javascript - NODE.JS - How to invoke correctly next() after multi REQUESTS are done?

Question

I am running over an ARRAY using forEach() loop. For every element , I need to invoke a request. I want to call the next() after ALL invoked requests are done (assume I have at least a 100 requests to invoke).

Any ideas how to approach this?

Here is a little sample code to demonstrate my question -

var arr = ["A" , "B", "C" , "D" , "E"];
arr.forEach(function (arrayItem) {
	var options = { 
        method: 'GET',
        url:"some_url",
        headers: {...} 
  };
  request(options, function (error, response, body)
  {
    if (error)  {next(error);}

    // DO_SOMETHING based on arrayItem

  }); // end of request()
});  //end of forEach

// WHERE should I place the next()?

Answer 1

There are a couple ways you can do this. Since all the calls to request() are asynchronous and thus will finish sometime in the future long after your loop has finished, you will have to keep track of them somehow. I'll show two methods, one using Promises and one using a counter.

Promises

// create wrapper function that returns a promise
function requestPromise(options) {
    return new Promise(function(resolve, reject) {
        request(options, function(error, response, body) {
            if (error) return reject(error);
            resolve({response: response, body: body});
        });
    });
}

var arr = ["A" , "B", "C" , "D" , "E"];

Promise.all(arr.map(function(item) {
    // create options object here for each request
    var options = { 
        method: 'GET',
        url:"some_url",
        headers: {...} 
    };

    return requestPromise(options);

})).then(function(results) {
    // process results here

    // call next() here because all processing is now done
    next();
}).catch(function(err) {
    // error happened
    next(err);
});

Manual Counter

var arr = ["A" , "B", "C" , "D" , "E"];
var errDone = false;
var cntr = 0;
arr.forEach(function (arrayItem) {
    var options = { 
        method: 'GET',
        url:"some_url",
        headers: {...} 
  };
  request(options, function (error, response, body) {
    if (error)  {
        if (!errDone) {
            // latch error so we don't call next(error) multiple times
            errDone = true;
            next(error);
        }
    } else {
        // process result here

        // check if this is the last response
        ++cntr;
        if (cntr === arr.length) {
            // all responses done here
            next();
        }
    }
  });