Question

I’m trying to get my head around when to use process.nextTick. Below I’m using async library to control my code flow and was wondering if I should be calling nextTick in the end callback or not.

async.parallel([
    function (callback) {
        // do something
        callback(data);
    },
    function (callback) {
        // do something
        callback(data);        
    }
], 
// callback
function (data) {
    process.nextTick(function () {
        // do something
    });
});

Answer 1

Without knowing exactly what your ‘do something’ is, I don’t see any reason that you’d need to use nextTick in this situation. You use nextTick when you have reason to defer execution of a function to the next iteration of the event loop. You might want to read Understanding process.nextTick for more detail.

Also, note that async’s parallel task completion callbacks take arguments err, data, so you should be doing:

async.parallel([
    function (callback) {
        // do something
        callback(null, data);
    },
    function (callback) {
        // do something
        callback(null, data);        
    }
], 
// callback
function (err, data) {
    if (err) throw err;
    // do something
});

The major difference being that the final callback is invoked immediately if one of the parallel tasks returns an error (calls callback with a truthy first argument).