Parallel function calls in Node.js

Problem

I need to do a few independent database queries in Node.js. After all queries are executed, response should be sent. My first try looks like this:

templateData = {};

model.getA(function(result) {
    templateData.A = result;

    model.getB(function(result) {
        templateData.B = result;

        model.getC(function(result) {
            templateData.C = result;

            response.send('template', templateData);
        })
    })
});

Of course, this approach in Node.js is not good at all, because all functions are called sequentially and I'm loosing advantages of asynchronous programming pattern. I'm new to Node.js and it's still unclear to me how to call getA(), getB() and getC() in parallel and send response just after everything is finished. Is there some really simple and common way to achieve this?

Problem courtesy of: Peter Krejci

Solution

Use the async package: (npm install async)

async.parallel([
    function(){ ... },
    function(){ ... }
], callback);

https://github.com/caolan/async#parallel

Alternatively, you can use promises:

Q.spread(
    [ model.getA(), model.getB(), model.getC() ],
    function(a, b, c) {
        // set templateData
        return templateData;
    }
).then(...);

(assuming that the get*() methods return promises)

Solution courtesy of: SLaks

Discussion

You could just use a function that checks if all the data is present as your callback to your queries, and if all the data is present then the response could be sent. So something like

function checkData(){
    if (templateData.A && templateData.B && templateData.C){
        //send your response
    }
}

then just don't nest your calls

model.getA(function(result){
    templateData.A = result;
    checkData();
}

model.getB(function(result){
    templateData.B = result;
    checkData();
}

model.getC(function(result){
    templateData.C = result;
    checkData();
}

When all three have completed your response will be sent.

Discussion courtesy of: hobberwickey

This recipe can be found in it's original form on Stack Over Flow.