async.each, that seems more adapted to use indexes, something like this :

var array = ['level 0', 'level 1', 'level 2', 'level 3', 'level 4'];
async.each(array, function(element, callback){
    console.log("element : "+ element);
    console.log("array.indexOf(element) : "+array.indexOf(element));
    /* query here */
    callback();
},function(){
    /* stuff there */
    console.log("done");
})

This is the console :

debugger listening on port 47025
element : level 0
array.indexOf(element) : 0
element : level 1
array.indexOf(element) : 1
element : level 2
array.indexOf(element) : 2
element : level 3
array.indexOf(element) : 3
element : level 4
array.indexOf(element) : 4
done

async parallel and using bind for iteration

1 Answers1