How to call callback inside other function

Question

How to call callback inside other function

I am trying to build a simple node waterfall via async module. I'm just getting started with asynchronous programming in node.

Basically - how to call callback() inside http.request function to continue waterfall AFTER response IS END?

async.waterfall([
            function (callback) {
                var req = http.request(options, function (response) {
                    var str = ''
                    response.on('data', function (chunk) {
                        str += chunk;
                    });

                    response.on('end', function () {
                        /* I want to call callback() AFTER `response` is END */
                    });
                });
                req.end();
            },
            function (response, callback) {

                // some operations WITH request `output`

            }], function (err, results) {
                // end
        });

javascript
node.js
asynchronous
waterfall

Answer 1

In JavaScript nested function see all parent variables. It is called closure.

That's why you can call your callback directly in the end event listener:

async.waterfall([
    function (callback) {
        var req = http.request(options, function (response) {
            var str = ''
            response.on('data', function (chunk) {
                str += chunk;
            });

            response.on('end', function () {
                /* I want to call callback() AFTER `response` is END */

                callback(null, str);
            });
        });
        req.end();
    },
    function (response, callback) {

    // some operations WITH request `output`

    }
], function (err, results) {
                // end
});