How to use aysnc module in node.js to loop over two arrays

Question

How to use aysnc module in node.js to loop over two arrays

I want to loop over an array and then write some data to the database. The code below shows how I would do it in a non-async way with a for loop. I know this is not the preferred way of doing it.

for(var x = 0;x < tt.matches.length;x++) //Match each player with a match and a playerId     from the tournament tree
    {
        if(tt.matches[x].p[0] !== -1)
        {
            var tmId = JSON.stringify(tt.matches[x].id);
            Player.update({ _id : grUpd.players[y] },{ tournamentMatchId : tmId, treeId : tt.matches[x].p[0], opponent : tt.matches[x].p[1] },{ safe : true }, function (err) {
                if(err)
                {
                    console.log(err);
                }
            });
            y++;
        }
        if(tt.matches[x].p[0] === -1)
        {
            byes++;

        }
        if(tt.matches[x].p[1] !== -1)
        {
            var tmId = JSON.stringify(tt.matches[x].id);
            Player.update({ _id : grUpd.players[y] },{ tournamentMatchId : tmId, treeId : tt.matches[x].p[1], opponent : tt.matches[x].p[0] },{ safe : true }, function (err) {
                if(err)
                {
                    console.log(err);
                }
            });
            y++;
        }
        if(tt.matches[x].p[1] === -1)
        {
            byes++;
        }
 }

I then need to perform the following, again shown in a 'traditional way'.

 for(var x = 0;x < plyrs.length;x++)
 {
     var nextMatch = JSON.stringify(tt.upcoming(plyrs[x].treeId)) ;
 Player.update({ _id : plyrs[x]._id },{ tournamentMatchId : nextMatch },{ safe : true }, function (err) {
        if(err)
            {
            console.log(err);
        }
    });
 }

arrays
node.js
asynchronous

Answer 1

You could do it by keeping a counter of open DB calls, then calling the next phase of your program when all of the calls have returned. See below.

There's a theoretical hole in this approach though, which is that if any of your Player.update() calls return before process.nextTick then the completion condition may be triggered early.

var activeCalls = 0;
for(var x = 0;x < tt.matches.length;x++) //Match each player with a match and a playerId     from the tournament tree
{
    if(tt.matches[x].p[0] !== -1)
    {
        var tmId = JSON.stringify(tt.matches[x].id);
        activeCalls++;
        Player.update({ _id : grUpd.players[y] },{ tournamentMatchId : tmId, treeId : tt.matches[x].p[0], opponent : tt.matches[x].p[1] },{ safe : true }, function (err) {
            activeCalls--;
            if(err)
            {
                console.log(err);
            }
            if ( activeCalls == 0 ) doNextThing()
        });
        y++;
    }
    if(tt.matches[x].p[0] === -1)
    {
        byes++;

    }
    if(tt.matches[x].p[1] !== -1)
    {
        var tmId = JSON.stringify(tt.matches[x].id);
        activeCalls++;
        Player.update({ _id : grUpd.players[y] },{ tournamentMatchId : tmId, treeId : tt.matches[x].p[1], opponent : tt.matches[x].p[0] },{ safe : true }, function (err) {
            activeCalls--;
            if(err)
            {
                console.log(err);
            }
            if ( activeCalls == 0 ) doNextThing()
        });
        y++;
    }
    if(tt.matches[x].p[1] === -1)
    {
        byes++;
    }
}