Node.js async loop with callback from function before next loop

Question

I have an async.whilst loop, which is for-looping through a string array called hashtagon each cycle.

During the for-loop, searchTwitter() function is fired. I need the for-loop to pause until a callback is returned from the searchTwitter() function, then there's a 2 second pause, then the for-loop continues to the next string in the hashtag array to pass to searchTwitter().

After for loop finishes, there's a timeout in the async.whilst loop, for 10 seconds and then whilst loop reloads the for-loop cycle again.

My code below is firing all the searchTwitter() functions immediately without waiting for callback or setTimeout(callback,2000) :

async.whilst(
    function () { return 0 == 0 },
    function (callback) {

        for (var i = 0; i < hashtag.length; i++) {
            searchTwitter(hashtag[i]);
            setTimeout(callback, 2000);
        }

        setTimeout(callback, 10000);
    },
    function (err) {
    }
);


function searchTwitter(tag, callback){

    T.get('search/tweets', { q: '#fun', count: 100 }, function(err, data, response) {
        console.log(data);
        callback();
    });

}

Anatoliy · Accepted Answer · 2014-09-14 22:37:40Z

2

async.whilst(
    function () { return 0 == 0 },
    function (callback) {
        async.eachSeries(hashtag, searchTwitter, callback);
    },
    function (err) {
    }
);

function searchTwitter(tag, done) {
    // search for `tag`
    // call `done` when done
}

Read docs: async#eachSeries

answered Sep 14, 2014 at 22:37

Anatoliy

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Comments

Mike S · Accepted Answer · 2014-09-14 22:47:02Z

Like @Anatoliy mentioned in his answer, async.eachSeries is going to help you out with that for loop. It will loop over your hashtag array, calling the next item only after you've called the callback function.

The trick to getting those delays in place is to call your setTimeout functions inside the callbacks from searchTwitter and the completion callback from eachSeries. Like so:

async.whilst(
    function () { return true }, // true makes more sense than 0 == 0, especially from a readability standpoint
    function (callback) {
        async.eachSeries(hashtag, function(tag, callback) {
            searchTwitter(tag, function() {
                setTimeout(callback, 2000); // Wait 2 seconds before going on to the next tag
            })
        }, function(err) {
            setTimeout(callback, 10000); // Wait 10 seconds before looping over the hashtags again
        });
    },
    function (err) {
    }
);

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